abstract: Identifying tradeoffs between hunting and scavenging in an ecological context is important for understanding predatory guilds. In the past century, the feeding strategy of one of the largest and best-known terrestrial carnivores, *Tyrannosaurus rex*, has been the subject of much debate: was it an active predator or an obligate scavenger? Here we look at the feasibility of an adult *T. rex* being an obligate scavenger in the environmental conditions of Late Cretaceous North America, given the size distributions of sympatric herbivorous dinosaurs and likely competition with more abundant small-bodied theropods. We predict that nearly 50 per cent of herbivores would have been within a 55–85 kg range, and calculate based on expected encounter rates that carcasses from these individuals would have been quickly consumed by smaller theropods. Larger carcasses would have been very rare and heavily competed for, making them an unreliable food source. The potential carcass search rates of smaller theropods are predicted to be 14–60 times that of an adult *T. rex*. Our results suggest that *T. rex* and other extremely large carnivorous dinosaurs would have been unable to compete as obligate scavengers and would have primarily hunted large vertebrate prey, similar to many large mammalian carnivores in modern-day ecosystems.
author: Chris Carbone; Samuel T. Turvey; Jon Bielby\*Author for correspondence (<email@example.com>).
date: 2011-09-07
institute: 1Institute of Zoology, Zoological Society of London, Regent's Park, London NW1 4RY, UK; 2Department of Infectious Disease Epidemiology, Imperial College London, St. Mary's Hospital, Norfolk Place, London W2 1PG, UK
references:
title: Intra-guild competition and its implications for one of the biggest terrestrial predators, *Tyrannosaurus rex*

# Introduction

Scavenging behaviour is associated with a diverse range of important ecological and behavioural processes including kleptoparasitism \[1\], disease transmission \[2\], species distributions \[3–5\] and nutrient cycling \[6\], but despite its importance it remains a poorly understood phenomenon \[7\]. Most carnivorous species today rely on a combination of hunting and scavenging \[7,8\], although some groups of species subsist almost exclusively on scavenging \[9\]. Effective scavengers must possess high movement speeds but low locomotion costs, and the ability to detect carcasses over long distances in order to outcompete other scavengers and predators \[10\]. These factors explain why vultures are such highly successful obligate scavengers, while sit-and-wait predators (e.g. viperid snakes) are reliant almost exclusively on hunting \[7\]. However, many physical features that help species scavenge successfully (e.g. powerful build; sharp teeth; fast speed; good vision and sense of smell) are also useful for hunting. As a result, it can be difficult to diagnose feeding strategies of species from their physical characteristics alone, especially for extinct carnivores known only from the fossil record \[11–13\].

In the past century, the feeding strategy of one of the best known and largest terrestrial carnivores, *Tyrannosaurus rex*, has been the subject of considerable debate and attention (e.g. \[14–20\]). Proposed support for the species being a primary or obligate scavenger includes its disproportionately tiny forelimbs and apparently reduced eyes \[16\], enlarged olfactory bulbs \[21\], and large body size and upright stance as a possible adaptation for finding carcasses, although the latter two traits may also have been an advantage for locating live prey \[15\]. However, enlarged olfactory bulbs may be associated with behaviours unrelated to food acquisition \[22,23\], and the eyes of *T. rex* are in fact large in both relative and absolute terms \[24\]. The binocular vision, bite force and impact-resistant teeth of *T. rex* suggest instead that it may have been better adapted for an active predatory lifestyle \[18,20,25,26\].

Nearly all of the debate over the feeding ecology of *T. rex* has focused on the interpretation of functional morphology rather than on energetic or ecosystem-level considerations. A recent attempt to determine the feasibility of *T. rex* being an obligate scavenger addressed the energetic costs of movement and the amount of meat available from carcasses \[17\]. Based on reasonable assumptions of energetic rates, travel speeds and carcass detection ability, this study concluded that *T. rex* could feasibly have survived as an obligate scavenger, although energetically it might have been a marginal strategy. However, as yet no analyses of the feeding ecology of *T. rex* have investigated wider patterns of theropod and other dinosaurian diversity in Late Cretaceous faunas \[27\], in particular, the impacts of competition from other sympatric carnivorous theropods, or the size distribution of carcasses available to potential scavengers of that period.

Most extant carnivore guilds contain a number of competing predatory/scavenging species \[28\], and the Late Cretaceous North American dinosaur fauna also consisted of several small-bodied non-avian carnivorous theropods in the Tyrannosauridae, Oviraptorosauria (Caenagnathidae), Deinonychosauria (Dromaeosauridae and Troodontidae) and Coelurosauria incertae sedis \[27\]. Although oviraptorosaurs and troodontids may not have been obligate carnivores \[29,30\], many of these sympatric theropods were largely or completely carnivorous, and there is evidence for both active predation and late-stage carcass consumption (probable scavenging) by dromaeosaurs \[31\]. In modern-day ecosystems, carcasses are typically a rare, widely dispersed and unpredictable resource, and competition among scavengers is often seen as a form of scramble competition \[7\]. If the same conditions applied in Late Cretaceous ecosystems, *T. rex* could have been in competition for resources with a range of different-sized theropods, also including juvenile and sub-adult *T. rex* individuals, which may have been relatively abundant compared with adults \[32\]. In our analysis, we focus on Late Cretaceous theropod species weighing 16–20 kg and up to the size of *T. rex*. Our lower size range was based on evidence from terrestrial mammalian carnivore guilds, because species of this weight range (e.g. wild dogs, jackals, hyenas) are extremely effective hunter/scavengers \[8\] (table 1). If the smaller, more abundant theropod species and *T. rex* age classes were therefore also primarily hunters that scavenged opportunistically, this might have placed extreme pressure on the resources available from scavenging in the Late Cretaceous theropod community.

Species and body masses of carnivorous non-avian theropod dinosaurs of Late Cretaceous North America. Midpoint taken when a range of body mass estimates is given in the literature. *Richardoestesia* is only known from jaws and teeth; since maximum tooth size is slightly smaller than that of *Saurornitholestes* \[33\], we estimate a body mass of *ca* 20 kg for this genus based on *Saurornitholestes* body mass estimates. Body mass estimates for *Albertosaurus sarcophagus* and *T. rex* taken as mean of all measurements for adult individuals that had reached somatic maturity given in Erickson *et al*. \[34\]. *Nanotyrannus* has been interpreted as a possible juvenile *Tyrannosaurus* by some authors \[35\], but we provisionally retain it here as a valid taxon following \[27\]. Mass categories and the estimated percentage that each size category contributes to the total carnivore guild are also presented (figure 1).

| species | family | mass (kg) | mass categories, kg (with estimated %) | reference |
|:---|:---|----|:---|:---|
| *Dromaeosaurus albertensis* | Dromaeosauridae | 16 | 20.6 (79.9%) | \[36\] |
| *Richardoestesia gilmorei* | Coelurosauria incertae sedis | 20 |  | see legend |
| *Richardoestesia isosceles* | Coelurosauria incertae sedis | 20 |  | see legend |
| *Saurornitholestes langstoni* | Dromaeosauridae | 23 |  | \[37\] |
| *Velociraptor* sp. | Dromaeosauridae | 24 |  | \[38\] |
| *Troodon formosus* | Troodontidae | 50 | 58.3 (19.0%) | \[39\] |
| *Chirostenotes elegans* | Caenagnathidae | 63 |  | \[40\] |
| *Chirostenotes pergracilis* | Caenagnathidae | 63 |  | \[40\] |
| *Nanotyrannus lancensis* | Tyrannosauridae | 1100 | 1123 (0.9%) | \[39\] |
| *Albertosaurus sarcophagus* | Tyrannosauridae | 1146 |  | \[34\] |
| *Tyrannosaurus rex* | Tyrannosauridae | 5347 | 5347 (0.1%) | \[34\] |

Interpreting patterns of relative abundance is notoriously difficult from the fossil record, due to taphonomic biases typically favouring the preservation of large-bodied species \[41\]. Skeletons of small-bodied Late Cretaceous dinosaurs are very rare and new species continue to be discovered, whereas the remains of large-bodied dinosaurs typically dominate Late Cretaceous fossil deposits \[38,42\]. However, even though they are recognized to have a much lower preservation potential, the observed fossil abundance of small putatively carnivorous theropods (dromaeosaurs and troodontids) is similar to that of tyrannosaurids in North American Late Cretaceous deposits \[42\], and many of these deposits show a relatively high abundance of both small theropods and small herbivorous dinosaurs (hypsilophodontids and pachycephalosaurs) \[43\]. This suggests that smaller carnivores and herbivores were proportionally much more abundant than tyrannosaurids in Late Cretaceous ecosystems. The abundance of small species within dinosaur guilds would be supplemented by the presence of earlier life stages of larger species. Body mass abundance patterns in fishes, another vertebrate group where individuals are independent before adulthood, are similar to those found in mammals when one considers the overall size classes rather than species weight classes \[44\]; assumptions about relationships between abundance and size at a faunal level are therefore likely to be robust to changes in the relative abundance of some of the smaller species.

