Dinosaur Body Temperatures: the Occurrence of Endothermy and Ectothermy
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Paleobiology, 29(1), 2003, pp. 105±122 Dinosaur body temperatures: the occurrence of endothermy and ectothermy Frank Seebacher Abstract.ÐDespite numerous studies, the thermal physiology of dinosaurs remains unresolved. Thus, perhaps the commonly asked question whether dinosaurs were ectotherms or endotherms is inappropriate, and it is more constructive to ask which dinosaurs were likely to have been en- dothermic and which ones ectothermic. Field data from crocodiles over a large size range show that body temperature ¯uctuations decrease with increasing body mass, and that average daily body temperatures increase with increasing mass. A biophysical model, the biological relevance of which was tested against ®eld data, was used to predict body temperatures of dinosaurs. However, rather than predicting thermal relations of a hypothetical dinosaur, the model considered correct paleogeographical distribution and climate to predict the thermal relations of a large number of dinosaurs known from the fossil record (.700). Many dinosaurs could have had ``high'' ($308C) and stable (daily amplitude #28C) body temperatures without metabolic heat production even in winter, so it is unlikely that selection pressure would have favored the evolution of elevated resting metabolic rates in those species. Recent evidence of ontogenetic growth rates indicates that even the juveniles of large species (3000±4000 kg) could have had biologically functional body temper- ature ranges during early development. Smaller dinosaurs (,100 kg) at mid to high latitudes (.458) could not have had high and stable body temperatures without metabolic heat production. How- ever, elevated metabolic rates were unlikely to have provided selective advantage in the absence of some form of insulation, so probably insulation was present before endothermy evolved, or else it coevolved with elevated metabolic rates. Superimposing these ®ndings onto a phylogeny of the Dinosauria suggests that endothermy most likely evolved among the Coelurosauria and, to a lesser extent, among the Hypsilophodontidae, but not among the Stegosauridae, Nodosauridae, Anky- losauridae, Hadrosauridae, Ceratopsidae, Prosauropoda, and Sauropoda. Frank Seebacher. School of Biological Sciences, A08, The University of Sydney, New South Wales 2006, Australia. E-mail: [email protected] Accepted: 20 June 2002 Introduction species (Paladino et al. 1990, 1997) to learn about dinosaur body temperatures (T ). Heat The thermal physiology of dinosaurs has b transfer relations between animals and their been the subject of much discussion for de- environment are extremely complex, and the cades (Colbert et al. 1946; Bakker 1972; Farlow exact value of some parameters, such as con- 1990; Seebacher et al. 1999; O'Connor and Dod- son 1999). Much of the debate has focused on vection coef®cients, can only be determined the question of whether dinosaurs were endo- empirically, as their complexity precludes the- therms or ectotherms. This, however, is not the oretical derivation (Mitchell 1976; Incropera appropriate question to ask. Given the enor- and DeWitt 1996). Hence, biophysical model- mous diversity of the Dinosauria, both in space ing of Tb in relation to environmental condi- and time, all dinosaurs are unlikely to have tions requires a number of simplifying as- possessed the same physiological make-up. sumptions and approximations (O'Connor Hence, it is more constructive to ask which di- and Spotila 1992). Predictions from theoretical nosaurs would have gained a selective advan- models, therefore, are informative only if their tage from elevated metabolic rate and its atten- underlying assumptions are valid in the bio- dant heat production, and which dinosaurs logical context under consideration, and the would have bene®ted more from ectothermy? only way to test the validity of theoretical Past research on dinosaur thermal physiol- models is to compare their predictions against ogy has used biophysical modeling (Spotila et empirical data. A major limitation of most bio- al. 1973; Dunham et al. 1989; O'Connor and physical studies of dinosaur Tb is that the rel- Dodson 1999) and comparisons with extant evance of their predictions has not been tested, q 2003 The Paleontological Society. All rights reserved. 