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615_4 | American Football Conference | then to Las Vegas in 2020, while the San Diego Chargers returned to Los Angeles in 2017 after 56 years in San Diego.
The Houston Oilers moved to Tennessee in 1997, where they were renamed the Tennessee Oilers. The team would change its name again, two years later, to the Tennessee Titans.
The NFL would again expand in 2002, adding the Houston Texans to the AFC. With the exception of the aforementioned relocations since that time, the divisional setup has remained static ever since.
Between 1995 and 2021, the AFC has sent only 9 of its 16 teams to the Super Bowl: New England Patriots (10 times), Denver Broncos (4 times), Pittsburgh Steelers (4 times), Baltimore Ravens (2 times), Indianapolis Colts (2 times), Kansas City Chiefs (2 times), Cincinnati Bengals (1 time), Las Vegas Raiders (1 time), Tennessee Titans (1 time). By contrast, the NFC has sent 13 of the 16 NFC teams during that same time frame with only the Detroit Lions, Minnesota Vikings, and Washington Commanders missing out on an appearance in the Super Bowl. 17 of the 19 AFC champions from 2001 to 2019 have started one of just three quarterbacks - | 260 | 1,128 |
615_5 | American Football Conference | Tom Brady, Peyton Manning and Ben Roethlisberger - in the Super Bowl. The AFC has started 7 quarterbacks in the last 20 Super Bowls, while the NFC has started 16.
Logo
The merged league created a new logo for the AFC that took elements of the old AFL logo, specifically the "A" and the six stars surrounding it. The AFC logo basically remained unchanged from 1970 to 2009. The 2010 NFL season introduced an updated AFC logo, with the most notable revision being the removal of two stars (leaving four representing the four divisions of the AFC), and moving the stars inside the letter, similar to the NFC logo.
Television
NBC aired the AFC's Sunday afternoon and playoff games from 1970 through the 1997 season. From 1998 to 2013, CBS was the primary broadcast rightsholder to the AFC; in those years, all interconference games in which the AFC team was the visiting team were broadcast on either NBC or CBS. Since 2014, the cross-flex policy allows select AFC games (that involve them playing an NFC team at home or intraconference games) to be moved from CBS to Fox. Since 1990, select AFC playoff games have been seen | 260 | 1,124 |
615_6 | American Football Conference | has started 7 quarterbacks in the last 20 Super Bowls, while the NFC has started 16.
Logo
The merged league created a new logo for the AFC that took elements of the old AFL logo, specifically the "A" and the six stars surrounding it. The AFC logo basically remained unchanged from 1970 to 2009. The 2010 NFL season introduced an updated AFC logo, with the most notable revision being the removal of two stars (leaving four representing the four divisions of the AFC), and moving the stars inside the letter, similar to the NFC logo.
Television
NBC aired the AFC's Sunday afternoon and playoff games from 1970 through the 1997 season. From 1998 to 2013, CBS was the primary broadcast rightsholder to the AFC; in those years, all interconference games in which the AFC team was the visiting team were broadcast on either NBC or CBS. Since 2014, the cross-flex policy allows select AFC games (that involve them playing an NFC team at home or intraconference games) to be moved from CBS to Fox. Since 1990, select AFC playoff games have been seen on ABC or ESPN.
References
National Football League
Conference
Sports organizations established in 1970 | 261 | 1,152 |
620_0 | Animal Farm | Animal Farm
Animal Farm is a satirical allegorical novella by George Orwell, first published in England on 17 August 1945. The book tells the story of a group of farm animals who rebel against their human farmer, hoping to create a society where the animals can be equal, free, and happy. Ultimately, the rebellion is betrayed, and the farm ends up in a state as bad as it was before, under the dictatorship of a pig named Napoleon.
According to Orwell, the fable reflects events leading up to the Russian Revolution of 1917 and then on into the Stalinist era of the Soviet Union. Orwell, a democratic socialist, was a critic of Joseph Stalin and hostile to Moscow-directed Stalinism, an attitude that was critically shaped by his experiences during the May Days conflicts between the POUM and Stalinist forces during the Spanish Civil War. In a letter to Yvonne Davet, Orwell described Animal Farm as a satirical tale against Stalin (""), and in his essay "Why I Write" (1946), wrote that Animal Farm was the first book in which he tried, with full consciousness of what he was doing, "to fuse political purpose and artistic purpose into one whole".
The original title was Animal Farm: A Fairy | 259 | 1,197 |
620_1 | Animal Farm | Story, but U.S. publishers dropped the subtitle when it was published in 1946, and only one of the translations during Orwell's lifetime, the Telugu version, kept it. Other titular variations include subtitles like "A Satire" and "A Contemporary Satire". Orwell suggested the title for the French translation, which abbreviates to URSA, the Latin word for "bear", a symbol of Russia. It also played on the French name of the Soviet Union, .
Orwell wrote the book between November 1943 and February 1944, when the United Kingdom was in its wartime alliance with the Soviet Union against Nazi Germany, and the British intelligentsia held Stalin in high esteem, a phenomenon Orwell hated. The manuscript was initially rejected by a number of British and American publishers, including one of Orwell's own, Victor Gollancz, which delayed its publication. It became a great commercial success when it did appear partly because international relations were transformed as the wartime alliance gave way to the Cold War.
Time magazine chose the book as one of the 100 best English-language novels (1923 to 2005); it also featured at number 31 on the Modern Library List of Best 20th-Century Novels, and number 46 | 260 | 1,208 |
620_2 | Animal Farm | 46 on the BBC's The Big Read poll. It won a Retrospective Hugo Award in 1996 and is included in the Great Books of the Western World selection.
Plot summary
The poorly-run Manor Farm near Willingdon, England, is ripened for rebellion from its animal populace by neglect at the hands of the irresponsible and alcoholic farmer, Mr. Jones. One night, the exalted boar, Old Major, holds a conference, at which he calls for the overthrow of humans and teaches the animals a revolutionary song called "Beasts of England". When Old Major dies, two young pigs, Snowball and Napoleon, assume command and stage a revolt, driving Mr. Jones off the farm and renaming the property "Animal Farm". They adopt the Seven Commandments of Animalism, the most important of which is, "All animals are equal". The decree is painted in large letters on one side of the barn. Snowball teaches the animals to read and write, while Napoleon educates young puppies on the principles of Animalism. To commemorate the start of Animal Farm, Snowball raises a green flag with a white hoof and horn. Food is plentiful, and the farm runs smoothly. The pigs elevate themselves to positions of leadership and set aside special food items, | 260 | 1,205 |
620_3 | Animal Farm | ostensibly for their personal health. Following an unsuccessful attempt by Mr. Jones and his associates to retake the farm (later dubbed the "Battle of the Cowshed"), Snowball announces his plans to modernise the farm by building a windmill. Napoleon disputes this idea, and matters come to head, which culminate in Napoleon's dogs chasing Snowball away and Napoleon declaring himself supreme commander.
Napoleon enacts changes to the governance structure of the farm, replacing meetings with a committee of pigs who will run the farm. Through a young porker named Squealer, Napoleon claims credit for the windmill idea, claiming that Snowball was only trying to win animals to his side. The animals work harder with the promise of easier lives with the windmill. When the animals find the windmill collapsed after a violent storm, Napoleon and Squealer persuade the animals that Snowball is trying to sabotage their project, and begin to purge the farm of animals accused by Napoleon of consorting with his old rival. When some animals recall the Battle of the Cowshed, Napoleon (who was nowhere to be found during the battle) gradually smears Snowball to the point of saying he is a collaborator of Mr. Jones, even dismissing the fact that Snowball was given an | 259 | 1,265 |
620_4 | Animal Farm | award of courage while falsely representing himself as the main hero of the battle. "Beasts of England" is replaced with "Animal Farm", while an anthem glorifying Napoleon, who appears to be adopting the lifestyle of a man ("Comrade Napoleon"), is composed and sung. Napoleon then conducts a second purge, during which many animals who are alleged to be helping Snowball in plots are executed by Napoleon's dogs, which troubles the rest of the animals. Despite their hardships, the animals are easily placated by Napoleon's retort that they are better off than they were under Mr. Jones, as well as by the sheep's continual bleating of “four legs good, two legs bad”.
Mr. Frederick, a neighbouring farmer, attacks the farm, using blasting powder to blow up the restored windmill. Although the animals win the battle, they do so at great cost, as many, including Boxer the workhorse, are wounded. Although he recovers from this, Boxer eventually collapses while working on the windmill (being almost 12 years old at that point). He is taken away in a knacker's van, and a donkey called Benjamin alerts the animals of this, but Squealer quickly waves off their alarm by persuading the animals that the | 259 | 1,201 |
620_5 | Animal Farm | van had been purchased from the knacker by an animal hospital and that the previous owner's signboard had not been repainted. Squealer subsequently reports Boxer's death and honours him with a festival the following day. (However, Napoleon had in fact engineered the sale of Boxer to the knacker, allowing him and his inner circle to acquire money to buy whisky for themselves.)
Years pass, the windmill is rebuilt, and another windmill is constructed, which makes the farm a good amount of income. However, the ideals that Snowball discussed, including stalls with electric lighting, heating, and running water, are forgotten, with Napoleon advocating that the happiest animals live simple lives. Snowball has been forgotten, alongside Boxer, with "the exception of the few who knew him". Many of the animals who participated in the rebellion are dead or old. Mr. Jones is also dead, saying he "died in an inebriates' home in another part of the country". The pigs start to resemble humans, as they walk upright, carry whips, drink alcohol, and wear clothes. The Seven Commandments are abridged to just one phrase: "All animals are equal, but some animals are more equal than others." The maxim "Four | 259 | 1,203 |
620_6 | Animal Farm | legs good, two legs bad" is similarly changed to "Four legs good, two legs better." Other changes include the Hoof and Horn flag being replaced with a plain green banner and Old Major's skull, which was previously put on display, being reburied.
Napoleon holds a dinner party for the pigs and local farmers, with whom he celebrates a new alliance. He abolishes the practice of the revolutionary traditions and restores the name "The Manor Farm". The men and pigs start playing cards, flattering and praising each other while cheating at the game. Both Napoleon and Mr. Pilkington, one of the farmers, play the Ace of Spades at the same time and both sides begin fighting loudly over who cheated first. When the animals outside look at the pigs and men, they can no longer distinguish between the two.
Characters
Pigs
Old Major – An aged prize Middle White boar provides the inspiration that fuels the rebellion. He is also called Willingdon Beauty when showing. He is an allegorical combination of Karl Marx, one of the creators of communism, and Vladimir Lenin, the communist leader of the Russian Revolution and the early Soviet nation, in that he draws up the principles of the revolution. His skull being put on revered public display recalls | 259 | 1,251 |
620_7 | Animal Farm | Lenin, whose embalmed body was left in indefinite repose. By the end of the book, the skull is reburied.
Napoleon – "A large, rather fierce-looking Berkshire boar, the only Berkshire on the farm, not much of a talker, but with a reputation for getting his own way". An allegory of Joseph Stalin, Napoleon is the leader of Animal Farm.
Snowball – Napoleon's rival and original head of the farm after Jones' overthrow. His life parallels that of Leon Trotsky, but may also combine elements from Lenin.
Squealer – A small, white, fat porker who serves as Napoleon's second-in-command and minister of propaganda, holding a position similar to that of Vyacheslav Molotov.
Minimus – A poetic pig who writes the second and third national anthems of Animal Farm after the singing of "Beasts of England" is banned. Literary theorist John Rodden compares him to the poet Vladimir Mayakovsky.
The piglets – Hinted to be the children of Napoleon and are the first generation of animals subjugated to his idea of animal inequality.
The young pigs – Four pigs who complain about Napoleon's takeover of the farm but are quickly silenced and later executed, the first | 259 | 1,161 |
620_8 | Animal Farm | animals killed in Napoleon's farm purge. Probably based on the Great Purge of Grigory Zinoviev, Lev Kamenev, Nikolai Bukharin, and Alexei Rykov.