In this paper, we explore the potential competitive ability of *T. rex* as an obligate scavenger. We take into account the presence of other carnivorous theropod individuals in Late Cretaceous ecosystems, and the estimated spatial distribution of carcasses of different herbivorous dinosaur species that would be available for scavengers. We make the assumption that, by analogy with vertebrate assemblages in modern-day terrestrial ecosystems, many predators are also opportunistic scavengers, so that there are more potential consumers of carcasses than carcasses available for these predatory guilds. We assess the likelihood that adult *T. rex* could have been an obligate scavenger based on expected carcass encounter rates for this species compared with those for sympatric smaller theropod dinosaurs. We follow Ruxton & Houston \[17\] in drawing explicitly on modern ecological analogues, by basing our analysis on the production rate of large carcasses estimated from the terrestrial large mammal community of the well-studied modern-day Serengeti ecosystem \[9\]. We also base our predictions of relative abundance and travel speed on well-established body size abundance and speed relationships found in terrestrial mammals \[45–47\].

# Methods

Lists of non-avian dinosaur species that were stratigraphically coeval and geographically sympatric with *T. rex* and which could potentially act as competitors or prey were collected from records of Late Cretaceous (late Campanian to late Maastrichtian) fossil formations in North America \[27\]. Only those dinosaur taxa that were positively identified to species or genus level from formations that also contained tyrannosaurid fossils identified as *T. rex* were categorized in our study as being sympatric; all provisional or tentative species or genus identifications were interpreted as being definite identifications for the purposes of analysis. Body mass estimates for these dinosaurs were obtained from the scientific literature (tables 1 and 2). We exclude the herbivorous ornithomimosaurs \[54\] from our analyses of carnivorous theropods but include caenagnathid oviraptorosaurs and troodontids, following earlier studies \[55–57\]. Although there may have been minor variations in dinosaur faunal composition across the geographical range of *T. rex* during the Late Cretaceous (e.g. titanosaurid sauropods were restricted to the southwestern United States \[27\]), our species list is treated as representing a consistent sympatric faunal unit across this region for the purposes of analysis. A slightly different species list is also available on the Paleobiology Database (<http://paleodb.org/cgi-bin/bridge.pl>). However, as our analysis of theropod performance focuses on a comparison between the largest and smallest theropod size classes (see below), it is robust to such minor changes in faunal composition.

Species and body masses of herbivorous dinosaurs of Late Cretaceous North America. Ornithischian and sauropod body masses taken from the midpoint of genus-level estimates in Peczkis \[48\]; ornithomimosaur body masses taken from Christiansen \[37\]. Species diversity from Weishampel *et al*. \[27\] revised following \[49–52\]; *Dyslocosaurus polyonychius* also excluded as the suggested Late Cretaceous occurrence of this species is probably erroneous \[53\]. Mass categories and the estimated percentage each category contributes to the total herbivore guild are based on herbivore mass–abundance relationships found in extant mammalian herbivores (see also figure 1).

| species | family | body mass (kg) | mass categories, kg (with estimated %) |
|:---|:---|----|:---|
| *Parksosaurus warreni* | Hypsilophodontidae | 55 | 75 (49.3%) |
| *Prenocephale edmontonensis* | Pachycephalosauridae | 85 |  |
| *Ornithomimus velox* | Ornithomimosauridae | 155 | 216 (36.8%) |
| *Struthiomimus* sp. | Ornithomimosauridae | 175 |  |
| *Thescelosaurus garbanii* | Hypsilophodontidae | 250 |  |
| *Thescelosaurus neglectus* | Hypsilophodontidae | 250 |  |
| *Leptoceratops gracilis* | Leptoceratopsidae | 250 |  |
| *Montanoceratops* sp. | Leptoceratopsidae | 550 | 700 (6.0%) |
| *Pachycephalosaurus wyomingensis* | Pachycephalosauridae | 850 |  |
| *Edmontosaurus annectens* | Hadrosauridae | 2500 | 2500 (6.7%) |
| *Edmontosaurus regalis* | Hadrosauridae | 2500 |  |
| *Edmontosaurus saskatchewanensis* | Hadrosauridae | 2500 |  |
| *Lambeosaurus* sp. | Hadrosauridae | 2500 |  |
| *Parasaurolophus walkeri* | Hadrosauridae | 2500 |  |
| *Edmontonia rugosidens* | Nodosauridae | 2500 |  |
| *Ankylosaurus magniventris* | Ankylosauridae | 5500 | 5000 (0.6%) |
| *Triceratops horridus* | Ceratopsidae | 8500 | 8500 (0.4%) |
| *Alamosaurus sanjuanensis* | Saltasauridae | 25 000 | 25 000 (0.2%) |

We based our estimates of carcass production on data from the Serengeti ecosystem \[17\]. We assume that this is a valid ecological comparison, because although ecosystems in the Late Cretaceous may have been more productive and contained more large-bodied species than terrestrial mammalian systems today, they would also be expected to contain a higher abundance of predators, and the fossil record of large-bodied species contains similar predator–prey ratios to those recorded in modern-day ecosystems \[58\].

Our approach relies on the use of allometric equations to estimate how factors such as abundance and search capacity vary with body mass. Many biological characteristics of animals are related to body mass and can be described with power equations taking the form of *Y* = *a* × *M*^*b*^, where *a* is a constant, *M* is mass (kg) and *b* represents the allometric exponent, which influences the relative change in the value *Y* with changes in body mass. These equations are commonly used in multi-species studies on the influence of body mass on species characteristics \[59\].

We assume that population densities of herbivorous and theropod dinosaurs, *N*~h~ and *N*~t~ (number of individuals km^−2^), are related to the following power equations:

![](rspb20102497-e1)

and

![](rspb20102497-e2)

where *M*~h~ and *M*~t~ are in kilograms, *c* and *e* are constants corresponding to the number of 1 kg animals km^−2^, and *d* and *f* are scaling factors.

To estimate the relative numbers of Late Cretaceous herbivorous dinosaurs, we used values of *c* = 100 and *d* = −0.75, similar to the values estimated from data on herbivorous mammal abundance \[45\]. Estimates of carnivorous theropod dinosaur abundances were derived from mammalian carnivore data \[47\], where *e* = 1.97 and *f* = −0.88. For the purposes of illustration, herbivorous and carnivorous dinosaur species, including different age classes of the larger species, were then grouped into seven and four size classes, respectively, corresponding to the natural body size groupings shown by the data (tables 1, 2 and figure 1).

To explore how the presence of sympatric competing theropods affects the likelihood of adult *T. rex* being an obligate scavenger, we used estimates of carcass encounter rate following the model proposed by Ruxton & Houston \[17\]. Our approach, however, differs from Ruxton & Houston in that we focus on estimating relative encounter rates of theropod scavengers accounting for size-dependent differences in estimated population density, travel speed and search capacity in relation to the size and likely distribution of available carcasses. Also unlike Ruxton & Houston, we do not explicitly attempt to estimate intake rates of a scavenging *T. rex* but focus our analysis on the relative search areas of an adult *T. rex* compared with other competing species and age classes. Following Ruxton & Houston, we assumed that the forager moves along a path where the search area is defined by the speed *S*~t~ (m s^−1^) and detection distance *D*~t~. Because we also need to estimate how these factors vary with body mass, we used the following allometric equations:

![](rspb20102497-e3)

where *g* and *h* represent the constant and exponent for speed, respectively, and:

![](rspb20102497-e4)

where *j* and *k* represent the constant and exponent for detection range, respectively.

In order to explore the potential impact of scavenging on carcasses, we calculated the total potential impact of the populations of different theropod species on carcass feeding by multiplying estimated population sizes by individual search rates (see above).

To estimate the population search rate *NA*~tot~ (km^2^ d^−1^) would be:

![](rspb20102497-e5)

which expands to:

![](rspb20102497-e6)

The constants *e*, *g* and *j* in this equation do not affect the relative impacts of scavenging by different theropods, but the scaling exponents have a critical influence on these relative values. Therefore, in order to compare these scanning rates we do not need to know the absolute values of the constants, which is useful as we do not have a good understanding of the absolute values of abundance, speed and detection range for any dinosaur species.