0094-8373/03/2901-0016/$1.00 106 FRANK SEEBACHER so that the results must be treated as specu- edge gained from other animals, such as tur- lative. tles and elephants, will be the best approach Comparisons with extant species are poten- to advance knowledge in this ®eld. tially useful, but the value of such compari- It was the aim of this study to explore body sons depends to a large extent on the choice of temperatures (Tb) of a large sample of dino- the model species, because no modern ani- saurs represented in the fossil record under the mals are quite like dinosaurs. Elephants, conditions prevalent at the time and place which have been considered an endothermic where they lived, in order to identify those model for dinosaurs (Paladino et al. 1997), can groups in which selection pressures favoring reach body mass comparable to medium- endothermy would have been greatest or least. sized dinosaurs, so studying elephants can re- The study uses crocodiles as a ``null-model,'' veal some of the challenges that large terres- and crocodiles are assumed to represent the trial animals must have faced. However, ele- simplest physiological and behavioral con®g- phants do not represent ``typical'' endo- uration possible for dinosaurs. The hypothesis therms. Their great mass places elephants at tested here is that as crocodile-like ectotherms one extreme of endothermic radiation, and that thermoregulated behaviorally by moving their morphological and behavioral features between heating and cooling environments that have evolved in order to dissipate heat (Seebacher and Grigg 1997; Grigg et al. 1998; (Wright 1984; Williams 1990) bear testimony Seebacher 1999) dinosaurs could have had high that endothermy in large terrestrial animals is and stable Tb (see below for de®nitions). Test- the exception rather than the rule. Because ing this hypothesis will reveal which dino- they reveal that it was of selective advantage saurs, living at a particular place and time, may for large endotherms to evolve heat-dissipat- have had high and stable Tb with an ectother- ing mechanisms, elephants are interesting, mic physiological make-up, so that selection but it would be illogical to use an endothermic pressures favoring endothermy would have exception to establish a rule for dinosaurs. been comparatively weak in those species, and, Similarly, marine turtles (Paladino et al. vice versa, which dinosaurs were unlikely to 1990) are atypical reptiles because they are have been simple ectotherms. wholly aquatic and the selection pressures Endothermy may not have evolved as a re- that acted to produce modern turtle species sult of selection pressures acting directly on must have been quite different from those that thermoregulatory ability. Rather, it has been acted on terrestrial species. Nonetheless, the suggested that selection pressures favored fact that leatherback turtles are able to main- high levels of activity (Bennett and Ruben tain Tb elevated above water temperatures (Pa- 1979; Hayes and Garland 1995) or increased ladino et al. 1990) is of interest and bears tes- capability for parental care (Farmer 2000; Ko- timony to the diversity of thermal relations teja 2000). In either scenario, high maximal among reptiles. aerobic metabolic rates would have been se- Several authors have used crocodilians as lected for and the increase in resting metabolic models to learn about dinosaur thermal rela- rate, which characterizes endothermy, would tions (Colbert et al. 1946; Spotila et al. 1973; have been a correlated response. However, Seebacher et al. 1999). Crocodiles are probably aerobic metabolic rates are dependent on the the most appropriate models for dinosaurs be- thermal sensitivity of underlying biochemical cause they are the last living non-avian archo- processes (Crawford et al. 1999; St. Pierre et al. saurs and are likely to be similar to the type 1998), so thermoregulation and high aerobic of basal archosaur from which dinosaurs metabolic rates cannot be viewed in isolation. evolved (Parrish 1997). In addition, crocodil- Moreover, animals that are ``warm'' without ians are the largest extant reptiles with an possessing metabolic rates typical of modern enormous ontogenetic size range. However, endotherms could also maximize aerobic met- this is not to say that crocodilians are the only abolic rates, because their Tb ranges would be useful model. Crocodilians may be the single optimal for related biochemical processes. most useful model, but a synthesis of knowl- Hence, it would be wrong to view endothermy DINOSAUR BODY TEMPERATURES 107 as ``superior'' to ectothermy, because both Materials and Methods have their advantages and disadvantages Crocodile Field Data. Field data and sam- (Pough 1980); in addition, selection pressures pling methods were reported previously (See- favoring endothermy would be relatively bacher and Grigg 1997; Grigg et al. 1998). Brief- weak in animals that could be ``warm'' with- ly, 11 free-ranging crocodiles (Crocodylus po- out incurring the high energetic cost of ele- rosus; 32±1010 kg) were studied in both winter vated, endothermic metabolic rates. and summer under seminatural conditions at In many respects endothermy and ectother- Edward River Crocodile Farm on Cape York my differ in degree