Pinkeye – A minor pig who is mentioned only once; he is the taste tester that samples Napoleon's food to make sure it is not poisoned, in response to rumours about an assassination attempt on Napoleon.
Humans
Mr. Jones – A heavy drinker who is the original owner of Manor Farm, a farm in disrepair with farmhands who often loaf on the job. He is an allegory of Russian Tsar Nicholas II, who abdicated following the February Revolution of 1917 and was murdered, along with the rest of his family, by the Bolsheviks on 17 July 1918. The animals revolt after Jones goes on a drinking binge, returns hungover the following day and neglects them completely. Jones is married, but his wife plays no active role in the book. She seems to live with her husband's drunkenness, going to bed while he stays up drinking till late into the night. In her only other appearance, she hastily throws a few things into a travel bag and flees when she sees that the animals | 259 | 1,106 |
620_9 | Animal Farm | are revolting. Towards the end of the book, one of the farm sows wears her old Sunday dress.
Mr. Frederick – The tough owner of Pinchfield Farm, a small but well-kept neighbouring farm, who briefly enters into an alliance with Napoleon. Animal Farm shares land boundaries with Pinchfield on one side and Foxwood on another, making Animal Farm a "buffer zone" between the two bickering farmers. The animals of Animal Farm are terrified of Frederick, as rumours abound of him abusing his animals and entertaining himself with cockfighting. Napoleon enters into an alliance with Frederick in order to sell surplus timber that Pilkington also sought, but is enraged to learn Frederick paid him in counterfeit money. Shortly after the swindling, Frederick and his men invade Animal Farm, killing many animals and destroying the windmill. The brief alliance and subsequent invasion may allude to the Molotov–Ribbentrop Pact and Operation Barbarossa.
Mr. Pilkington – The easy-going but crafty and well-to-do owner of Foxwood Farm, a large neighbouring farm overgrown with weeds. Pilkington is wealthier than Frederick and owns more land, but his farm is in need of care as opposed to Frederick's smaller but | 259 | 1,205 |
620_10 | Animal Farm | more efficiently run farm. Although on bad terms with Frederick, Pilkington is also concerned about the animal revolution that deposed Jones and worried that this could also happen to him.
Mr. Whymper – A man hired by Napoleon to act as the liaison between Animal Farm and human society. At first, he is used to acquire necessities that cannot be produced on the farm, such as dog biscuits and paraffin wax, but later he procures luxuries like alcohol for the pigs.
Equines
Boxer – A loyal, kind, dedicated, extremely strong, hard-working, and respectable cart-horse, although quite naive and gullible. Boxer does a large share of the physical labour on the farm. He is shown to hold the belief that "Napoleon is always right." At one point, he had challenged Squealer's statement that Snowball was always against the welfare of the farm, earning him an attack from Napoleon's dogs. But Boxer's immense strength repels the attack, worrying the pigs that their authority can be challenged. Boxer has been compared to Alexey Stakhanov, a diligent and enthusiastic role model of the Stakhanovite movement. He has been described as "faithful and strong"; he | 259 | 1,157 |
620_11 | Animal Farm | believes any problem can be solved if he works harder. When Boxer is injured, Napoleon sells him to a local knacker to buy himself whisky, and Squealer gives a moving account, falsifying Boxer's death.
Mollie – A self-centred, self-indulgent, and vain young white mare who quickly leaves for another farm after the revolution, in a manner similar to those who left Russia after the fall of the Tsar. She is only once mentioned again.
Clover – A gentle, caring mare, who shows concern especially for Boxer, who often pushes himself too hard. Clover can read all the letters of the alphabet, but cannot "put words together". She seems to catch on to the sly tricks and schemes set up by Napoleon and Squealer.
Benjamin – A donkey, one of the oldest, wisest animals on the farm, and one of the few who can read properly. He is sceptical, temperamental and cynical: his most frequent remark is, "Life will go on as it has always gone on – that is, badly." The academic Morris Dickstein has suggested there is "a touch of Orwell himself in this creature's timeless scepticism" and indeed, friends called Orwell "Donkey George", | 259 | 1,127 |
620_12 | Animal Farm | "after his grumbling donkey Benjamin, in Animal Farm."
Other animals
Muriel – A wise old goat who is friends with all of the animals on the farm. Similarly to Benjamin, Muriel is one of the few animals on the farm who is not a pig but can read.
The puppies – Offspring of Jessie and Bluebell, the puppies were taken away at birth by Napoleon and raised by him to serve as his powerful security force.
Moses – The Raven, "Mr. Jones's especial pet, was a spy and a tale-bearer, but he was also a clever talker." Initially following Mrs. Jones into exile, he reappears several years later and resumes his role of talking but not working. He regales Animal Farm's denizens with tales of a wondrous place beyond the clouds called "Sugarcandy Mountain, that happy country where we poor animals shall rest forever from our labours!" Orwell portrays established religion as "the black raven of priestcraft – promising pie in the sky when you die, and faithfully serving whoever happens to be in power." His preaching to the animals heartens them, and Napoleon allows Moses to reside at the farm "with an allowance of a gill of beer daily", akin to how | 259 | 1,148 |
620_13 | Animal Farm | Stalin brought back the Russian Orthodox Church during the Second World War.
The sheep – They are not given individual names or personalities. They show limited understanding of Animalism and the political atmosphere of the farm, yet nonetheless they are the voice of blind conformity as they bleat their support of Napoleon's ideals with jingles during his speeches and meetings with Snowball. Their constant bleating of "four legs good, two legs bad" was used as a device to drown out any opposition or alternative views from Snowball, much as Stalin used hysterical crowds to drown out Trotsky. Towards the end of the book, Squealer (the propagandist) trains the sheep to alter their slogan to "four legs good, two legs better", which they dutifully do.
The hens – Also unnamed, the hens are promised at the start of the revolution that they will get to keep their eggs, which are stolen from them under Mr. Jones. However, their eggs are soon taken from them under the premise of buying goods from outside Animal Farm. The hens are among the first to rebel, albeit unsuccessfully, against Napoleon.
The cows – Also unnamed, the cows are enticed into the revolution by promises that their milk will not be stolen but can be used to | 259 | 1,239 |
620_14 | Animal Farm | raise their own calves. Their milk is then stolen by the pigs, who learn to milk them. The milk is stirred into the pigs' mash every day, while the other animals are denied such luxuries.
The cat – Unnamed and never seen to carry out any work, the cat is absent for long periods and is forgiven because her excuses are so convincing and she "purred so affectionately that it was impossible not to believe in her good intentions." She has no interest in the politics of the farm, and the only time she is recorded as having participated in an election, she is found to have actually "voted on both sides."
The ducks – Also unnamed.
The roosters – One arranges to wake Boxer early, and a black one acts as a trumpeter for Napoleon.
The geese – Also unnamed. One gander commits suicide by eating nightshade berries.
Genre and style
George Orwell's Animal Farm is an example of a political satire that was intended to have a "wider application", according to Orwell himself, in terms of its relevance. Stylistically, the work shares many similarities with some of Orwell's other works, most notably 1984, as both have been considered works of Swiftian Satire. Furthermore, | 259 | 1,176 |
620_15 | Animal Farm | these two prominent works seem to suggest Orwell's bleak view of the future for humanity; he seems to stress the potential/current threat of dystopias similar to those in Animal Farm and 1984. In these kinds of works, Orwell distinctly references the disarray and traumatic conditions of Europe following the Second World War. Orwell's style and writing philosophy as a whole were very concerned with the pursuit of truth in writing. Orwell was committed to communicating in a way that was straightforward, given the way that he felt words were commonly used in politics to deceive and confuse. For this reason, he is careful, in Animal Farm, to make sure the narrator speaks in an unbiased and uncomplicated fashion. The difference is seen in the way that the animals speak and interact, as the generally moral animals seem to speak their minds clearly, while the wicked animals on the farm, such as Napoleon, twist language in such a way that it meets their own insidious desires. This style reflects Orwell's close proximation to the issues facing Europe at the time and his determination to comment critically on Stalin's Soviet Russia.
Background
Origin and writing
George Orwell wrote the manuscript between November 1943 and February 1944 after his experiences during the Spanish Civil War, which he described in Homage to | 259 | 1,332 |
620_16 | Animal Farm | Catalonia (1938). In the preface of a 1947 Ukrainian edition of Animal Farm, he explained how escaping the communist purges in Spain taught him "how easily totalitarian propaganda can control the opinion of enlightened people in democratic countries." This motivated Orwell to expose and strongly condemn what he saw as the Stalinist corruption of the original socialist ideals. Homage to Catalonia sold poorly; after seeing Arthur Koestler's best-selling, Darkness at Noon, about the Moscow Trials, Orwell decided that fiction was the best way to describe totalitarianism.
Immediately prior to writing the book, Orwell had quit the BBC. He was also upset about a booklet for propagandists the Ministry of Information had put out. The booklet included instructions on how to quell ideological fears of the Soviet Union, such as directions to claim that the Red Terror was a figment of Nazi imagination.
In the preface, Orwell described the source of the idea of setting the book on a farm:
In 1944, the manuscript was almost lost when a German V-1 flying bomb destroyed his London home. Orwell spent hours sifting through the rubble to find the pages intact.
Publication
Publishing
Orwell initially encountered difficulty getting the manuscript published, largely due to fears that the book might upset the | 259 | 1,312 |
620_17 | Animal Farm | alliance between Britain, the United States, and the Soviet Union. Four publishers refused to publish Animal Farm, yet one had initially accepted the work, but declined it after consulting the Ministry of Information. Eventually, Secker and Warburg published the first edition in 1945.
During the Second World War, it became clear to Orwell that anti-Soviet literature was not something which most major publishing houses would touch – including his regular publisher Gollancz. He also submitted the manuscript to Faber and Faber, where the poet T. S. Eliot (who was a director of the firm) rejected it; Eliot wrote back to Orwell praising the book's "good writing" and "fundamental integrity", but declared that they would only accept it for publication if they had some sympathy for the viewpoint "which I take to be generally Trotskyite". Eliot said he found the view "not convincing", and contended that the pigs were made out to be the best to run the farm; he posited that someone might argue "what was needed ... was not more communism but more public-spirited pigs". Orwell let André Deutsch, who was working for Nicholson & Watson in 1944, read the typescript, and Deutsch was convinced that Nicholson & | 259 | 1,214 |
620_18 | Animal Farm | Watson would want to publish it; however, they did not, and "lectured Orwell on what they perceived to be errors in Animal Farm." In his London Letter on 17 April 1944 for Partisan Review, Orwell wrote that it was "now next door to impossible to get anything overtly anti-Russian printed. Anti-Russian books do appear, but mostly from Catholic publishing firms and always from a religious or frankly reactionary angle."
The publisher Jonathan Cape, who had initially accepted Animal Farm, subsequently rejected the book after an official at the British Ministry of Information warned him off – although the civil servant who it is assumed gave the order was later found to be a Soviet spy. Writing to Leonard Moore, a partner in the literary agency of Christy & Moore, publisher Jonathan Cape explained that the decision had been taken on the advice of a senior official in the Ministry of Information. Such flagrant anti-Soviet bias was unacceptable, and the choice of pigs as the dominant class was thought to be especially offensive. It may reasonably be assumed that the "important official" was a man named Peter Smollett, who was later unmasked as a Soviet agent. Orwell was suspicious of Smollett/Smolka, and he would be one of the names Orwell included | 259 | 1,263 |
620_19 | Animal Farm | in his list of Crypto-Communists and Fellow-Travellers sent to the Information Research Department in 1949. The publisher wrote to Orwell, saying:
Frederic Warburg also faced pressures against publication, even from people in his own office and from his wife Pamela, who felt that it was not the moment for ingratitude towards Stalin and the heroic Red Army, which had played a major part in defeating Adolf Hitler. A Russian translation was printed in the paper Posev, and in giving permission for a Russian translation of Animal Farm, Orwell refused in advance all royalties. A translation in Ukrainian, which was produced in Germany, was confiscated in large part by the American wartime authorities and handed over to the Soviet repatriation commission.