We initially chose variables for the constants and exponents to match Ruxton & Houston's \[17\] predictions for *T. rex* (table 3, scenario 1), but also used alternative values based on more extreme assumptions about relative differences between theropod species (table 3, scenarios 2–5). In scenario 1, we set the speed constant *g* = 11.9 and the speed scaling factor *h* = 0.05. We set the detection range constant *j* = 0.034 and the detection range scaling factor *k* = 0.1. These values predict a daily distance for an individual *T. rex* of 18.3 km and a detection distance of 80 m (approximately the values estimated in \[17\]), compared with 13.8 km d^−1^ and a 50 m detection range for a 20 kg theropod. We increased the exponents for the scaling of both speed and/or detection range with mass (scenarios 2–4) to increase the relative speed and detection distance for *T. rex*. Here we used combinations of values of 0.16 and 0.25 for the speed and detection range scaling factors *h* and *k*, respectively (broadly similar to values described in scaling studies on mammals \[46,60\]; table 3). For the purposes of illustration, we calculated an extreme case (scenario 5) where *T. rex* performs at the same level as smaller theropod species, with scaling factor values *h* = 0.35 and *k* = 0.54.

Sensitivity analysis of the impact of varying speed and detection range for 'small' (a hypothetical 20 kg species) and 'large' (*T. rex*) theropod dinosaur populations. Scenario 5 shows extreme scaling where *T. rex*-sized theropods would be predicted to be able to cover the same areas as all other theropod species. In this case the constant *sc* was set to 6.6 to reduce the km d^−1^ estimate for *T. rex*.

|  | speed |  | detection |  | theropod mass (kg) | km d^−1^ | *S*~t~ (km h^−1^) | *D*~t~ (km) | ratio^a^ |
|:---|----|----|:---|:---|:---|----|----|----|----|
| scenario | const. *g* | exp. *h* | const. *j* | exp. *k* |  |  |  |  |  |
| 1 | 11.9 | 0.05 | 0.034 | 0.1 | 20 | 13.8 | 1.2 | 0.05 | 60.1 |
|  |  |  |  |  | 5347 | 18.3 | 1.5 | 0.08 |  |
| 2 | 11.9 | 0.16 | 0.034 | 0.1 | 20 | 19.2 | 1.6 | 0.05 | 32.5 |
|  |  |  |  |  | 5347 | 47.0 | 3.9 | 0.08 |  |
| 3 | 11.9 | 0.05 | 0.034 | 0.25 | 20 | 13.8 | 1.2 | 0.07 | 26 |
|  |  |  |  |  | 5347 | 18.3 | 1.5 | 0.29 |  |
| 4 | 11.9 | 0.16 | 0.034 | 0.25 | 20 | 19.2 | 1.6 | 0.07 | 14 |
|  |  |  |  |  | 5347 | 47.0 | 3.9 | 0.29 |  |
| 5 | 6.6 | 0.35 | 0.034 | 0.54 | 20 | 18.8 | 1.6 | 0.17 | 1.0 |
|  |  |  |  |  | 5347 | 133.2 | 11.1 | 3.50 |  |

^a^Ratio of the daily area covered by members of a species of 20 kg over that predicted for the *T. rex* population, based on an estimated density of 177.4 per 1000 km^2^ for the small species and 2.9 per 1000 km^2^ for *T. rex* (see text for details).

# Results

Based on size–abundance relationships found in modern-day herbivorous mammals \[45\], we predict that just under 50 per cent of herbivorous carcasses in the *T. rex* community would have been in the 55–85 kg size range (figure 1). A productive modern-day ecosystem such as the Serengeti generates 4.38 kg of carcasses km^−2^ d^−1^ \[9\]. If we can assume that levels of carcass abundance were similar in Mesozoic terrestrial ecosystems, a substantial number of dinosaur carcasses would therefore have been present in Late Cretaceous North America, enough to sustain an individual adult *T. rex*. However, it becomes clear that finding these carcasses becomes a problem if we estimate the distributions of different-sized carcasses given this predicted rate of supply. Using the simplified assumption that all size classes of species produce carcasses at the same rate \[47\], we can easily calculate an expected distribution based on the Serengeti production rate as an even fraction of the number of herbivorous dinosaur size classes. If every carcass lasted 7 days before becoming consumed (or 14 days at 50% of the original body mass), most carcasses would be very widely dispersed indeed: on average there would be just one 75 kg carcass every 17 km^2^, one 700 kg carcass every 160 km^2^, and one 5 tonne carcass every 1000 km^2^, with a 25 tonne carcass every 5000 km^2^. Clearly, these carcasses would be difficult for individual scavengers to find without travelling great distances.

Based on Ruxton & Houston's calculations \[17\], we expect an adult *T. rex* to have covered just under 3 km^2^ a day (18.3 km daily distance × 2 × 0.08 km detection distance). Using our estimates of carcass distributions described above, we predict that *T. rex* would have been able to find a 75 kg carcass on average in just under 6 days, and a 700 kg carcass in approximately 55 days; on average, it would have had to search for over a year to find a 5 tonne carcass and five times that to find a 25 ton carcass (figure 2). The potential absence of titanosaurid sauropods, the largest size class of herbivorous dinosaurs, from part of the geographical range of *T. rex* \[27\] therefore has little bearing on the availability of carcasses in Late Cretaceous ecosystems.

How might populations of other smaller carnivorous theropod dinosaurs have impacted these carcasses during the time *T. rex* spent searching for them? Given the much higher predicted relative abundances of smaller theropod species and age classes compared with an adult *T. rex*, one might expect individuals of these species to have had a considerable effect on the amount of meat supplied by carcasses. In figure 2, we show the average expected numbers of theropod dinosaurs that would arrive at a carcass before an adult *T. rex* could reach it. The smaller, more abundant species would be expected to take less time to locate and arrive at a carcass. For a 75 kg carcass, we predict that on average five *Dromaeosaurus*-sized individuals and one *Troodon*-sized individual would already have arrived first, and would be expected to quickly consume most or all of the carcass. Larger carcasses would have attracted more theropods, and again would probably have been fully consumed before the arrival of *T. rex*. Overall for the *T. rex* community, we predict that the smallest carnivorous theropods collectively would have had carcass search rates over 60 times higher than adult *T. rex* under a model of modest scaling exponents for travel speed and detection range (i.e. small differences between theropods), and between 26–33 times higher given moderate scaling exponents (table 3). Only if we assume extreme scaling exponents, where an adult *T. rex* travelled over 130 km d^−1^ and had a detection range of around 3.5 km, could it have had the same search rate as smaller theropod species (table 3). However, even under these extreme and implausible parameters, competition would have been intense across the Late Cretaceous theropod guild. Although we do not show these calculations, if an adult *T. rex* had been even larger than our body mass estimate and instead nearer to 7 tonnes \[34\] following alternative estimates, we would expect approximately 30–73-fold lower search rates in this species given conditions in scenarios 1–3.

# Discussion

Our results highlight the importance of taking both intraspecific and interspecific competition and the distribution of food resources into account when investigating the ecological viability of alternative carnivore foraging strategies. Ultimately, the nutritional ecology of a carnivore will be determined by the costs and benefits of different available strategies. Most terrestrial mammalian carnivores are primarily hunters, but scavenge opportunistically when carcasses become available \[7\]. A heavy reliance on scavenging will only be viable if this strategy can provide a reliable source of food. While our analysis does not examine these costs/benefits in relation to hunting at an individual level, we clearly show that given the distribution of carcasses and the potential for competition with other members of the carnivorous theropod guild, it is extremely unlikely that an adult *T. rex* could use scavenging as a long-term sustainable foraging strategy. The simple reason for this lies in the predicted overwhelming abundance of smaller-bodied competing theropods, with the smallest of these in the 16–25 kg range estimated to represent approximately 80 per cent of individuals in the guild (table 2).

In modern-day carnivore guilds, groups of African wild dogs (*Lycaon pictus*), wolves (*Canis lupus*) and spotted hyena (*Crocuta crocuta*) weighing 20–60 kg can reduce a 70+ kg carcass to scraps of skin and bone extremely rapidly, certainly in under an hour \[61,62\]. If similar-sized carnivorous theropods had the same ecological impact in the Late Cretaceous, this would have severely reduced the availability of the most common carcass size available to larger carnivorous species. Although a fully grown *T. rex*, being such a large carnivore, may have dominated carcasses once it had arrived \[63,64\], there would have been a limited number of carcasses persisting in the environment to support such a giant obligate scavenger. Even without the impact of small theropods, predicted travel speeds and detection ranges indicate that carcasses would have rotted by the time an adult *T. rex* was typically able to find them. Furthermore, whereas our analysis has only considered potential dinosaur competitors, *T. rex* may also have competed with other species such as giant azhdarchid pterosaurs for access to carcasses \[12,65\], suggesting that Late Cretaceous North America would have been even less likely to support a giant obligate terrestrial scavenger than we have predicted. Decomposers would also have played their part in further reducing the amount of meat available to vertebrate scavengers \[7\]. In addition, our analyses made the conservative assumption that theropod speed increases with size. Recent studies on theropod locomotion, however, suggest that smaller species may in fact have been faster and more mobile \[66,67\]. This is consistent with patterns of home range size in modern carnivores, where the largest home ranges are found in the intermediate body size range \[68\]. If movement rates of smaller species were in fact higher than we have assumed, the difference in search rates would be even more extreme.

The question of whether dinosaurs were endothermic or ectothermic remains one of the great unresolved controversies of palaeontology, and has important implications for reconstructing aspects of dinosaurian trophic ecology such as metabolic requirements and predator : prey ratios \[69–71\]. Our comparison of the search rates of theropod dinosaurs is sensitive to changes in relative abundance, speed of movement and detection distances. While these variables would be affected by metabolic rate, our main conclusions regarding competition between theropods would not be affected by their thermoregulatory status if we assume that all theropods in the *T. rex* community had similar metabolic rates, a plausible assumption following several lines of evidence (e.g. phylogeny, shared possession of feathers or protofeathers, bipedal stance \[27,70\]). If theropods were ectotherms, we would expect *T. rex* to be able to tolerate longer periods of fasting, but we might also expect it to move more slowly and have a lower search rate \[17\]. We might expect ectothermic theropods to obtain higher densities for a given density of prey/carcasses, but this would also lead to higher levels of competition for each carcass. These tradeoffs therefore suggest that our conclusions about the trophic status of *T. rex* should hold true whether it was endothermic or ectothermic.

In a wider ecological context, it should not be surprising that a fully grown *T. rex* cannot be realistically interpreted as an obligate scavenger. Large-bodied theropods (e.g. neoceratosaurs, carnosaurs, megalosaurs, tyrannosauroids) are characteristic members of all well-studied diverse Jurassic and Cretaceous dinosaur assemblages, and almost all of these species other than *T. rex* have been interpreted uncontroversially as active terrestrial predators \[24\]. The only large-bodied theropods that probably had a markedly different nutritional ecology are the fish-eating spinosaurs, which unlike *T. rex* have been reconstructed on the basis of several independent lines of evidence (functional morphology, isotope signatures and stomach contents \[72–74\]). Several Jurassic and Cretaceous predator guilds (e.g. Morrison Formation, Bahariya Formation) contained multiple lineages of extremely large sympatric predatory theropods \[24,75\], for which direct competition for prey must have been mitigated by morphologically or ecologically mediated niche partitioning \[56\]. In contrast, tyrannosaurids are the only large-bodied theropods in Late Cretaceous North America, with *T. rex* as the only giant carnivore (see \[34\] for tyrannosaurid body masses and growth rates). Since the *T. rex* community also contained a wide range of large-bodied potential prey species, it can be considered extremely unlikely on comparative ecological grounds that this system for some reason lacked a similar theropod 'super-predator'.

Many extant mammalian carnivores feed in groups, especially in open and highly competitive ecosystems, and recent work on Pleistocene carnivores suggests that sociality may make species more effective at defending scavenged carcasses from competitors \[64,76\]. If intra-guild competition was similarly high in Late Cretaceous terrestrial ecosystems, we might also expect selection for social grouping behaviour in carnivorous theropods. Interestingly, although reconstructing social behaviour for extinct species in the fossil record is an extremely difficult process, dromaeosaurs have often been interpreted as social carnivores ('pack hunters') on the basis of *Deinonychus*–*Tenontosaurus* fossil assemblages \[77\] (although see \[78\]), and there is also some taphonomic evidence to suggest gregarious behaviour in the small tyrannosaurid *Albertosaurus* \[79\].

What can our analysis tell us about other aspects of the predatory behaviour of *T. rex* and other extremely large theropods? Clearly, *T. rex* would have been able to dominate almost all other terrestrial predators at any time or any place. However, given its large mass and the fact that it would have been vastly outnumbered by smaller theropods, it was probably a poor competitor in indirect scramble competition. We have evaluated the potential impact of intra- and interspecific competition between sympatric theropods on scavenging opportunities for *T. rex*, but similar arguments could also be applied to its ability to find hatchling or juvenile dinosaurs or other sources of food if these had an unpredictable and ephemeral distribution, even though these are sometimes interpreted as possible specialist tyrannosaurid prey items \[80\]. Given its physical dominance, being one of the largest known theropods \[36\], *T. rex* would have been better able to compete for more predictable food resources, and as a large prey specialist it would have had a unique position in its guild at being able to effectively hunt herbivorous dinosaurs that were far larger than would be available to other theropod species. Indeed, although pack hunting may have allowed dromaeosaurs to target larger prey species, taphonomic evidence suggests that small Cretaceous deinonychosaurian theropods probably hunted relatively small-bodied herbivorous dinosaurs such as basal neoceratopsians or hypsilophodontids, or dinosaur hatchlings \[31,57,77\], rather than the larger hadrosaurs, neoceratopsians or titanosaurs. As an active large prey specialist, feeding on herbivores of similar or greater mass, *T. rex* would have had feeding habits consistent with prey size selection patterns found in extant mammalian carnivores \[81,82\]. We propose that this is the most likely feeding strategy for *T. rex*. Future research into the energetic and behavioural constraints on this extreme giant predator might also help to better understand the evolutionary constraints on carnivory.

## Acknowledgements

We thank Marcus Clauss, James Farlow and John R. Hutchinson for their helpful comments on previous versions of the manuscript and thank Tom Kennedy for help with the figures.

C.C. acknowledges funding provided by the Deutsche Forschungsgemeinschaft (DFG) for publishing this manuscript. This is contribution no. 104 of the DFG Research Unit 533 'Biology of the sauropod dinosaurs: the evolution of gigantism'.

# References


abstract: Speculations on the potential impact of climate change on human health frequently focus on malaria. Predictions are common that in the coming decades, tens – even hundreds – of millions more cases will occur in regions where the disease is already present, and that transmission will extend to higher latitudes and altitudes. Such predictions, sometimes supported by simple models, are persuasive because they are intuitive, but they sidestep factors that are key to the transmission and epidemiology of the disease: the ecology and behaviour of both humans and vectors, and the immunity of the human population. A holistic view of the natural history of the disease, in the context of these factors and in the precise setting where it is transmitted, is the only valid starting point for assessing the likely significance of future changes in climate.
author: Paul Reiter
date: 2008
institute: 1Insects and Infectious Disease Unit, Institut Pasteur, 25-28 rue du Dr Roux, 75724 Paris, France
references:
title: Global warming and malaria: knowing the horse before hitching the cart

*Malaria is ... a thousand different diseases and epidemiological puzzles. Like chess, it is played with a few pieces, but is capable of an infinite variety of situations*.

*Malaria in Europe, An Ecological Study* \[1\]

Man-made climate change has become a defining moral and political issue of our age. Speculations on its potential impact often focus on infectious diseases, and on malaria in particular. Predictions are common that in the coming decades, tens – even hundreds – of millions more cases will occur in regions where the disease is already present, and that the vectors and the pathogens will move to higher latitudes and altitudes.

Numerous review publications \[2-34\] and substantial media attention have had a major impact on public perceptions of the issue. In most cases, these publications make brief mention of where malaria occurs and how it is transmitted, followed by a succession of statements on the action of temperature, rainfall and other climate variables on specific components of the transmission cycle. These statements – often valid in themselves – are used to justify disquieting predictions that are persuasive because they are intuitive. Some are based on mathematical models that select a climate variable (usually temperature), propose a direct interaction with a transmission parameter (e.g. multiplication of the parasite, survival of the vector), and inevitably arrive at the same conclusions. Many focus on the vulnerability of people in poorer countries and place the blame squarely on the activities of the industrial nations. A deplorable trend in the scientific press is the inclusion of a political message, much as in the popular media.

The great majority of these publications sidestep four factors that are key to the transmission and epidemiology of the disease: the ecology and behaviour of humans, and the ecology and behaviour of the vectors. There is rarely any mention of a fifth, the immunity of the host and of the host population. These factors interact with each other, and are themselves made up of an intricate network of highly variable parameters. The true dynamics of transmission can only be assessed by taking this daunting complexity into account. Moreover, the key climate variables – temperature, rainfall and humidity – cannot be viewed independently; the effects of temperature are modified by humidity, the daily range of each may be more significant than the daily mean, brief periods of atypical heat or cold can be more significant than long-term averages, heavy storms can have a different impact than light prolonged rainfall, one year's events may have a significant impact on subsequent years, and so on.

In summary, a holistic view of the complex natural history of malaria in the precise setting where it is transmitted is mandatory for any speculation on the role of climate variables; the significance of these variables, and their putative role in future climates, can only be assessed in the perspective of this complexity.

# Mathematical models

The mathematical modelling of malaria began with Ronald Ross, soon after he had demonstrated the transmission cycle of the parasite. Fifty years later, Macdonald modified his approach to build a more comprehensive model that proved useful in developing strategies for control. Both men were aware that their approach was limited; they recognized that factors such as immunity, behaviour and seasonality were not included, mainly because they are hard to quantify. Nevertheless, many of the claims that are made in the climate change debate blithely present Macdonald's simple deterministic model, list its component variables, point out that the extrinsic incubation period is sensitive to temperature and thereby reason that higher temperatures will mean higher rates of transmission. To the layman, this naïve approach may be persuasive because it is intuitive, but Ronald Ross and Macdonald would certainly not have approved.