In October 1945, Orwell wrote to Frederic Warburg expressing interest in pursuing the possibility that the political cartoonist David Low might illustrate Animal Farm. Low had written a letter saying that he had had "a good time with Animal Farm – an excellent bit of satire – it would illustrate perfectly." Nothing came of this, and a trial issue produced by Secker & Warburg in 1956 illustrated by John Driver was abandoned, but the Folio Society published an edition in 1984 illustrated by Quentin Blake and an edition illustrated | 259 | 1,295 |
620_20 | Animal Farm | by the cartoonist Ralph Steadman was published by Secker & Warburg in 1995 to celebrate the fiftieth anniversary of the first edition of Animal Farm.
Preface
Orwell originally wrote a preface complaining about British self-censorship and how the British people were suppressing criticism of the USSR, their World War II ally:
Although the first edition allowed space for the preface, it was not included, and as of June 2009 most editions of the book have not included it.
Secker and Warburg published the first edition of Animal Farm in 1945 without an introduction. However, the publisher had provided space for a preface in the author's proof composited from the manuscript. For reasons unknown, no preface was supplied, and the page numbers had to be renumbered at the last minute.
In 1972, Ian Angus found the original typescript titled "The Freedom of the Press", and Bernard Crick published it, together with his own introduction, in The Times Literary Supplement on 15 September 1972 as "How the essay came to be written". Orwell's essay criticised British self-censorship by the press, specifically the suppression of unflattering descriptions of Stalin and the Soviet government. The | 259 | 1,199 |
620_21 | Animal Farm | same essay also appeared in the Italian 1976 edition of Animal Farm with another introduction by Crick, claiming to be the first edition with the preface. Other publishers were still declining to publish it.
Reception
Contemporary reviews of the work were not universally positive. Writing in the American New Republic magazine, George Soule expressed his disappointment in the book, writing that it "puzzled and saddened me. It seemed on the whole dull. The allegory turned out to be a creaking machine for saying in a clumsy way things that have been said better directly." Soule believed that the animals were not consistent enough with their real-world inspirations, and said, "It seems to me that the failure of this book (commercially it is already assured of tremendous success) arises from the fact that the satire deals not with something the author has experienced, but rather with stereotyped ideas about a country which he probably does not know very well".
The Guardian on 24 August 1945 called Animal Farm "a delightfully humorous and caustic satire on the rule of the many by the few". Tosco Fyvel, writing in Tribune on the same day, called the book "a gentle satire on a certain State and on the illusions | 259 | 1,226 |
620_22 | Animal Farm | of an age which may already be behind us." Julian Symons responded, on 7 September, "Should we not expect, in Tribune at least, acknowledgement of the fact that it is a satire not at all gentle upon a particular State – Soviet Russia? It seems to me that a reviewer should have the courage to identify Napoleon with Stalin, and Snowball with Trotsky, and express an opinion favourable or unfavourable to the author, upon a political ground. In a hundred years time perhaps, Animal Farm may be simply a fairy story; today it is a political satire with a good deal of point." Animal Farm has been subject to much comment in the decades since these early remarks.
The CIA, from 1952 to 1957 in Operation Aedinosaur, sent millions of balloons carrying copies of the novel into Poland, Hungary and Czechoslovakia, whose air forces tried to shoot the balloons down.
Time magazine chose Animal Farm as one of the 100 best English-language novels (1923 to 2005); it also featured at number 31 on the Modern Library List of Best 20th-Century Novels. It won a Retrospective Hugo Award in 1996 and is included in the Great Books of the Western World selection.
Popular | 259 | 1,161 |
620_23 | Animal Farm | reading in schools, Animal Farm was ranked the UK's favourite book from school in a 2016 poll.
Animal Farm has also faced an array of challenges in school settings around the US. The following are examples of this controversy that has existed around Orwell's work:
The John Birch Society in Wisconsin challenged the reading of Animal Farm in 1965 because of its reference to masses revolting.
New York State English Council's Committee on Defense Against Censorship found that in 1968, Animal Farm had been widely deemed a "problem book".
A censorship survey conducted in DeKalb County, Georgia, relating to the years 1979–1982, revealed that many schools had attempted to limit access to Animal Farm due to its "political theories".
A superintendent in Bay County, Florida, banned Animal Farm at the middle school and high school levels in 1987.
The Board quickly brought back the book, however, after receiving complaints of the ban as "unconstitutional".
Animal Farm was removed from the Stonington, Connecticut school district curriculum in 2017.
Animal Farm has also faced similar forms of resistance in other countries. The ALA also mentions the way that the book was prevented from being featured at the International Book Fair in Moscow, Russia, in 1977 | 261 | 1,271 |
620_24 | Animal Farm | 1977 and banned from schools in the United Arab Emirates for references to practices or actions that defy Arab or Islamic beliefs, such as pigs or alcohol.
In the same manner, Animal Farm has also faced relatively recent issues in China. In 2018, the government made the decision to censor all online posts about or referring to Animal Farm. However the book itself, as of 2019, remains sold in stores. Amy Hawkins and Jeffrey Wasserstrom of The Atlantic stated in 2019 that the book is widely available in Mainland China for several reasons: censors believe the general public is unlikely to read a highbrow book
, because the elites who do read books feel connected to the ruling party anyway, and because the Communist Party sees being too aggressive in blocking cultural products as a liability. The authors stated "It was—and remains—as easy to buy 1984 and Animal Farm in Shenzhen or Shanghai as it is in London or Los Angeles."
An enhanced version of the book, launched in India in 2017, was widely praised for capturing the author's intent, by republishing the proposed preface of the First Edition and the preface he wrote for the Ukrainian edition.
Analysis
Animalism
The pigs Snowball, Napoleon, and Squealer | 261 | 1,225 |
620_25 | Animal Farm | Squealer adapt Old Major's ideas into "a complete system of thought", which they formally name Animalism, an allegoric reference to Communism, not to be confused with the philosophy Animalism. Soon after, Napoleon and Squealer partake in activities associated with the humans (drinking alcohol, sleeping in beds, trading), which were explicitly prohibited by the Seven Commandments. Squealer is employed to alter the Seven Commandments to account for this humanisation, an allusion to the Soviet government's revising of history in order to exercise control of the people's beliefs about themselves and their society.
The original commandments are:
Whatever goes upon two legs is an enemy.
Whatever goes upon four legs, or has wings, is a friend.
No animal shall wear clothes.
No animal shall sleep in a bed.
No animal shall drink alcohol.
No animal shall kill any other animal.
All animals are equal.
These commandments are also distilled into the maxim "Four legs good, two legs bad!" which is primarily used by the sheep on the farm, often to disrupt discussions and disagreements between animals on the nature of Animalism.
Later, Napoleon and his pigs secretly revise some commandments to clear themselves of accusations of law-breaking. The changed commandments are as follows, | 261 | 1,294 |
620_26 | Animal Farm | with the changes bolded:
Eventually, these are replaced with the maxims, "All animals are equal, but some animals are more equal than others", and "Four legs good, two legs better" as the pigs become more human. This is an ironic twist to the original purpose of the Seven Commandments, which were supposed to keep order within Animal Farm by uniting the animals together against the humans and preventing animals from following the humans' evil habits. Through the revision of the commandments, Orwell demonstrates how simply political dogma can be turned into malleable propaganda.
Significance and allegory
Orwell biographer Jeffrey Meyers has written, "virtually every detail has political significance in this allegory." Orwell himself wrote in 1946, "Of course I intended it primarily as a satire on the Russian revolution ... [and] that kind of revolution (violent conspiratorial revolution, led by unconsciously power-hungry people) can only lead to a change of masters [-] revolutions only effect a radical improvement when the masses are alert." In a preface for a 1947 Ukrainian edition, he stated, "for the past ten years I have been convinced that the destruction of the Soviet myth was essential if we wanted a revival of the socialist | 259 | 1,253 |
620_27 | Animal Farm | movement. On my return from Spain [in 1937] I thought of exposing the Soviet myth in a story that could be easily understood by almost anyone and which could be easily translated into other languages."
The revolt of the animals against Farmer Jones is Orwell's analogy with the October 1917 Bolshevik Revolution. The Battle of the Cowshed has been said to represent the allied invasion of Soviet Russia in 1918, and the defeat of the White Russians in the Russian Civil War. The pigs' rise to preeminence mirrors the rise of a Stalinist bureaucracy in the USSR, just as Napoleon's emergence as the farm's sole leader reflects Stalin's emergence. The pigs' appropriation of milk and apples for their own use, "the turning point of the story" as Orwell termed it in a letter to Dwight Macdonald, stands as an analogy for the crushing of the left-wing 1921 Kronstadt revolt against the Bolsheviks, and the difficult efforts of the animals to build the windmill suggest the various Five Year Plans. The puppies controlled by Napoleon parallel the nurture of the secret police in the Stalinist structure, and the pigs' treatment of the other animals on the farm recalls the internal terror faced by the populace in the 1930s. In | 259 | 1,227 |
620_28 | Animal Farm | chapter seven, when the animals confess their non-existent crimes and are killed, Orwell directly alludes to the purges, confessions and show trials of the late 1930s. These contributed to Orwell's conviction that the Bolshevik revolution had been corrupted and the Soviet system become rotten.
Peter Edgerly Firchow and Peter Davison contend that the Battle of the Windmill, specifically referencing the Battle of Stalingrad and the Battle of Moscow, represents World War II. During the battle, Orwell first wrote, "All the animals, including Napoleon" took cover. Orwell had the publisher alter this to "All the animals except Napoleon" in recognition of Stalin's decision to remain in Moscow during the German advance. Orwell requested the change after he met Józef Czapski in Paris in March 1945. Czapski, a survivor of the Katyn Massacre and an opponent of the Soviet regime, told Orwell, as Orwell wrote to Arthur Koestler, that it had been "the character [and] greatness of Stalin" that saved Russia from the German invasion.
Other connections that writers have suggested illustrate Orwell's telescoping of Russian history from 1917 to 1943 include the wave of rebelliousness that ran through the | 259 | 1,207 |
620_29 | Animal Farm | countryside after the Rebellion, which stands for the abortive revolutions in Hungary and in Germany (Ch IV); the conflict between Napoleon and Snowball (Ch V), parallelling "the two rival and quasi-Messianic beliefs that seemed pitted against one another: Trotskyism, with its faith in the revolutionary vocation of the proletariat of the West; and Stalinism with its glorification of Russia's socialist destiny"; Napoleon's dealings with Whymper and the Willingdon markets (Ch VI), paralleling the Treaty of Rapallo; and Frederick's forged bank notes, parallelling the Hitler-Stalin pact of August 1939, after which Frederick attacks Animal Farm without warning and destroys the windmill.
The book's close, with the pigs and men in a kind of rapprochement, reflected Orwell's view of the 1943 Tehran Conference that seemed to display the establishment of "the best possible relations between the USSR and the West" – but in reality were destined, as Orwell presciently predicted, to continue to unravel. The disagreement between the allies and the start of the Cold War is suggested when Napoleon and Pilkington, both suspicious, each "played an ace of spades simultaneously".
Similarly, the music in the novel, starting with | 259 | 1,230 |
620_30 | Animal Farm | "Beasts of England" and the later anthems, parallels "The Internationale" and its adoption and repudiation by the Soviet authorities as the anthem of the USSR in the 1920s and 1930s.
Adaptations
Stage productions
In 2021, the National Youth Theatre toured a stage version of Animal Farm.
A solo version, adapted and performed by Guy Masterson, premièred at the Traverse Theatre Edinburgh in January 1995 and has toured worldwide since.