The advent of low-cost computers has propelled mathematical modelling into a major role in the description of complex systems, including ecology, epidemiology and public health \[35\]. It is now relatively simple to run stochastic models, built of a selected set of variables, with interactions driven by sets of differential equations. Complex systems imply the need for a large number of variables and operators, but as these numbers increase, so does the variance – and the uncertainty – of the models. In a sense, therefore, they remain an extension of the intuitive approach because the selection of variables, the assumptions of the frequency distributions that are involved, the mathematical descriptors of the operators, and the constraints on both are made by the modeller. This is not to say that such models are inappropriate. On the contrary, they have become a central component of infectious disease epidemiology, mathematical representations that can be used to explore the dynamics of transmission and the fit of such models to field data. Nevertheless, an inescapable constraint in the case of vector-borne diseases is that the natural history and ecology, of both vector and host, are so complex that it is facile to predict future prevalence and incidence on the basis of temperature.

# Common misconceptions

There is a widespread misconception that mosquito-borne diseases require tropical temperatures, or at least the temperatures of the warmer temperate regions. A glance at a map of global isotherms reveals that summer temperatures in many temperate regions are at least as high as in the warmest seasons of many regions in the tropics. The crucial difference is that the tropics do not have cold winters. Moreover, if tropical mosquito-borne pathogens are introduced to temperate regions in the right season, they can be transmitted, if suitable vectors are present \[34\].

There is also a misconception that mosquitoes die in winter, and that more die in colder winters, but it is obvious that mosquitoes – and indeed all life-forms that are native to temperate regions – have evolved strategies to survive low temperatures. In the tropics, comparable adaptations are necessary for survival during unfavorable dry periods that may last for several years. In both cases, such adaptations merely impose seasonality on transmission. In southern Europe, for example, *Plasmodium falciparum* (the most pathogenic species of malarial pathogen) was once transmitted from July to September \[36\]. In Mali, where the disease is still endemic, it is limited to the same three months, the wet season \[37\].

The physical environment is an important modifier of local climate. *Anopheles arabiensis*, an important vector of malaria in Africa, can survive in the Sudan when outdoor temperatures are above 55°C by hiding in the thatch of buildings in the daytime, feeding after mid-night, and ovipositing at dawn or dusk \[38\]. In Lapland, in the past, anopheline species survived the winter in houses and stables, feeding occasionally, and transmitting malaria when outdoor temperatures were below -40°C \[39\].

These examples underline the limited value of meteorological variables as a guide to the behaviour and geographic range of vector species, and of the pathogens they transmit.

# Malaria in temperate regions

Few people are aware that it is less than forty years since the final eradication of malaria in Europe and the United States. Indeed, the disease was common in the period from the 16^th^ to 18^th^ centuries that climatologists term the Little Ice Age \[33\], and data from burial records around the Thames estuary reveal that mortality in "marsh parishes" of England was comparable to that in areas of transmission in sub-Saharan Africa today \[40,41\].

Until the mid-19^th^ century, the northern limit of transmission was roughly defined by the present 15°C July isotherm. Denmark and parts of Sweden suffered devastating epidemics until the 1860s. Incidence diminished thereafter and the disease had essentially disappeared around the turn of the 20^th^ Century. The same was true in Finland, except for a brief recrudescence in 1941, during the Russo-Finnish war. Figure 1 shows the distribution of malaria cases in Norway between 1860 and 1920. In England, there was a gradual decrease in transmission until the 1880s, after which it dropped precipitously and became relatively rare, except in a short period following World War I. In Germany, transmission also diminished rapidly. After World War I it was mainly confined to a few marshy localities \[36\].

This regional decline is attributed to a combination of factors:

## Ecological change

Improved drainage, reclamation of swampy land for cultivation and the adoption of new farming methods all served to eliminate mosquito habitat (there is an old Italian saying: "malaria flees before the plough").

## New farm crops

New root crops such as turnips and mangel-wurzels were adopted as winter fodder. These enabled farmers to maintain larger numbers of animals, particularly cattle, throughout the year. European vectors of malaria readily bite cattle, so the increasing size of herds diverted mosquitoes from feeding on humans. Human malaria parasites cannot infect cattle, so this diversion reduced the number of infected mosquitoes and infective bites.

## New rearing practices

Selective breeding of cattle, and new introductions (e.g. the Chinese domestic pig), in combination with the new fodder crops, enabled farmers to keep large populations of stock in farm buildings rather than in open fields and woodland. These buildings provided attractive sites for adult mosquitoes to rest and feed, diverting them from human habitation.

## Urbanization and mechanization

Rural populations declined as industrialization drew people to urban areas. The increased ratio of cattle to people further reduced the attack rates of mosquitoes on humans.

## Human living conditions

New building materials and improvements in construction methods made houses more mosquito-proof, especially in winter, another factor that reduced contact with the vector.

## Medical care

Greater access to medical care, and wider use of quinine (in part due to a major reduction in price) reduced the survival rate of the malaria parasite in its human host, thus limiting the infection rate of mosquitoes.

These factors are a classic illustration of the role of ecology and behaviour in the prevalence and incidence of a mosquito-borne disease. Moreover, the decline cannot be attributed to climate change, for it occurred at the start of the current warming phase. Nor can it be attributed, as is often stated, to deliberate mosquito control, for it came before recognition of the role of the vector.

In contrast, in countries that lagged in these changes, malaria did not decline "spontaneously" \[36\]. In the Soviet-block countries, for example, from Poland to eastern Siberia, major epidemics occurred throughout the 19^th^ century and the disease remained one of the principal public health problems for the entire first half of the 20^th^ century. Indeed, in the 1920s, in the wake of massive social and economic disruption, a pandemic swept through the entire Soviet Union. Official figures for 1923–25 listed 16.5 million cases, of which not less than 600,000 were fatal \[36\]. Tens of thousands of infections, many caused by *P. falciparum*, occurred as far north as the Arctic seaport of Arkhangelsk (61° 30^'^N).

The advent of DDT revolutionized malaria control. Cheap, safe, effective applications of the chemical could be targeted at the site where most infections occur – in the home. Initial efforts in Italy, Cyprus and Greece were so successful that a decision was made to eradicate the disease from all of Europe. The entire continent was finally declared free of endemic malaria in 1975. One of the last countries affected was Holland.

Several efficient malaria vectors are indigenous to North America, and are common in many areas. The pathogen probably arrived in the hemisphere in infected European colonists and African slaves. The history of its decline is similar to that of Europe. In the 1880s, it was widespread in nearly all states east of the Rocky Mountains, from the semitropical Gulf Coast states to the northern border and into Canada. It was also present west of the Rocky Mountains, particularly in areas where rainfall is abundant. As living conditions improved, and anti-malarial drugs became more widely available, the incidence of the disease declined. During World War II, the US Military created an Office of Malaria Control in War Areas, responsible for military bases throughout the country. In a symposium published in 1941 \[42\], the disease was described as "of moderate endemic intensity in the sixteen south-eastern states...with scattered epidemics at intervals of approximately seven years." In 1946 the United States Congress established a new agency, the Communicable Disease Center. This was the forerunner of the U.S. Centers for Disease Control and Prevention (CDC), and its principal mission was to eradicate malaria from the entire country. Its headquarters were in Atlanta, Georgia, because the southern states were the main region still affected by the disease. When operations began, foci of transmission were already hard to find, but the disease was not totally eliminated until the late 1950s. Today, as in Europe, transmission cycles have been disrupted and the pathogens are absent.

Malaria vectors are, of course, still widespread in Europe and North America, and there are occasional cases of autochthonous transmission when an infected traveler infects local mosquitoes. Given the efficacy of anti-malarial therapy, however, and the increasing sophistication of disease surveillance, it is safe to say that there is no chance of significant transmission arising from such cases, and certainly no chance of the disease becoming endemic, at least under current economic circumstances.

# Malaria in the tropics

Ninety percent of the estimated 300–500 million cases of malaria worldwide occur each year in sub-Saharan Africa. Statements on climate change and human health often focus on this region, with predictions that, by the mid-21^st^ century, tens of millions more cases will occur there as a direct result of increasing temperatures (see for example \[2\]).

A critical aspect of malaria epidemiology is the concept of *stability*. In much of sub-Saharan Africa, parts of northern India, Indonesia, South America and elsewhere, transmission is termed *stable* because it is fairly constant from year to year; the disease is endemic, but epidemics are uncommon. In other regions, including parts of India, Southeast Asia, Central and South America, the disease is also endemic but is termed *unstable* because transmission can vary greatly from year to year, and the potential for epidemics is high. These terms are, of course, a simplification; there is a wide range of degrees of stability, depending on complex factors in local circumstances.