A theatrical version, with music by Richard Peaslee and lyrics by Adrian Mitchell, was staged at the National Theatre London on 25 April 1984, directed by Peter Hall. It toured nine cities in 1985.
A new adaptation written and directed by Robert Icke, designed by Bunny Christie with puppetry designed and directed by Toby Olié opened at the Birmingham Repertory Theatre in January 2022 before touring the UK.
Films
Animal Farm has been adapted to film twice. Both differ from the novel and have been accused of taking significant liberties, including sanitising some aspects.
Animal Farm (1954) is an animated film, in which Napoleon is eventually overthrown in a second revolution. In 1974, E. Howard | 259 | 1,146 |
620_31 | Animal Farm | Hunt revealed that he had been sent by the CIA's Psychological Warfare department to obtain the film rights from Orwell's widow, and the resulting 1954 animation was funded by the agency.
Animal Farm (1999) is a live-action TV version that shows Napoleon's regime collapsing in on itself, with the farm having new human owners, reflecting the collapse of Soviet communism.
Andy Serkis is directing a film adaptation for Netflix, with Matt Reeves producing. Serkis began work on the film after finishing directing duties for Venom: Let There Be Carnage.
Radio dramatisations
A BBC radio version, produced by Rayner Heppenstall, was broadcast in January 1947. Orwell listened to the production at his home in Canonbury Square, London, with Hugh Gordon Porteous, amongst others. Orwell later wrote to Heppenstall that Porteous, "who had not read the book, grasped what was happening after a few minutes."
A further radio production, again using Orwell's own dramatisation of the book, was broadcast in January 2013 on BBC Radio 4. Tamsin Greig narrated, and the cast included Nicky Henson as Napoleon, Toby Jones as the propagandist Squealer, and Ralph | 259 | 1,154 |
620_32 | Animal Farm | Ineson as Boxer.
Comic strip
In 1950, Norman Pett and his writing partner Don Freeman were secretly hired by the Information Research Department (IRD), a secret wing of the British Foreign Office, to adapt Animal Farm into a comic strip. This comic was not published in the U.K. but ran in Brazilian and Burmese newspapers.
See also
Information Research Department
Authoritarian personality
History of Soviet Russia and the Soviet Union (1917–1927)
History of the Soviet Union (1927–1953)
Ideocracy
New class
Anthems in Animal Farm
Animals, an album based on Animal Farm
Books
Gulliver's Travels was a favourite book of Orwell's. Swift reverses the role of horses and human beings in the fourth book. Orwell brought to Animal Farm "a dose of Swiftian misanthropy, looking ahead to a time 'when the human race had finally been overthrown.'"
Bunt (Revolt), published in 1924, is a book by Polish Nobel laureate Władysław Reymont with a theme similar to Animal Farms.
White Acre vs. Black Acre, published in 1856 and written by William M. Burwell, is a satirical novel | 259 | 1,081 |
620_33 | Animal Farm | that features allegories for slavery in the United States similar to Animal Farms portrayal of Soviet history.
George Orwell's own Nineteen Eighty-Four, a classic dystopian novel about totalitarianism.
References
Explanatory notes
Citations
General sources
Further reading
O'Neill, Terry, Readings on Animal Farm (1998), Greenhaven Press. .
External links
Animal Farm Book Notes from Literapedia
Excerpts from Orwell's letters to his agent concerning Animal Farm
Literary Journal review
Orwell's original preface to the book
Animal Farm Revisited by John Molyneux, International Socialism, 44 (1989)
Animal Farm at the British Library
Animal Farm (1954)
1945 British novels
Allegory
British novellas
British novels adapted into films
British novels adapted into plays
British novels adapted into television shows
British political novels
British satirical novels
Cats in literature
Cattle in literature
Censored books
Dogs in literature
Dystopian novels
English novels
Hugo Award for Best Novella winning works
Novels about animals
Novels about propaganda
Novels about revolutionaries
Novels about totalitarianism
Novels adapted into comics
Novels | 262 | 1,175 |
620_34 | Animal Farm | totalitarianism.
References
Explanatory notes
Citations
General sources
Further reading
O'Neill, Terry, Readings on Animal Farm (1998), Greenhaven Press. .
External links
Animal Farm Book Notes from Literapedia
Excerpts from Orwell's letters to his agent concerning Animal Farm
Literary Journal review
Orwell's original preface to the book
Animal Farm Revisited by John Molyneux, International Socialism, 44 (1989)
Animal Farm at the British Library
Animal Farm (1954)
1945 British novels
Allegory
British novellas
British novels adapted into films
British novels adapted into plays
British novels adapted into television shows
British political novels
British satirical novels
Cats in literature
Cattle in literature
Censored books
Dogs in literature
Dystopian novels
English novels
Hugo Award for Best Novella winning works
Novels about animals
Novels about propaganda
Novels about revolutionaries
Novels about totalitarianism
Novels adapted into comics
Novels adapted into radio programs
Novels by George Orwell
Pigs in literature
Political literature
Roman à clef novels
Satirical novels
Secker & Warburg books | 260 | 1,141 |
621_0 | Amphibian | Amphibian
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this.
The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. They are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.
The earliest amphibians ("crown") evolved in the Carboniferous period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became | 261 | 1,248 |
621_1 | Amphibian | dominant during the Carboniferous and Permian periods, but were later displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia.
The three modern orders of amphibians are Anura (the frogs), Urodela (the salamanders), and Apoda (the caecilians). The number of known amphibian species is approximately 8,000, of which nearly 90% are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just . The largest living amphibian is the South China giant salamander (Andrias sligoi), but this is dwarfed by the extinct Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.
Classification
The word amphibian is derived from the Ancient Greek term (), which means 'both kinds of life', meaning 'of both kinds' and meaning 'life'. The term was initially used as a general adjective for animals that | 261 | 1,080 |
621_2 | Amphibian | could live on land or in water, including seals and otters. Traditionally, the class Amphibia includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest sense () was divided into three subclasses, two of which are extinct:
Subclass Lepospondyli† (small Paleozoic group, which are more closely related to amniotes than Lissamphibia)
Subclass Temnospondyli† (diverse Paleozoic and early Mesozoic grade)
Subclass Lissamphibia (all modern amphibians, including frogs, toads, salamanders, newts and caecilians)
Salientia (frogs, toads and relatives): Jurassic to present—7,360 current species in 53 families
Caudata (salamanders, newts and relatives): Jurassic to present—764 current species in 9 families
Gymnophiona (caecilians and relatives): Jurassic to present—215 current species in 10 families
Allocaudata† (Albanerpetontidae) Middle Jurassic - Early Pleistocene
The actual number of species in each group depends on the taxonomic classification followed. The two most | 261 | 999 |
621_3 | Amphibian | common systems are the classification adopted by the website AmphibiaWeb, University of California, Berkeley and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database "Amphibian Species of the World". The numbers of species cited above follows Frost and the total number of known amphibian species as of March 31, 2019 is exactly 8,000, of which nearly 90% are frogs.
With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called Lissamphibia. The phylogeny of Paleozoic amphibians is uncertain, and Lissamphibia may possibly fall within extinct groups, like the Temnospondyli (traditionally placed in the subclass | 261 | 1,206 |
621_4 | Amphibian | Labyrinthodontia) or the Lepospondyli, and in some analyses even in the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were formerly placed in Amphibia in Linnaean taxonomy, and included them elsewhere under cladistic taxonomy. If the common ancestor of amphibians and amniotes is included in Amphibia, it becomes a paraphyletic group.
All modern amphibians are included in the subclass Lissamphibia, which is usually considered a clade, a group of species that have evolved from a common ancestor. The three modern orders are Anura (the frogs), Caudata (or Urodela, the salamanders), and Gymnophiona (or Apoda, the caecilians). It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes. Although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is Prosalirus bitis, from the Early | 261 | 1,100 |
621_5 | Amphibian | Jurassic Kayenta Formation of Arizona. It is anatomically very similar to modern frogs. The oldest known caecilian is another Early Jurassic species, Eocaecilia micropodia, also from Arizona. The earliest salamander is Beiyanerpeton jianpingensis from the Late Jurassic of northeastern China.
Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. The Lissamphibia are traditionally divided into three orders, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of Lissamphibia alongside the superorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, Triadobatrachus.
Evolutionary history
The first major groups of amphibians ("stem") developed in the Devonian period, around 370 million years ago, from lobe-finned fish which were similar to the modern coelacanth and lungfish. These ancient lobe-finned fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs that help them | 261 | 1,145 |
621_6 | Amphibian | breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all tetrapods, including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric tetrapodomorph fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills.
Many examples of species showing transitional features have been discovered. Ichthyostega was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, Eusthenopteron. Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land. They developed "hands" and "feet" with five or more digits; the skin became more capable of retaining body fluids and resisting desiccation. | 262 | 1,238 |
621_7 | Amphibian | desiccation. The fish's hyomandibula bone in the hyoid region behind the gills diminished in size and became the stapes of the amphibian ear, an adaptation necessary for hearing on dry land. An affinity between the amphibians and the teleost fish is the multi-folded structure of the teeth and the paired supra-occipital bones at the back of the head, neither of these features being found elsewhere in the animal kingdom.
At the end of the Devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates, though some, such as Ichthyostega, may have sometimes hauled themselves out of the water. It is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the elephant seal. In the early Carboniferous (360 to 345 million years ago), the climate became wet and warm. Extensive swamps developed with mosses, ferns, horsetails and calamites. Air-breathing arthropods evolved and invaded the land where they provided food for the carnivorous amphibians that began to adapt to the terrestrial environment. There were no other | 264 | 1,196 |
621_8 | Amphibian | tetrapods on the land and the amphibians were at the top of the food chain, occupying the ecological position currently held by the crocodile. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. They were the top land predators, sometimes reaching several metres in length, preying on the large insects of the period and the many types of fish in the water. They still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. It was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their dominance in the period that followed.
After the Carboniferous rainforest collapse amphibian dominance gave way to reptiles, and amphibians were further devastated by the Permian–Triassic extinction event. During the Triassic Period (250 to 200 million years ago), the reptiles continued to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the biosphere. According to the fossil record, Lissamphibia, which includes all modern | 261 | 1,240 |
621_9 | Amphibian | amphibians and is the only surviving lineage, may have branched off from the extinct groups Temnospondyli and Lepospondyli at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence precludes precise dating, but the most recent molecular study, based on multilocus sequence typing, suggests a Late Carboniferous/Early Permian origin for extant amphibians.
The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A 2005 molecular phylogeny, based on rDNA analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs. It also appears that the divergence of the three groups took place in the Paleozoic or early Mesozoic (around 250 million years ago), before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fish. The briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils. There are large gaps in the fossil record, but the discovery of a Gerobatrachus hottoni from the Early Permian in | 261 | 1,189 |
621_10 | Amphibian | Texas in 2008 provided a missing link with many of the characteristics of modern frogs. Molecular analysis suggests that the frog–salamander divergence took place considerably earlier than the palaeontological evidence indicates. Newer research indicates that the common ancestor of all Lissamphibians lived about 315 million years ago, and that stereospondyls are the closest relatives to the caecilians.
As they evolved from lunged fish, amphibians had to make certain adaptations for living on land, including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward, but on land, quite different mechanisms were required. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their lateral line systems and adapted their sensory systems to receive stimuli via the medium of the air. They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviours suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.
Characteristics | 263 | 1,363 |
621_11 | Amphibian | loss.
Characteristics
The superclass Tetrapoda is divided into four classes of vertebrate animals with four limbs. Reptiles, birds and mammals are amniotes, the eggs of which are either laid or carried by the female and are surrounded by several membranes, some of which are impervious. Lacking these membranes, amphibians require water bodies for reproduction, although some species have developed various strategies for protecting or bypassing the vulnerable aquatic larval stage. They are not found in the sea with the exception of one or two frogs that live in brackish water in mangrove swamps; the Anderson's salamander meanwhile occurs in brackish or salt water lakes. On land, amphibians are restricted to moist habitats because of the need to keep their skin damp.