## Stable endemic malaria

In regions where the principal malaria vectors are anthropophilic (i.e. prefer to feed on humans) and have a high survival rate, transmission is usually stable. The disease is hard to control because transmission is efficient and transmission rates are so high that most people experience many infective bites per year. Severe illness and mortality occurs mainly among "new arrivals," i.e. children and non-immune immigrants. Older inhabitants have survived multiple infections and maintain a degree of immunity by repeated re-infection. They can have bouts of illness that may be life-threatening, but are usually relatively mild.

## Unstable endemic malaria

This generally occurs in regions where the anophelines are zoophilic (i.e. bite animals as well as humans), or their survival rates are low, or where both apply. Transmission can vary greatly from year to year, with epidemics separated by many years of relatively low activity during which the overall immunity of the population declines. The factors that precipitate such epidemics are often difficult to identify. The disease may appear suddenly, for no apparent reason, only to disappear again without obvious cause. In general, transmission is relatively inefficient and, therefore, requires high mosquito populations. In theory this implies that the disease is easier to control. In practice, it can be catastrophic, not least because the attack rate is not restricted to "new arrivals".

The vast majority of people in sub-Saharan Africa live in regions of stable transmission. In other words, throughout their lives, people living in the shaded areas of the map (Figure 2) are regularly exposed to multiple bites from infective mosquitoes, as many as 300 infective bites per year in some regions. Under such circumstances, just as it is impossible to pour more water into a glass that is already full, it is illogical to suggest that increased temperatures will result in an increased incidence of infections. On the other hand, changes in rainfall patterns could alter stability and distribution.

Thus, as in temperate regions, the behaviour and ecology of vector and host are the dominant factors in transmission. In many cases, these are attributable to explosive population growth and poverty.

## Birth rate

The world's population has grown from 2.5 billion in 1950 to 6.7 billion at the time of writing (September 2008). In sub-Saharan Africa, there are now nearly five times as many people (ca. 750 million) as there were in 1955. In some countries, more than half the population is under 15 years of age. High birth rates often give rise to larger communities with higher densities of people, which can lead to a higher attack rate; clinical studies in some parts of Africa quote 998 infections per 1,000 infants.

## Forest clearance

Many malaria vectors breed in open, sunlit pools. Forest clearance provides abundant new habitat for these species, a classic cause of the emergence of malaria problems \[43\].

## Agriculture

Irrigation creates an ideal habitat for mass-production of mosquitoes, as can construction of dams for hydroelectric power. Rice cultivation provides an environment for many of the most efficient malaria vectors. Conversely, the cultivation of ground depressions can suppress such vectors and thereby reduce transmission \[\].

## Movement of people

Infected people in pursuit of work can introduce malaria to areas where it is rare. Non-immune people are at high risk if they move to areas of transmission. Extensive road building and modern transportation have greatly exacerbated this factor.

## Urbanization

Water storage and inadequate water disposal can provide habitat for mosquitoes, particularly in rapidly expanding urban areas. The absence of cattle can promote stable transmission by forcing zoophilic species to feed on people. Moreover, many tropical cities are surrounded by densely-populated, satellite settlements that are essentially rural in nature.

## Insecticide resistance

*Physiological resistance* to insecticides is common in many regions. *Behavioural resistance* can also be a major problem: species that prefer to feed and rest indoors (*endophilic*) can switch to outdoor (*exophilic*) activity in response to treatment of indoor surfaces with insecticides.

## Drug resistance

In many parts of the world, the malaria parasite has evolved resistance to commonly used anti-malarial drugs. Substitutes are available, but are much more expensive.

## Degradation of the health infrastructure

Lack of funding, institutional difficulties, rapid urbanization and other problems associated with rapid development have eroded the public health sector of many countries. In addition, the AIDS pandemic has overwhelmed the ability of authorities to deal with other diseases.

## War and civil strife

In times of conflict, mass movements of people, e.g. soldiers and refugees, often promote malaria transmission. The breakdown of public health services, damage to water distribution and drainage systems, and the destruction of homes often exacerbate the situation. High concentrations of people in camps for displaced persons can also be disastrous.

# Highland malaria in the tropics

A topic that is repeatedly cited in the climate change debate, both in the scientific and the popular press, is that warmer temperatures will drive malaria transmission to higher altitudes in the tropical highlands, particularly East Africa. Indeed, environmental activists frequently state that this is already happening.

It is certainly true that the incidence of malaria has increased in some highland areas, and it is perfectly acceptable to cite temperature as a limiting factor in transmission; vectors such as *Anopheles gambiae* have been reported at altitudes up to 3,000 m above sea level, but endemic malaria disappears above 1,800–2,000 m. What is rarely mentioned is that less than two percent of the African continent (including North Africa) is above 2,000 m, and that much of this is so arid that it offers little opportunity for cultivation. Moreover, the history of malaria in highland areas is a compelling example of the dominant role of human behaviour and human ecology, not climate, as the driving factors in the dynamics of transmission.

Two examples illustrate this point: the Kenya Highlands and the New Guinea Highlands.

# Kenya Highlands

Nairobi, capital of Kenya, was founded in 1899 during the construction of a railway from Mombassa, on the coast, to Lake Victoria. The site was chosen because it was on the last stretch of level ground before the steep descent into the Rift Valley. It was a swampy area, and had always been known as an unhealthy locality "swarming with mosquitoes" \[45\]. Indeed, in 1904, when the town had already grown substantially, a committee of doctors petitioned the colonial administration that the entire municipality be relocated because it was a spawning ground for disease. At that time it marked the upper limits (1,068 m) of malaria transmission in the region, but the disease began to appear at higher altitudes after the clearance of forests for the development of tea estates and the importation of infected labourers \[46\]. The first sizeable epidemic, shortly after World War I, was attributed to the return of local soldiers from Tanzania. A major epidemic in 1926 led to recognition that economic development was a key factor in the proliferation of mosquito breeding sites, and hence the source of the increasingly serious problem:

*That there have been no notable general alterations in the domestic environment of the natives of these reserves during recent years is true, but on the other hand it is to be remembered that in every direction roads, and to a lesser extent, railways, have been carried into and through these areas, and always where there are roads, artificial and un-drained excavations are to be found* \[47\] quoted by Snow *et al* \[48\]

In the following year, the colonial administration and the Municipal Corporation of Nairobi agreed to spend £40,000 (the equivalent of nearly £800,000 (US\$ 1,400,000) in current currency) for eradication of anopheline breeding sites in the Nairobi area. Nevertheless, there were six major epidemics in the city between the two World Wars, with serious rates of transmission extending to the Londiani district (2,250–2,490 m) and even at a farm near Mount Timboroa, at 2,490–2,550 m \[49\].

The fundamental cause of the upward advance of the disease was widespread deforestation and development as the areas were opened up for large farming ventures. As in the quote above, the construction of roads and railways created innumerable flooded "borrow pits" (depressions left by excavation for materials) and also contributed to the dispersal of the mosquito. The introduction of the ox-wagon caused a proliferation of rough cart roads; water in the wheel ruts provided a prolific breeding site for vectors. Mill-dams on rivers interfered with natural drainage \[49\].

These and many other factors were components of a drastic ecological change, and it was this change that brought transmission to the Highlands. The disease continued to be a serious public health problem until the 1950s, when the administration organized an extensive control programme, mainly based on DDT, after which the area was essentially malaria-free until the 1970s.

The tea-growing estates in the Kericho district (1,780–2,225 m) have an extensive medical service for employees and their dependents that was initiated in 1925. Health care at the central hospital of Unilever Kenya Ltd. is extended to some 100,000 inhabitants of the region. However, there is no attempt at mosquito control, and malaria has re-emerged as a serious problem. Epidemics occurred almost every year from 1990 to 1997, with a mean annual attack rate of around 50 percent \[50\]. Peak transmission was from May to July, after the principal rainy season and before mean monthly temperatures drop below 18°C. A questionnaire survey (June 1997) indicated that only 8 percent of patients had travelled to areas with known malaria transmission in the previous 30 days.

The main factor in this recrudescence may be increased resistance to anti-malarial drugs, as well as the unsupervised use of ineffective medications, but the picture is not entirely clear \[51\]. Whatever the cause, the history of multiple epidemics in the earlier part of the century, including many at higher altitudes, makes it unnecessary to infer climate change as a contributory factor. Moreover, a set of well-maintained meteorological records shows no significant change in temperature over recent decades \[52\]. Indeed, in a detailed report to the World Health Organization, a group of malaria specialists based in Nairobi dismissed those who claim a global warming link as "scientific Nostradamuses" \[48\].

# New Guinea Highlands

In the early 1930s, a human population estimated at 1,000,000, previously unknown to outsiders, was contacted in the mountains of New Guinea. It appeared that these so-called "Stone Age people" were malaria-free, and this was attributed to their unique state of isolation. By contrast, the lowland coastal regions were highly malarious.