Modern amphibians have a simplified anatomy compared to their ancestors due to paedomorphosis, caused by two evolutionary trends: miniaturization and an unusually large genome, which result in a slower growth and development rate compared to other vertebrates. Another reason for their size is associated with their rapid metamorphosis, which seems to have evolved only in the ancestors of lissamphibia; in all other known lines the development was much more gradual. Because a remodeling of the feeding apparatus means they | 263 | 1,302 |
621_12 | Amphibian | don't eat during the metamorphosis, the metamorphosis has to go faster the smaller the individual is, so it happens at an early stage when the larvae are still small. (The largest species of salamanders don't go through a metamorphosis.) Amphibians that lay eggs on land often go through the whole metamorphosis inside the egg. An anamniotic terrestrial egg is less than 1 cm in diameter due to diffusion problems, a size which puts a limit on the amount of posthatching growth.
The smallest amphibian (and vertebrate) in the world is a microhylid frog from New Guinea (Paedophryne amauensis) first discovered in 2012. It has an average length of and is part of a genus that contains four of the world's ten smallest frog species. The largest living amphibian is the Chinese giant salamander (Andrias davidianus) but this is a great deal smaller than the largest amphibian that ever existed—the extinct Prionosuchus, a crocodile-like temnospondyl dating to 270 million years ago from the middle Permian of Brazil. The largest frog is the African Goliath frog (Conraua goliath), | 264 | 1,082 |
621_13 | Amphibian | goliath), which can reach and weigh .
Amphibians are ectothermic (cold-blooded) vertebrates that do not maintain their body temperature through internal physiological processes. Their metabolic rate is low and as a result, their food and energy requirements are limited. In the adult state, they have tear ducts and movable eyelids, and most species have ears that can detect airborne or ground vibrations. They have muscular tongues, which in many species can be protruded. Modern amphibians have fully ossified vertebrae with articular processes. Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are often incompletely ossified. Their skin contains little keratin and lacks scales, apart from a few fish-like scales in certain caecilians. The skin contains many mucous glands and in some species, poison glands (a type of granular gland). The hearts of amphibians have three chambers, two atria and one ventricle. They have a urinary bladder and nitrogenous waste products are excreted primarily as urea. Most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis to become terrestrial adults. Amphibians breathe by means of a pump | 262 | 1,228 |
621_14 | Amphibian | action in which air is first drawn into the buccopharyngeal region through the nostrils. These are then closed and the air is forced into the lungs by contraction of the throat. They supplement this with gas exchange through the skin.
Anura
The order Anura (from the Ancient Greek a(n)- meaning "without" and oura meaning "tail") comprises the frogs and toads. They usually have long hind limbs that fold underneath them, shorter forelimbs, webbed toes with no claws, no tails, large eyes and glandular moist skin. Members of this order with smooth skins are commonly referred to as frogs, while those with warty skins are known as toads. The difference is not a formal one taxonomically and there are numerous exceptions to this rule. Members of the family Bufonidae are known as the "true toads". Frogs range in size from the Goliath frog (Conraua goliath) of West Africa to the Paedophryne amauensis, first described in Papua New Guinea in 2012, which is also the smallest known vertebrate. Although most species are associated with water and damp habitats, some are specialised to live in trees or in deserts. | 261 | 1,119 |
621_15 | Amphibian | They are found worldwide except for polar areas.
Anura is divided into three suborders that are broadly accepted by the scientific community, but the relationships between some families remain unclear. Future molecular studies should provide further insights into their evolutionary relationships. The suborder Archaeobatrachia contains four families of primitive frogs. These are Ascaphidae, Bombinatoridae, Discoglossidae and Leiopelmatidae which have few derived features and are probably paraphyletic with regard to other frog lineages. The six families in the more evolutionarily advanced suborder Mesobatrachia are the fossorial Megophryidae, Pelobatidae, Pelodytidae, Scaphiopodidae and Rhinophrynidae and the obligatorily aquatic Pipidae. These have certain characteristics that are intermediate between the two other suborders. Neobatrachia is by far the largest suborder and includes the remaining families of modern frogs, including most common species. Ninety-six percent of the over 5,000 extant species of frog are neobatrachians.
Caudata
The order Caudata (from the Latin cauda meaning "tail") consists of the salamanders—elongated, | 264 | 1,152 |
621_16 | Amphibian | salamanders—elongated, low-slung animals that mostly resemble lizards in form. This is a symplesiomorphic trait and they are no more closely related to lizards than they are to mammals. Salamanders lack claws, have scale-free skins, either smooth or covered with tubercles, and tails that are usually flattened from side to side and often finned. They range in size from the Chinese giant salamander (Andrias davidianus), which has been reported to grow to a length of , to the diminutive Thorius pennatulus from Mexico which seldom exceeds in length. Salamanders have a mostly Laurasian distribution, being present in much of the Holarctic region of the northern hemisphere. The family Plethodontidae is also found in Central America and South America north of the Amazon basin; South America was apparently invaded from Central America by about the start of the Miocene, 23 million years ago. Urodela is a name sometimes used for all the extant species of salamanders. Members of several salamander families have become paedomorphic and either fail to complete their metamorphosis or retain some larval characteristics as adults. Most salamanders are under long. They may be terrestrial or aquatic | 265 | 1,202 |
621_17 | Amphibian | and many spend part of the year in each habitat. When on land, they mostly spend the day hidden under stones or logs or in dense vegetation, emerging in the evening and night to forage for worms, insects and other invertebrates.
The suborder Cryptobranchoidea contains the primitive salamanders. A number of fossil cryptobranchids have been found, but there are only three living species, the Chinese giant salamander (Andrias davidianus), the Japanese giant salamander (Andrias japonicus) and the hellbender (Cryptobranchus alleganiensis) from North America. These large amphibians retain several larval characteristics in their adult state; gills slits are present and the eyes are unlidded. A unique feature is their ability to feed by suction, depressing either the left side of their lower jaw or the right. The males excavate nests, persuade females to lay their egg strings inside them, and guard them. As well as breathing with lungs, they respire through the many folds in their thin skin, which has capillaries close to the surface.
The suborder Salamandroidea contains the advanced salamanders. They differ from the cryptobranchids by having fused | 261 | 1,161 |
621_18 | Amphibian | prearticular bones in the lower jaw, and by using internal fertilisation. In salamandrids, the male deposits a bundle of sperm, the spermatophore, and the female picks it up and inserts it into her cloaca where the sperm is stored until the eggs are laid. The largest family in this group is Plethodontidae, the lungless salamanders, which includes 60% of all salamander species. The family Salamandridae includes the true salamanders and the name "newt" is given to members of its subfamily Pleurodelinae.
The third suborder, Sirenoidea, contains the four species of sirens, which are in a single family, Sirenidae. Members of this order are eel-like aquatic salamanders with much reduced forelimbs and no hind limbs. Some of their features are primitive while others are derived. Fertilisation is likely to be external as sirenids lack the cloacal glands used by male salamandrids to produce spermatophores and the females lack spermathecae for sperm storage. Despite this, the eggs are laid singly, a behaviour not conducive for external fertilisation.
Gymnophiona | 263 | 1,070 |
621_19 | Amphibian | fertilisation.
Gymnophiona
The order Gymnophiona (from the Greek gymnos meaning "naked" and ophis meaning "serpent") or Apoda comprises the caecilians. These are long, cylindrical, limbless animals with a snake- or worm-like form. The adults vary in length from 8 to 75 centimetres (3 to 30 inches) with the exception of Thomson's caecilian (Caecilia thompsoni), which can reach . A caecilian's skin has a large number of transverse folds and in some species contains tiny embedded dermal scales. It has rudimentary eyes covered in skin, which are probably limited to discerning differences in light intensity. It also has a pair of short tentacles near the eye that can be extended and which have tactile and olfactory functions. Most caecilians live underground in burrows in damp soil, in rotten wood and under plant debris, but some are aquatic. Most species lay their eggs underground and when the larvae hatch, they make their way to adjacent bodies of water. Others brood their eggs and the larvae undergo metamorphosis before the eggs hatch. A few species give birth to live young, nourishing them with glandular secretions while they are in the oviduct. | 270 | 1,166 |
621_20 | Amphibian | oviduct. Caecilians have a mostly Gondwanan distribution, being found in tropical regions of Africa, Asia and Central and South America.
Anatomy and physiology
Skin
The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly cornified outer layers, renewed periodically through a moulting process controlled by the pituitary and thyroid glands. Local thickenings (often called warts) are common, such as those found on toads. The outside of the skin is shed periodically mostly in one piece, in contrast to mammals and birds where it is shed in flakes. Amphibians often eat the sloughed skin. Caecilians are unique among amphibians in having mineralized dermal scales embedded in the dermis between the furrows in the skin. The similarity of these to the scales of bony fish is largely superficial. Lizards and some frogs have somewhat similar osteoderms forming bony deposits in the dermis, but this is an example of convergent evolution with similar structures having arisen independently in diverse vertebrate lineages.
Amphibian skin is permeable to water. Gas exchange can take place through the skin (cutaneous respiration) and this allows adult amphibians to | 262 | 1,243 |
621_21 | Amphibian | respire without rising to the surface of water and to hibernate at the bottom of ponds. To compensate for their thin and delicate skin, amphibians have evolved mucous glands, principally on their heads, backs and tails. The secretions produced by these help keep the skin moist. In addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. Some amphibian toxins can be lethal to humans while others have little effect. The main poison-producing glands, the parotoids, produce the neurotoxin bufotoxin and are located behind the ears of toads, along the backs of frogs, behind the eyes of salamanders and on the upper surface of caecilians.
The skin colour of amphibians is produced by three layers of pigment cells called chromatophores. These three cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing many granules, producing a blue-green colour) and the lipophores (yellow, the most superficial layer). The colour change displayed by many species is initiated by hormones secreted by the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system, | 261 | 1,227 |
621_22 | Amphibian | and this results in the colour change taking place more slowly than happens in fish. A vividly coloured skin usually indicates that the species is toxic and is a warning sign to predators.
Skeletal system and locomotion
Amphibians have a skeletal system that is structurally homologous to other tetrapods, though with a number of variations. They all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs. The bones are hollow and lightweight. The musculoskeletal system is strong to enable it to support the head and body. The bones are fully ossified and the vertebrae interlock with each other by means of overlapping processes. The pectoral girdle is supported by muscle, and the well-developed pelvic girdle is attached to the backbone by a pair of sacral ribs. The ilium slopes forward and the body is held closer to the ground than is the case in mammals.
In most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either. Some salamanders have fewer digits and the amphiumas are eel-like in appearance with tiny, stubby legs. The sirens are aquatic salamanders | 263 | 1,169 |
621_23 | Amphibian | salamanders with stumpy forelimbs and no hind limbs. The caecilians are limbless. They burrow in the manner of earthworms with zones of muscle contractions moving along the body. On the surface of the ground or in water they move by undulating their body from side to side.
In frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. In the walkers and runners the hind limbs are not so large, and the burrowers mostly have short limbs and broad bodies. The feet have adaptations for the way of life, with webbing between the toes for swimming, broad adhesive toe pads for climbing, and keratinised tubercles on the hind feet for digging (frogs usually dig backwards into the soil). In most salamanders, the limbs are short and more or less the same length and project at right angles from the body. Locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing. In their normal gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish. Some salamanders in the genus Aneides | 264 | 1,159 |
621_24 | Amphibian | Aneides and certain plethodontids climb trees and have long limbs, large toepads and prehensile tails. In aquatic salamanders and in frog tadpoles, the tail has dorsal and ventral fins and is moved from side to side as a means of propulsion. Adult frogs do not have tails and caecilians have only very short ones.
Salamanders use their tails in defence and some are prepared to jettison them to save their lives in a process known as autotomy. Certain species in the Plethodontidae have a weak zone at the base of the tail and use this strategy readily. The tail often continues to twitch after separation which may distract the attacker and allow the salamander to escape. Both tails and limbs can be regenerated. Adult frogs are unable to regrow limbs but tadpoles can do so.
Circulatory system
Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, and is spread around the body and back to the heart in a | 262 | 1,128 |
621_25 | Amphibian | single loop. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumped through the pulmonary artery to the lungs. Continued contraction then pumps oxygenated blood around the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.
Nervous and sensory systems
The nervous system is basically the same as in other vertebrates, with a central brain, a spinal cord, and nerves throughout the body. The amphibian brain is less well developed than that of reptiles, birds and mammals but is similar in morphology and function to that of a fish. It is believed amphibians are capable of perceiving pain. The brain consists of equal parts, cerebrum, midbrain and cerebellum. Various parts of the cerebrum process sensory input, such as smell in the olfactory lobe and sight in the optic lobe, and it is additionally the centre of behaviour and learning. The cerebellum is the center of muscular coordination and the medulla oblongata controls some organ functions including heartbeat and respiration. The brain sends | 261 | 1,215 |
621_26 | Amphibian | signals through the spinal cord and nerves to regulate activity in the rest of the body. The pineal body, known to regulate sleep patterns in humans, is thought to produce the hormones involved in hibernation and aestivation in amphibians.
Tadpoles retain the lateral line system of their ancestral fishes, but this is lost in terrestrial adult amphibians. Some caecilians possess electroreceptors that allow them to locate objects around them when submerged in water. The ears are well developed in frogs. There is no external ear, but the large circular eardrum lies on the surface of the head just behind the eye. This vibrates and sound is transmitted through a single bone, the stapes, to the inner ear. Only high-frequency sounds like mating calls are heard in this way, but low-frequency noises can be detected through another mechanism. There is a patch of specialized haircells, called papilla amphibiorum, in the inner ear capable of detecting deeper sounds. Another feature, unique to frogs and salamanders, is the columella-operculum complex adjoining the auditory capsule which is involved in the transmission of both airborne and seismic signals. The ears of salamanders and caecilians are less highly developed than those of frogs as they do not normally | 261 | 1,271 |
621_27 | Amphibian | communicate with each other through the medium of sound.
The eyes of tadpoles lack lids, but at metamorphosis, the cornea becomes more dome-shaped, the lens becomes flatter, and eyelids and associated glands and ducts develop. The adult eyes are an improvement on invertebrate eyes and were a first step in the development of more advanced vertebrate eyes. They allow colour vision and depth of focus. In the retinas are green rods, which are receptive to a wide range of wavelengths.
Digestive and excretory systems
Many amphibians catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth before seizing the item with their jaws. Some use inertial feeding to help them swallow the prey, repeatedly thrusting their head forward sharply causing the food to move backwards in their mouth by inertia. Most amphibians swallow their prey whole without much chewing so they possess voluminous stomachs. The short oesophagus is lined with cilia that help to move the food to the stomach and mucus produced by glands in the mouth and pharynx eases its passage. The enzyme chitinase produced in the stomach helps digest the chitinous cuticle of arthropod prey.
Amphibians | 263 | 1,217 |
621_28 | Amphibian | prey.
Amphibians possess a pancreas, liver and gall bladder. The liver is usually large with two lobes. Its size is determined by its function as a glycogen and fat storage unit, and may change with the seasons as these reserves are built or used up. Adipose tissue is another important means of storing energy and this occurs in the abdomen (in internal structures called fat bodies), under the skin and, in some salamanders, in the tail.
There are two kidneys located dorsally, near the roof of the body cavity. Their job is to filter the blood of metabolic waste and transport the urine via ureters to the urinary bladder where it is stored before being passed out periodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen as ammonia in large quantities of dilute urine, while terrestrial species, with a greater need to conserve water, excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their metabolic waste as uric acid.
Respiratory system
The lungs in amphibians are primitive compared to those of amniotes, possessing few internal septa and large alveoli, and consequently having a comparatively slow diffusion rate for oxygen entering the blood. Ventilation | 265 | 1,262 |
621_29 | Amphibian | is accomplished by buccal pumping. Most amphibians, however, are able to exchange gases with the water or air via their skin. To enable sufficient cutaneous respiration, the surface of their highly vascularised skin must remain moist to allow the oxygen to diffuse at a sufficiently high rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the Titicaca water frog and the hellbender salamander. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders, which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the axolotl) retaining gills as aquatic adults.
Reproduction
For the purpose of reproduction most amphibians require fresh water although some lay their eggs on land and have developed various means of keeping them moist. A few (e.g. Fejervarya raja) can inhabit brackish water, but there are no true marine amphibians. There are reports, however, of particular amphibian populations unexpectedly | 261 | 1,217 |
621_30 | Amphibian | invading marine waters. Such was the case with the Black Sea invasion of the natural hybrid Pelophylax esculentus reported in 2010.
Several hundred frog species in adaptive radiations (e.g., Eleutherodactylus, the Pacific Platymantis, the Australo-Papuan microhylids, and many other tropical frogs), however, do not need any water for breeding in the wild. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet tropical rainforests and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Reproductive success of many amphibians is dependent not only on the quantity of rainfall, but the seasonal timing.
In the tropics, many amphibians breed continuously or at any time of year. In temperate regions, breeding is mostly seasonal, usually in the spring, and is triggered by increasing day length, rising temperatures or rainfall. Experiments have shown the importance of temperature, but the trigger event, especially in arid regions, is often a storm. In anurans, males usually arrive at the breeding sites before females and the vocal chorus they | 261 | 1,254 |
621_31 | Amphibian | produce may stimulate ovulation in females and the endocrine activity of males that are not yet reproductively active.
In caecilians, fertilisation is internal, the male extruding an intromittent organ, the , and inserting it into the female cloaca. The paired Müllerian glands inside the male cloaca secrete a fluid which resembles that produced by mammalian prostate glands and which may transport and nourish the sperm. Fertilisation probably takes place in the oviduct.
The majority of salamanders also engage in internal fertilisation. In most of these, the male deposits a spermatophore, a small packet of sperm on top of a gelatinous cone, on the substrate either on land or in the water. The female takes up the sperm packet by grasping it with the lips of the cloaca and pushing it into the vent. The spermatozoa move to the spermatheca in the roof of the cloaca where they remain until ovulation which may be many months later. Courtship rituals and methods of transfer of the spermatophore vary between species. In some, the spermatophore may be placed directly into the female cloaca while in others, the female may be guided to the spermatophore | 263 | 1,161 |
621_32 | Amphibian | spermatophore or restrained with an embrace called amplexus. Certain primitive salamanders in the families Sirenidae, Hynobiidae and Cryptobranchidae practice external fertilisation in a similar manner to frogs, with the female laying the eggs in water and the male releasing sperm onto the egg mass.
With a few exceptions, frogs use external fertilisation. The male grasps the female tightly with his forelimbs either behind the arms or in front of the back legs, or in the case of Epipedobates tricolor, around the neck. They remain in amplexus with their cloacae positioned close together while the female lays the eggs and the male covers them with sperm. Roughened nuptial pads on the male's hands aid in retaining grip. Often the male collects and retains the egg mass, forming a sort of basket with the hind feet. An exception is the granular poison frog (Oophaga granulifera) where the male and female place their cloacae in close proximity while facing in opposite directions and then release eggs and sperm simultaneously. The tailed frog (Ascaphus truei) exhibits internal fertilisation. The "tail" is only possessed by the male and is an extension of the | 263 | 1,168 |
621_33 | Amphibian | cloaca and used to inseminate the female. This frog lives in fast-flowing streams and internal fertilisation prevents the sperm from being washed away before fertilisation occurs. The sperm may be retained in storage tubes attached to the oviduct until the following spring.
Most frogs can be classified as either prolonged or explosive breeders. Typically, prolonged breeders congregate at a breeding site, the males usually arriving first, calling and setting up territories. Other satellite males remain quietly nearby, waiting for their opportunity to take over a territory. The females arrive sporadically, mate selection takes place and eggs are laid. The females depart and territories may change hands. More females appear and in due course, the breeding season comes to an end. Explosive breeders on the other hand are found where temporary pools appear in dry regions after rainfall. These frogs are typically fossorial species that emerge after heavy rains and congregate at a breeding site. They are attracted there by the calling of the first male to find a suitable place, perhaps a pool that forms in the same place each rainy season. The assembled frogs may call in unison and frenzied activity ensues, the males scrambling to mate with the usually smaller number of females.
There is a direct competition between males to win | 261 | 1,345 |
621_34 | Amphibian | the attention of the females in salamanders and newts, with elaborate courtship displays to keep the female's attention long enough to get her interested in choosing him to mate with. Some species store sperm through long breeding seasons, as the extra time may allow for interactions with rival sperm.
Life cycle
Most amphibians go through metamorphosis, a process of significant morphological change after birth. In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads and salamanders all hatch from the egg as larvae with external gills. Metamorphosis in amphibians is regulated by thyroxine concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts thyroxine's effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, it is subject to many adaptations due to specific environmental circumstances. For this reason tadpoles can have horny ridges instead of Teeth, whisker-like skin extensions or fins. They also make use of a sensory lateral line organ similar to that of fish. After metamorphosis, these organs become redundant and will be reabsorbed by controlled cell death, called apoptosis. | 261 | 1,286 |
621_35 | Amphibian | The variety of adaptations to specific environmental circumstances among amphibians is wide, with many discoveries still being made.
Eggs
The egg of an amphibian is typically surrounded by a transparent gelatinous covering secreted by the oviducts and containing mucoproteins and mucopolysaccharides. This capsule is permeable to water and gases, and swells considerably as it absorbs water. The ovum is at first rigidly held, but in fertilised eggs the innermost layer liquefies and allows the embryo to move freely. This also happens in salamander eggs, even when they are unfertilised. Eggs of some salamanders and frogs contain unicellular green algae. These penetrate the jelly envelope after the eggs are laid and may increase the supply of oxygen to the embryo through photosynthesis. They seem to both speed up the development of the larvae and reduce mortality. Most eggs contain the pigment melanin which raises their temperature through the absorption of light and also protects them against ultraviolet radiation. Caecilians, some plethodontid salamanders and certain frogs lay eggs underground that are unpigmented. In the wood frog (Rana sylvatica), the interior of the globular egg cluster has been found to be up | 261 | 1,232 |
621_36 | Amphibian | to warmer than its surroundings, which is an advantage in its cool northern habitat.
The eggs may be deposited singly or in small groups, or may take the form of spherical egg masses, rafts or long strings. In terrestrial caecilians, the eggs are laid in grape-like clusters in burrows near streams. The amphibious salamander Ensatina attaches its similar clusters by stalks to underwater stems and roots. The greenhouse frog (Eleutherodactylus planirostris) lays eggs in small groups in the soil where they develop in about two weeks directly into juvenile frogs without an intervening larval stage. The tungara frog (Physalaemus pustulosus) builds a floating nest from foam to protect its eggs. First a raft is built, then eggs are laid in the centre, and finally a foam cap is overlaid. The foam has anti-microbial properties. It contains no detergents but is created by whipping up proteins and lectins secreted by the female.
Larvae
The eggs of amphibians are typically laid in water and hatch into free-living larvae that complete their development in water and later transform into either aquatic or terrestrial adults. In many species of frog and in most lungless salamanders (Plethodontidae), | 268 | 1,207 |
621_37 | Amphibian | (Plethodontidae), direct development takes place, the larvae growing within the eggs and emerging as miniature adults. Many caecilians and some other amphibians lay their eggs on land, and the newly hatched larvae wriggle or are transported to water bodies. Some caecilians, the alpine salamander (Salamandra atra) and some of the African live-bearing toads (Nectophrynoides spp.) are viviparous. Their larvae feed on glandular secretions and develop within the female's oviduct, often for long periods. Other amphibians, but not caecilians, are ovoviviparous. The eggs are retained in or on the parent's body, but the larvae subsist on the yolks of their eggs and receive no nourishment from the adult. The larvae emerge at varying stages of their growth, either before or after metamorphosis, according to their species. The toad genus Nectophrynoides exhibits all of these developmental patterns among its dozen or so members.