At first, the highlanders became a new source of labour for the coastal plantations, but after World War II there was a rapid increase in the number labourers from the lowlands employed by landowners growing *Arabica* coffee and other crops in the mountains. In the late 1940s, government scientists warned that the increasing contact between the regions could bring disaster, for epidemic malaria had already appeared in several highland areas at around 1,500 m \[53\]. By the mid-1950s several alarming outbreaks lead to the enforcement of a law that required employers of highlanders working on the coast to supply them with anti-malarials, and to ensure that the medications were actually taken. On repatriation, highlanders were held by the government in compulsory quarantine for two weeks and given curative malaria therapy \[54,55\].

These regulations failed to stop the emergence of the disease and its spread to many isolated valleys. As in Kenya, the increase in prevalence was clearly attributable to a rapid increase of anopheline populations after forest clearance, the influx of people from malarious areas, and the construction of roads, airstrips, plantations, mines, water impoundmentsand other human artifacts. Several vectors were involved, including *Anopheles farauti* and *Anopheles punctulatus* \[56\]. These species breed in open sunlit pools and are essentially the local equivalent of *An. gambiae* and *Anopheles funestus* in sub-Saharan Africa.

Subsequent studies suggest that the parasite may have arrived in the highlands as early as the 1940s but did not become evident until forest clearance and development were more widespread. Whatever the chronology, the history of the disease is clearly attributable to the introduction of the parasite to non-immune populations and the proliferation of its vectors as a result of large-scale ecological change.

# Conclusion

Simplistic reasoning on the future prevalence of malaria is ill-founded; malaria is not limited by climate in most temperate regions, nor in the tropics, and in nearly all cases, "new" malaria at high altitudes is well below the maximum altitudinal limits for transmission. Future changes in climate may alter the prevalence and incidence of the disease, but obsessive emphasis on "global warming" as a dominant parameter is indefensible; the principal determinants are linked to ecological and societal change, politics and economics. There is a critical need for cheap, effective control campaigns, as were implemented during the DDT era. A creative and organized search for new strategies, perhaps based on new technologies, is urgently required, irrespective of future climate change.

# Competing interests

The authors declare that they have no competing interests.

### Acknowledgements

This article has been published as part of *Malaria Journal* Volume 7 Supplement 1, 2008: Towards a research agenda for global malaria elimination. The full contents of the supplement are available online at <http://www.malariajournal.com/supplements/7/S1>


author: David S Pisetsky
date: 2012
institute: 1Duke University Medical Center, 151G, Durham VA Medical Center, Durham, NC 27705, USA
title: The LE cell: crime scene or crime stopper?

I have often said that the lupus erythematosus (LE) cell is the most important cell in all of medicine. This judgment does not reflect the cell's putative function in auto-immunity but rather the insights it has provided into the mechanisms of autoimmunity. Discovered fortuitously by Hargraves and colleagues (see Suggested reading below) at the Mayo Clinic, the test to detect this cell is very simple. A sample of peripheral blood is disrupted, incubated, and then spread onto a slide for inspection, after Wright's staining, under an old-fashioned light microscope. If the patient has lupus, then, *voilà*, the LE cell stands out among the mundane and ordinary red and white cells that populate the blood.

No doubt, my view of the significance of the LE cell is parochial and relates to my identity as a rheumatologist who has spent a career trying to solve the enigmas of anti-DNA antibodies. The competition for the LE cell is also not that stiff. 'Tart' cells are now forgotten and the Reed-Sternberg cell, though valuable for the diagnosis of Hodgkin's disease, has a very confusing phenotype, representing a B cell of some kind. Truly, what has the Reed-Sternberg cell revealed about the basis of malignancy? In contrast, the study of the LE cell has been a bonanza for autoimmunity.

As is now recognized, the LE cell phenomenon involves, in the presence of anti-nuclear antibodies (ANAs) and complement, the phagocytosis of a cell nucleus. In peripheral blood, the cell doing the engulfing is likely a neutrophil. For many years, the focus on the LE cell phenomenon was the ANA component and specificities that can bind nuclear molecules; this focus spawned simpler and more widely used tests such as the fluorescence anti-nuclear antibody (FANA) test. Other elements of the LE cell phenomenon raise salient issues that remain prime topics for investigation: the role of complement in the binding and clearance of nuclear material; the activation of neutrophils by nuclear molecules; and, perhaps most important, the existence of nuclear material outside cells.

Of events key to the creation of an LE cell, the translocation of an intact nucleus from the inside to the outside of a cell is perhaps the most mysterious and, for many years, attracted little investigative interest. Physiological enucleation is thought to occur essentially only in the bone marrow during red cell development and in the eye during the formation of the lens. Otherwise, cells quite reasonably hold onto their nuclei with their treasures of genetic information. As suggested by the apparent supply of nuclei available for the LE cell phenomenon, enucleation of hematopoietic cells perhaps occurs readily in the blood, a feature of a cell death process yet to be described and defined. If such a process exists among blood cells, it differs from apoptosis, in which nuclei condense and fragment for safe disposal in the form of apoptotic bodies and microparticles.

The enucleation central to the LE phenomenon also must differ from the extrusion of nuclear material from neutrophils as neutrophil extracellular traps (NETs). NETs are a unique form of DNA designed to entrap and ensnare bacteria and fungi. The process of the formation of NETs is called NETosis. During NETosis, which can be triggered by a variety of stimuli (including cytokines and Toll-like receptor (TLR) ligands), the nuclear membrane breaks down to allow mixing of DNA with cytoplasmic and granule contents which emerge from cells as strands or mesh. A NET should differ in appearance from the seemingly intact nucleus inside the LE cell. The striking images of NETs by fluorescence microscopy may not be the true story, however. Perhaps, during NETosis, the neutrophil ejects a more intact structure that unravels and disassembles over time once outside the cell to produce the now-classic image of a NET.

Whatever the mechanism for the generation of an LE cell, its close association with lupus, especially the more severe manifestations, has enforced the idea that it represents a pathological process linked to inflammation. Furthermore, the demonstration of LE cells in pleural effusions, for example, places the LE cell directly at the site of pathology. Even if the LE cell is an *in vitro* finding, the evidence for its involvement in pathogenicity, though circumstantial, is compelling. In other words, the LE cell has the tell-tale signs of a crime scene. All that is missing is the yellow tape the police use to keep gawkers away.

I do not watch the *CSI* shows, nor am I an expert in forensics. But I am not ready to declare the case closed and the LE cell guilty as charged. I think that another interpretation is possible, and the LE cell as shown under the microscope signifies something quite different. Let me reconstruct the crime scene in a different way, which I hope will encourage a more general examination of the role of ANAs in lupus. This interpretation stems from exciting studies on the treatment of immune-mediated disease by monoclonal antibodies to nuclear molecules.

As shown now in many studies, the boundary between foreign (non-self) and self in the induction of innate immunity is not that sharp, and many self molecules can acquire immunological activity. This acquisition occurs during the exposure of these molecules to the immune system during cell injury and death, and the death processes often alter their structure by cleavage, denaturation, or post-translational modification. Such self molecules go by a number of terms; 'danger' molecules is the most dramatic and vivid description of their ability to signal that the organism is in dire straits. These molecules can also alert the immune system to trouble, hence the term alarmin. Finally, these molecules, by analogy to PAMPs, or pathogen-associated molecular patterns, can be called DAMPs, or damage-associated (or death-associated) molecular patterns. Indeed, DAMPs and PAMPs can trigger the same set of sensors, such as the TLRs. The interaction of DAMPs and PAMPs with the same receptors indicates that the immune system does not really discriminate self from foreign or non-self but rather recognizes patterns that are structurally similar despite disparate origins.

Among the most prominent and potent DAMPs are nuclear molecules, and HMGB1 (high-mobility group box 1 protein) is at the top of the list. HMGB1 is a nonhistone nuclear protein that can bind DNA and serves as the prototype alarmin. HMGB1 leaves cells during cell death (apoptosis and necrosis), activation (especially of macrophages), and NETosis, and its extracellular activity is determined in part by its redox state. In addition, HMGB1 can partner with other molecules, such as lipopolysaccharide or cytokines like interleukin-1, to create new immunostimulatory moieties. Relevant to this discussion are observations that antibodies to HMGB1 are highly effective therapies in animal models of sepsis and arthritis. Coupled with the observations that HMGB1 is elevated in the blood or tissue during inflammatory disease, these findings have made HMGB1 a compelling target in new treatments.

Other nuclear components with immunological activity are nucleosomes, histones, and DNA, although DNA may need a partner like HMGB1 or LL37 for its immunostimulatory activity. Interestingly, histones can directly inflict tissue injury, and monoclonal anti-histone (H1 and H3) antibodies are effective in blocking models of thrombosis, shock, and liver injury in mice. Together, these considerations suggest that nuclear material can be pathogenic and that ANAs (anti-HMGB1 and anti-histones) can be therapeutic. Thus, the binding of an ANA can prevent disease as well as cause it.

In light of these data, the LE cell can be reconceptualized as a beneficial response, in which the binding of the nucleus facilitates neutralization or removal of a source of DAMPs. This process would be directly analogous to the opsonization and phagocytosis of a bacterium to remove a bundle of PAMPs before they cause harm, such as shock. Another way to look at this is that, if the target antigen in the LE cell were a bacterium instead of nucleus, you would say, 'Way to go, immune system. Get that critter.' Because the target is a nucleus, however, you say, 'Omigod, it's lupus. Get steroids'.

Although this line of thought sounds reasonable (at least to me), one can argue that there is an obvious flaw since the LE cell is associated with a more serious set of disease manifestations. If LE cells were preventing inflammation and damage, the opposite relationship could be expected. Thus, owing to a robust protective response, individuals with high numbers of LE cells (a sign of anti-nuclear defense) should display disease that is less severe, not more severe. I would counter this argument by saying that, in those individuals with active disease with LE cells, the system is working in overdrive but has been overwhelmed with nuclear material. This situation may be exacerbated in lupus by inadequate handling of the burden of dangerous molecules. Clearance defects abound in patients with lupus, providing plausibility for this model. In this scenario, the presence of LE cells at sites of inflammation does not mean that the cells are committing a crime but that they are trying to stop or prevent one.

A further issue in conceptualizing the LE cell relates to the heterogeneity in the serological response of patients with lupus. These responses are highly diverse, targeting a multitude of nuclear antigens. Importantly, many ANAs have never been definitively associated with clinical disease manifestations, and some patients can remain 'serologically positive, clinically negative' for many years, seemingly spared active disease despite abundant ANA production. In such patients, this discordancy suggests that the ANAs, though pathological, are neither pathogenic nor nephritogenic. Mouse transfer models show clearly that, although some anti-DNAs can deposit in the kidney, many cannot despite *bona fide* DNA binding. Clearly, interaction with a nuclear antigen does not necessitate a role in promoting disease, or exclude a role in preventing disease. It is of interest, therefore, that between 15% and 25% of otherwise healthy individuals can express an ANA when blood is assayed by sensitive techniques. A high frequency of ANA expression in normal individuals could signify a propensity to autoreactivity in the general population but could also indicate the ordinary operation of immunoregulatory mechanisms to constrain immune responses to nuclear molecules. Unfortunately, the specificity of the ANAs in normal individuals is usually not known, although the characterization of these ANAs could provide additional insights into the way that the immune system protects against the activity of nuclear molecules. Therefore, it is of great interest that, in a study by Li and colleagues (see Suggested reading below), healthy controls without ANA reactivity showed more evidence of upregulated gene expression by array analysis of peripheral blood cells than healthy controls with ANA positivity; those with ANA positivity, however, showed evidence of upregulation of genes associated with the interferon signature. Therefore, further genetic and genomic studies of patients and controls should be informative in elucidating immune disturbances underlying lupus, especially if studies allow for the possibilities that ANAs can act defensively as well as offensively and that serological disturbances can indicate a response to impending danger and not necessarily its cause.

The remarkable success of anti-HMGB1 antibodies in animal models of disease and the equally impressive but more limited studies on anti-histone antibodies indicate that some ANAs are protective. This protection could result from increased clearance of nuclear antigens or blockade of TLR binding. Since many nuclear molecules with DAMP activity function in the context of a nanostructure or microstructure (for example, microparticles or microvesicles), protective ANAs could indeed pro-mote their phagocystosis after opsonization, effectively reproducing the LE cell phenomenon. This discussion is not just for intellectual amusement but rather to suggest new lines of research into the serology of lupus and other diseases characterized by ANA production. Given the possibility that ANAs can be protective and not just pathogenic (or non-pathogenic), the relationship between serology and clinical disease events may be clarified. Furthermore, the possibility that some ANAs are protective could spur inquiry into new biological therapies based on the paradigm, now so well established with anti-HMGB1 antibodies, that ANAs can block disease by eliminating deleterious nuclear material. If the LE cell phenomenon provided evidence for the prevention of disease and not its induction, then a venerable but clunky assay could have a revival as a screen for agents to stop the immunological crime known as lupus.

# Abbreviations

ANA: anti-nuclear antibody; DAMP: damage-associated (or death-associated) molecular pattern; HMGB1: high-mobility group box 1 protein; LE: lupus erythematosus; NET: neutrophil extracellular trap; PAMP: pathogen-associated molecular pattern; TLR: Toll-like receptor.

# Suggested reading

Andersson U, Tracey KJ: **HMGB1 is a therapeutic target for sterile inflammation and infection**. *Annu Rev Immunol* 2011, **29:**139-162.

Farrell K, Jarrett RF: **The molecular pathogenesis of Hodgkin lymphoma**. *Histopathology* 2011, **58:**15-25.

Griffith TS, Ferguson TA: **Cell death in the maintenance and abrogation of tolerance: the five Ws of dying cells**. *Immunity* 2011, **35:**456-466.

Gulhane S, Gangane N: **Detection of lupus erythematosus cells in pleural effusion: an unusual presentation of systemic lupus erythematosus**. *J Cytol* 2012, **29:**77-79.

Hargraves M, Richmond H, Morton R: **Presentation of two bone marrow elements; the tart**. *Proc Staff Meet Mayo Clin* 1948, **23:**25-28.

Harris HE, Andersson U, Pisetsky DS: **HMGB1: a multifunctional alarmin driving autoimmune and inflammatory disease**. *Nat Rev Rheumatol* 2012, **8:**195-202.

Hebederen historical series: **The LE cell**. *Rheumatology* 2001, **40:**826-827.

Hreggvidsdottir HS, Lundberg AM, Aveberger AC, Klevenvall L, Andersson U, Harris HE: **High mobility group box protein 1 (HMGB1)-partner molecule complexes enhance cytokine production by signaling through the partner molecule receptor**. *Mol Med* 2012, **18:**224-230.

Keerthivasan G, Wickrema A, Crispino JD: **Erythroblast enucleation**. *Stem Cells Intl* 2011, 139851.

Kruse K, Janko C, Urbonaviciute V, Mierke CT, Winkler TH, Voll RE, Schett G, Munoz LE, Hermann M: **Inefficient clearance of dying cells in patients with SLE; anti-dsDNA autoantibodies, MFG-E8, HMGB-1 and other players**. *Apoptosis* 2010, **15:**1098-1113.

Li Q-Z, Zhou J, Lian Y, Zhang B, Branch VK, Carr-Johnson F, Karp DR, Mohan C, Wakeland EK, Olsen NJ: **Interferon signature gene expression is correlated with autoantibody profiles in patients with incomplete lupus syndromes**. *Clin Exp Immunol* 2009, **159:**281-291.

Mortensen ES, Rekvig OP: **Nephritogenic potential of anti-DNA antibodies against necrotic nucleosomes**. *J Am Soc Nephrol* 2009, **20:**696-704.

Nakano T, Goto S, Lai C-Y, Hsu L-W, Takaoka Y, Kawamoto S, Chiang K-C, Shimada Y, Ohmori N, Goto T, Sato S, Ono K, Cheng Y-F, Chen C-L: **Immunological aspects and therapeutic significance of an autoantibody against histone H1 in a rat model of concanavalin A-induced hepatitis**. *Immunology* 2010, **129:**547-555.

Quan-Zhen L, Karp DR, Quan J, Branch VK, Zhou J, Lian Y, Chong BF, Wakeland EK, Olsen NJ: **Risk factors for ANA positivity in healthy persons**. *Arthritis Res Ther* 2011, **13:**1-11.

Pisetsky D: **Cell death in the pathogenesis of immune-mediated diseases: the role of HMGB1 and DAMP-PAMP complexes**. *Swiss Med Wkly* 2011, **141:**w13256.

Park JY, Malik A, Dumoff KL, Gupta PK: **Case report and review of lupus erythematosus cells in cytology fluids**. *Cytopathol* 2007, **35:**806-809.

Ramijsen Q, Kuijpers TW, Wirawan E, Lippens S, vandenabeele P, Vanden-Berghe T: **Dying for a cause: NETosis, mechanisms behind an antimicrobial cell death modality**. *Cell Death Differ* 2011, **18:**581-588.

Xu J, Zhang X, Pelayo R, Monestier M, Ammollo CT, Semeraro F, Taylor FB, Esmon NL, Lupu F, Esmon CT: **Extracellular histones are major mediators of death in sepsis**. *Nature Med* 2009, **15:**1318-1322.

Xu J, Zhang X, Monestier M, Esmon NL, Esmon CT: **Extracellular histones are mediators of death through TLR2 and TLR4 in mouse fatal liver injury**. *J Immunol* 2011, **187:**2626-2631.