Frogs
Frog larvae are known as tadpoles and typically have oval bodies and long, vertically flattened tails with fins. The free-living larvae are normally fully aquatic, but the | 262 | 1,110 |
621_38 | Amphibian | tadpoles of some species (such as Nannophrys ceylonensis) are semi-terrestrial and live among wet rocks. Tadpoles have cartilaginous skeletons, gills for respiration (external gills at first, internal gills later), lateral line systems and large tails that they use for swimming. Newly hatched tadpoles soon develop gill pouches that cover the gills. The lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place. While tadpoles do not have true teeth, in most species, the jaws have long, parallel rows of small keratinized structures called keradonts surrounded by a horny beak. Front legs are formed under the gill sac and hind legs become visible a few days later.
Iodine and T4 (over stimulate the spectacular apoptosis [programmed cell death] of the cells of the larval gills, tail and fins) also stimulate the evolution of nervous systems transforming the aquatic, vegetarian tadpole into the terrestrial, | 261 | 1,178 |
621_39 | Amphibian | carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting.
In fact, tadpoles developing in ponds and streams are typically herbivorous. Pond tadpoles tend to have deep bodies, large caudal fins and small mouths; they swim in the quiet waters feeding on growing or loose fragments of vegetation. Stream dwellers mostly have larger mouths, shallow bodies and caudal fins; they attach themselves to plants and stones and feed on the surface films of algae and bacteria. They also feed on diatoms, filtered from the water through the gills, and stir up the sediment at bottom of the pond, ingesting edible fragments. They have a relatively long, spiral-shaped gut to enable them to digest this diet. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles and fish. Young of the Cuban tree frog (Osteopilus septentrionalis) can occasionally be cannibalistic, the younger tadpoles attacking a larger, more developed tadpole when it is undergoing metamorphosis.
At metamorphosis, rapid changes in the body take place as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is reabsorbed | 261 | 1,197 |
621_40 | Amphibian | together with the spiral gut. The animal develops a large jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, and a tongue is formed. There are associated changes in the neural networks such as development of stereoscopic vision and loss of the lateral line system. All this can happen in about a day. A few days later, the tail is reabsorbed, due to the higher thyroxine concentration required for this to take place.
Salamanders
At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills, a somewhat laterally flattened body and a long tail with dorsal and ventral fins. The forelimbs may be partially developed and the hind limbs are rudimentary in pond-living species but may be rather more developed in species that reproduce in moving water. Pond-type larvae often have a pair of balancers, rod-like structures on either side of the head that may prevent the gills from becoming clogged up with sediment. Some members of the genera Ambystoma and Dicamptodon have larvae that never fully develop into the adult form, but this varies with species and with populations. The northwestern | 262 | 1,202 |
621_41 | Amphibian | northwestern salamander (Ambystoma gracile) is one of these and, depending on environmental factors, either remains permanently in the larval state, a condition known as neoteny, or transforms into an adult. Both of these are able to breed. Neoteny occurs when the animal's growth rate is very low and is usually linked to adverse conditions such as low water temperatures that may change the response of the tissues to the hormone thyroxine. Other factors that may inhibit metamorphosis include lack of food, lack of trace elements and competition from conspecifics. The tiger salamander (Ambystoma tigrinum) also sometimes behaves in this way and may grow particularly large in the process. The adult tiger salamander is terrestrial, but the larva is aquatic and able to breed while still in the larval state. When conditions are particularly inhospitable on land, larval breeding may allow continuation of a population that would otherwise die out. There are fifteen species of obligate neotenic salamanders, including species of Necturus, Proteus and Amphiuma, and many examples of facultative ones that adopt this strategy under appropriate environmental circumstances.
Lungless salamanders in the family Plethodontidae | 262 | 1,226 |
621_42 | Amphibian | are terrestrial and lay a small number of unpigmented eggs in a cluster among damp leaf litter. Each egg has a large yolk sac and the larva feeds on this while it develops inside the egg, emerging fully formed as a juvenile salamander. The female salamander often broods the eggs. In the genus Ensatinas, the female has been observed to coil around them and press her throat area against them, effectively massaging them with a mucous secretion.
In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems. Their lungs are functional early, but the larvae do not make as much use of them as do tadpoles. Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of the skin, the development of eyelids, and certain changes in dentition and tongue structure. Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are | 261 | 1,195 |
621_43 | Amphibian | encumbrances on land. Adult salamanders often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxine. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
Caecilians
Most terrestrial caecilians that lay eggs do so in burrows or moist places on land near bodies of water. The development of the young of Ichthyophis glutinosus, a species from Sri Lanka, has been much studied. The eel-like larvae hatch out of the eggs and make their way to water. They have three pairs of external red feathery gills, a blunt head with two rudimentary eyes, a lateral line system and a short tail with fins. They swim by undulating their body from side to side. They are mostly active at night, soon lose their gills and make sorties onto land. Metamorphosis is gradual. By the age of about ten months they have developed a pointed head with sensory tentacles near the mouth and lost their eyes, lateral line systems and tails. The skin thickens, embedded | 261 | 1,161 |
621_44 | Amphibian | scales develop and the body divides into segments. By this time, the caecilian has constructed a burrow and is living on land.
In the majority of species of caecilians, the young are produced by viviparity. Typhlonectes compressicauda, a species from South America, is typical of these. Up to nine larvae can develop in the oviduct at any one time. They are elongated and have paired sac-like gills, small eyes and specialised scraping teeth. At first, they feed on the yolks of the eggs, but as this source of nourishment declines they begin to rasp at the ciliated epithelial cells that line the oviduct. This stimulates the secretion of fluids rich in lipids and mucoproteins on which they feed along with scrapings from the oviduct wall. They may increase their length sixfold and be two-fifths as long as their mother before being born. By this time they have undergone metamorphosis, lost their eyes and gills, developed a thicker skin and mouth tentacles, and reabsorbed their teeth. A permanent set of teeth grow through soon after birth.
The ringed caecilian (Siphonops annulatus) has developed a | 261 | 1,108 |
621_45 | Amphibian | unique adaptation for the purposes of reproduction. The progeny feed on a skin layer that is specially developed by the adult in a phenomenon known as maternal dermatophagy. The brood feed as a batch for about seven minutes at intervals of approximately three days which gives the skin an opportunity to regenerate. Meanwhile, they have been observed to ingest fluid exuded from the maternal cloaca.
Parental care
The care of offspring among amphibians has been little studied but, in general, the larger the number of eggs in a batch, the less likely it is that any degree of parental care takes place. Nevertheless, it is estimated that in up to 20% of amphibian species, one or both adults play some role in the care of the young. Those species that breed in smaller water bodies or other specialised habitats tend to have complex patterns of behaviour in the care of their young.
Many woodland salamanders lay clutches of eggs under dead logs or stones on land. The black mountain salamander (Desmognathus welteri) does this, the mother brooding the eggs and guarding them from predation as the embryos feed on the yolks of their eggs. When fully developed, they break their way out of the egg capsules and disperse as juvenile | 261 | 1,236 |
621_46 | Amphibian | salamanders. The male hellbender, a primitive salamander, excavates an underwater nest and encourages females to lay there. The male then guards the site for the two or three months before the eggs hatch, using body undulations to fan the eggs and increase their supply of oxygen.
The male Colostethus subpunctatus, a tiny frog, protects the egg cluster which is hidden under a stone or log. When the eggs hatch, the male transports the tadpoles on his back, stuck there by a mucous secretion, to a temporary pool where he dips himself into the water and the tadpoles drop off. The male midwife toad (Alytes obstetricans) winds egg strings round his thighs and carries the eggs around for up to eight weeks. He keeps them moist and when they are ready to hatch, he visits a pond or ditch and releases the tadpoles. The female gastric-brooding frog (Rheobatrachus spp.) reared larvae in her stomach after swallowing either the eggs or hatchlings; however, this stage was never observed before the species became extinct. The tadpoles secrete a hormone that inhibits digestion in the mother whilst they develop by consuming their very large yolk | 261 | 1,145 |
621_47 | Amphibian | supply. The pouched frog (Assa darlingtoni) lays eggs on the ground. When they hatch, the male carries the tadpoles around in brood pouches on his hind legs. The aquatic Surinam toad (Pipa pipa) raises its young in pores on its back where they remain until metamorphosis. The granular poison frog (Oophaga granulifera) is typical of a number of tree frogs in the poison dart frog family Dendrobatidae. Its eggs are laid on the forest floor and when they hatch, the tadpoles are carried one by one on the back of an adult to a suitable water-filled crevice such as the axil of a leaf or the rosette of a bromeliad. The female visits the nursery sites regularly and deposits unfertilised eggs in the water and these are consumed by the tadpoles.
Genetics and genomics
Amphibians are notable among vertebrates for their diversity of chromosomes and genomes. The karyotypes (chromosomes) have been determined for at least 1,193 (14.5%) of the ~8,200 known (diploid) species, including 963 anurans, 209 salamanders, | 262 | 1,013 |
621_48 | Amphibian | salamanders, and 21 caecilians. Generally, the karyotypes of diploid amphibians are characterized by 20–26 bi-armed chromosomes. Amphibians have also very large genomes compared to other taxa of vertebrates and corresponding variation in genome size (C-value: picograms of DNA in haploid nuclei). The genome sizes range from 0.95 to 11.5 pg in frogs, from 13.89 to 120.56 pg in salamanders, and from 2.94 to 11.78 pg in caecilians.
The large genome sizes have prevented whole-genome sequencing of amphibians although a number of genomes have been published recently. The 1.7GB draft genome of Xenopus tropicalis was the first to be reported for amphibians in 2010. Compared to some salamanders this frog genome is tiny. For instance, the genome of the Mexican axolotl turned out to be 32 Gb, which is more than 10 times larger than the human genome (3GB).
Feeding and diet
With a few exceptions, adult amphibians are predators, feeding on virtually anything that moves that they can swallow. The diet mostly consists of small prey that do not move too fast | 264 | 1,062 |
621_49 | Amphibian | such as beetles, caterpillars, earthworms and spiders. The sirens (Siren spp.) often ingest aquatic plant material with the invertebrates on which they feed and a Brazilian tree frog (Xenohyla truncata) includes a large quantity of fruit in its diet. The Mexican burrowing toad (Rhinophrynus dorsalis) has a specially adapted tongue for picking up ants and termites. It projects it with the tip foremost whereas other frogs flick out the rear part first, their tongues being hinged at the front.
Food is mostly selected by sight, even in conditions of dim light. Movement of the prey triggers a feeding response. Frogs have been caught on fish hooks baited with red flannel and green frogs (Rana clamitans) have been found with stomachs full of elm seeds that they had seen floating past. Toads, salamanders and caecilians also use smell to detect prey. This response is mostly secondary because salamanders have been observed to remain stationary near odoriferous prey but only feed if it moves. Cave-dwelling amphibians normally hunt by smell. Some salamanders seem to have learned to recognize immobile prey when it has no smell, even in complete darkness.
Amphibians | 266 | 1,173 |
621_50 | Amphibian | darkness.
Amphibians usually swallow food whole but may chew it lightly first to subdue it. They typically have small hinged pedicellate teeth, a feature unique to amphibians. The base and crown of these are composed of dentine separated by an uncalcified layer and they are replaced at intervals. Salamanders, caecilians and some frogs have one or two rows of teeth in both jaws, but some frogs (Rana spp.) lack teeth in the lower jaw, and toads (Bufo spp.) have no teeth. In many amphibians there are also vomerine teeth attached to a facial bone in the roof of the mouth.
The tiger salamander (Ambystoma tigrinum) is typical of the frogs and salamanders that hide under cover ready to ambush unwary invertebrates. Others amphibians, such as the Bufo spp. toads, actively search for prey, while the Argentine horned frog (Ceratophrys ornata) lures inquisitive prey closer by raising its hind feet over its back and vibrating its yellow toes. Among leaf litter frogs in Panama, frogs that actively hunt prey have narrow mouths and are slim, often brightly coloured and toxic, while ambushers have wide | 262 | 1,105 |
621_51 | Amphibian | mouths and are broad and well-camouflaged. Caecilians do not flick their tongues, but catch their prey by grabbing it with their slightly backward-pointing teeth. The struggles of the prey and further jaw movements work it inwards and the caecilian usually retreats into its burrow. The subdued prey is gulped down whole.
When they are newly hatched, frog larvae feed on the yolk of the egg. When this is exhausted some move on to feed on bacteria, algal crusts, detritus and raspings from submerged plants. Water is drawn in through their mouths, which are usually at the bottom of their heads, and passes through branchial food traps between their mouths and their gills where fine particles are trapped in mucus and filtered out. Others have specialised mouthparts consisting of a horny beak edged by several rows of labial teeth. They scrape and bite food of many kinds as well as stirring up the bottom sediment, filtering out larger particles with the papillae around their mouths. Some, such as the spadefoot toads, have strong biting jaws and are carnivorous or even cannibalistic.
Vocalization
The calls made by caecilians and salamanders are limited to occasional soft squeaks, | 263 | 1,192 |
621_52 | Amphibian | squeaks, grunts or hisses and have not been much studied. A clicking sound sometimes produced by caecilians may be a means of orientation, as in bats, or a form of communication. Most salamanders are considered voiceless, but the California giant salamander (Dicamptodon ensatus) has vocal cords and can produce a rattling or barking sound. Some species of salamander emit a quiet squeak or yelp if attacked.
Frogs are much more vocal, especially during the breeding season when they use their voices to attract mates. The presence of a particular species in an area may be more easily discerned by its characteristic call than by a fleeting glimpse of the animal itself. In most species, the sound is produced by expelling air from the lungs over the vocal cords into an air sac or sacs in the throat or at the corner of the mouth. This may distend like a balloon and acts as a resonator, helping to transfer the sound to the atmosphere, or the water at times when the animal is submerged. The main vocalisation is the male's loud advertisement call which seeks to both encourage a female to approach and discourage other males from intruding on its territory. This call is modified to a quieter courtship | 263 | 1,208 |
621_53 | Amphibian | courtship call on the approach of a female or to a more aggressive version if a male intruder draws near. Calling carries the risk of attracting predators and involves the expenditure of much energy. Other calls include those given by a female in response to the advertisement call and a release call given by a male or female during unwanted attempts at amplexus. When a frog is attacked, a distress or fright call is emitted, often resembling a scream. The usually nocturnal Cuban tree frog (Osteopilus septentrionalis) produces a rain call when there is rainfall during daylight hours.
Territorial behaviour
Little is known of the territorial behaviour of caecilians, but some frogs and salamanders defend home ranges. These are usually feeding, breeding or sheltering sites. Males normally exhibit such behaviour though in some species, females and even juveniles are also involved. Although in many frog species, females are larger than males, this is not the case in most species where males are actively involved in territorial defence. Some of these have specific adaptations such as enlarged teeth for biting or spines on the chest, arms or thumbs.
In salamanders, defence of a territory involves adopting an aggressive posture and if necessary attacking the intruder. This may involve snapping, | 262 | 1,308 |
621_54 | Amphibian | chasing and sometimes biting, occasionally causing the loss of a tail. The behaviour of red back salamanders (Plethodon cinereus) has been much studied. 91% of marked individuals that were later recaptured were within a metre (yard) of their original daytime retreat under a log or rock. A similar proportion, when moved experimentally a distance of , found their way back to their home base. The salamanders left odour marks around their territories which averaged in size and were sometimes inhabited by a male and female pair. These deterred the intrusion of others and delineated the boundaries between neighbouring areas. Much of their behaviour seemed stereotyped and did not involve any actual contact between individuals. An aggressive posture involved raising the body off the ground and glaring at the opponent who often turned away submissively. If the intruder persisted, a biting lunge was usually launched at either the tail region or the naso-labial grooves. Damage to either of these areas can reduce the fitness of the rival, either because of the need to regenerate tissue or because it impairs its ability to detect food.
In frogs, male territorial behaviour is often observed at breeding locations; calling is both an announcement of ownership of part of this resource and | 261 | 1,295 |
621_55 | Amphibian | an advertisement call to potential mates. In general, a deeper voice represents a heavier and more powerful individual, and this may be sufficient to prevent intrusion by smaller males. Much energy is used in the vocalization and it takes a toll on the territory holder who may be displaced by a fitter rival if he tires. There is a tendency for males to tolerate the holders of neighbouring territories while vigorously attacking unknown intruders. Holders of territories have a "home advantage" and usually come off better in an encounter between two similar-sized frogs. If threats are insufficient, chest to chest tussles may take place. Fighting methods include pushing and shoving, deflating the opponent's vocal sac, seizing him by the head, jumping on his back, biting, chasing, splashing, and ducking him under the water.
Defence mechanisms
Amphibians have soft bodies with thin skins, and lack claws, defensive armour, or spines. Nevertheless, they have evolved various defence mechanisms to keep themselves alive. The first line of defence in salamanders and frogs is the mucous secretion that they produce. This keeps their skin moist and makes them slippery and difficult to grip. The secretion is often sticky and distasteful or toxic. Snakes have been observed | 261 | 1,279 |
621_56 | Amphibian | yawning and gaping when trying to swallow African clawed frogs (Xenopus laevis), which gives the frogs an opportunity to escape. Caecilians have been little studied in this respect, but the Cayenne caecilian (Typhlonectes compressicauda) produces toxic mucus that has killed predatory fish in a feeding experiment in Brazil. In some salamanders, the skin is poisonous. The rough-skinned newt (Taricha granulosa) from North America and other members of its genus contain the neurotoxin tetrodotoxin (TTX), the most toxic non-protein substance known and almost identical to that produced by pufferfish. Handling the newts does not cause harm, but ingestion of even the most minute amounts of the skin is deadly. In feeding trials, fish, frogs, reptiles, birds and mammals were all found to be susceptible. The only predators with some tolerance to the poison are certain populations of common garter snake (Thamnophis sirtalis).
In locations where both snake and salamander co-exist, the snakes have developed immunity through genetic changes and they feed on the amphibians with impunity. Coevolution occurs with the newt increasing its toxic capabilities at the same | 261 | 1,167 |
621_57 | Amphibian | rate as the snake further develops its immunity. Some frogs and toads are toxic, the main poison glands being at the side of the neck and under the warts on the back. These regions are presented to the attacking animal and their secretions may be foul-tasting or cause various physical or neurological symptoms. Altogether, over 200 toxins have been isolated from the limited number of amphibian species that have been investigated.
Poisonous species often use bright colouring to warn potential predators of their toxicity. These warning colours tend to be red or yellow combined with black, with the fire salamander (Salamandra salamandra) being an example. Once a predator has sampled one of these, it is likely to remember the colouration next time it encounters a similar animal. In some species, such as the fire-bellied toad (Bombina spp.), the warning colouration is on the belly and these animals adopt a defensive pose when attacked, exhibiting their bright colours to the predator. The frog Allobates zaparo is not poisonous, but mimics the appearance of other toxic species in its locality, a strategy that may deceive predators.
Many amphibians are nocturnal and hide during the day, thereby avoiding diurnal predators that hunt by sight. Other | 261 | 1,260 |
621_58 | Amphibian | amphibians use camouflage to avoid being detected. They have various colourings such as mottled browns, greys and olives to blend into the background. Some salamanders adopt defensive poses when faced by a potential predator such as the North American northern short-tailed shrew (Blarina brevicauda). Their bodies writhe and they raise and lash their tails which makes it difficult for the predator to avoid contact with their poison-producing granular glands. A few salamanders will autotomise their tails when attacked, sacrificing this part of their anatomy to enable them to escape. The tail may have a constriction at its base to allow it to be easily detached. The tail is regenerated later, but the energy cost to the animal of replacing it is significant.
Some frogs and toads inflate themselves to make themselves look large and fierce, and some spadefoot toads (Pelobates spp) scream and leap towards the attacker. Giant salamanders of the genus Andrias, as well as Ceratophrine and Pyxicephalus frogs possess sharp teeth and are capable of drawing blood with a defensive bite. The blackbelly salamander (Desmognathus quadramaculatus) can bite an | 261 | 1,158 |
621_59 | Amphibian | attacking common garter snake (Thamnophis sirtalis) two or three times its size on the head and often manages to escape.
Cognition
In amphibians, there is evidence of habituation, associative learning through both classical and instrumental learning, and discrimination abilities.
In one experiment, when offered live fruit flies (Drosophila virilis), salamanders chose the larger of 1 vs 2 and 2 vs 3. Frogs can distinguish between low numbers (1 vs 2, 2 vs 3, but not 3 vs 4) and large numbers (3 vs 6, 4 vs 8, but not 4 vs 6) of prey. This is irrespective of other characteristics, i.e. surface area, volume, weight and movement, although discrimination among large numbers may be based on surface area.
Conservation
Dramatic declines in amphibian populations, including population crashes and mass localized extinction, have been noted since the late 1980s from locations all over the world, and amphibian declines are thus perceived to be one of the most critical threats to global biodiversity. In 2004, the International Union for Conservation of Nature (IUCN) reported stating that currently birds, | 262 | 1,113 |
621_60 | Amphibian | birds, mammals, and amphibians extinction rates were at minimum 48 times greater than natural extinction rates—possibly 1,024 times higher.
In 2006, there were believed to be 4,035 species of amphibians that depended on water at some stage during their life cycle. Of these, 1,356 (33.6%) were considered to be threatened and this figure is likely to be an underestimate because it excludes 1,427 species for which there was insufficient data to assess their status. A number of causes are believed to be involved, including habitat destruction and modification, over-exploitation, pollution, introduced species, global warming, endocrine-disrupting pollutants, destruction of the ozone layer (ultraviolet radiation has shown to be especially damaging to the skin, eyes, and eggs of amphibians), and diseases like chytridiomycosis. However, many of the causes of amphibian declines are still poorly understood, and are a topic of ongoing discussion.
With their complex reproductive needs and permeable skins, amphibians are often considered to be ecological indicators. In many terrestrial ecosystems, they constitute one of the largest parts of the vertebrate biomass. Any decline in amphibian numbers will affect the patterns of predation. The loss of | 262 | 1,255 |
621_61 | Amphibian | carnivorous species near the top of the food chain will upset the delicate ecosystem balance and may cause dramatic increases in opportunistic species. In the Middle East, a growing appetite for eating frog legs and the consequent gathering of them for food was linked to an increase in mosquitoes. Predators that feed on amphibians are affected by their decline. The western terrestrial garter snake (Thamnophis elegans) in California is largely aquatic and depends heavily on two species of frog that are decreasing in numbers, the Yosemite toad (Bufo canorus) and the mountain yellow-legged frog (Rana muscosa), putting the snake's future at risk. If the snake were to become scarce, this would affect birds of prey and other predators that feed on it. Meanwhile, in the ponds and lakes, fewer frogs means fewer tadpoles. These normally play an important role in controlling the growth of algae and also forage on detritus that accumulates as sediment on the bottom. A reduction in the number of tadpoles may lead to an overgrowth of algae, resulting in depletion of oxygen in the water when the algae later die and decompose. Aquatic invertebrates and fish might then die and there would be unpredictable ecological consequences.
A global strategy to | 261 | 1,256 |
Subsets and Splits