Dinosaurs: Warm Blooded or Cold Blooded? Guðmundur Freyr Matthíasson Laufásvegur 42, 101 Reykjavík 09.60.31 Jarðsaga 1. Haust 2003

During the Mesozoic era, dinosaurs reigned supreme on the planet. Many people have wondered exactly how they managed that. As the common thought is that they were cold blooded and therefore had a considerable disatvantace compared to mammals. New researches seem to suggest that this was not so, that dinosaurs were, indeed, warm-blooded. Several arguments have been put forth by both camps in which they try to support their case. This is likely to remain unresolved for the time being, as the evidence put forth so far is, for the most part, inferential.

Introduction: constant value is a very large advantage. This article will try to summarise the Not only does it mean that the animal is various theories and arguments that have not dependent on the environmental been put forth in the dinosaur debate. It temperature but can hunt at any time of the will start with some basic definitions that day (or night), or in any season, but it also are important to the subject. Then it will means operating at maximum efficiency. go into detail about the various arguments All creatures, whether warm- or cold- that have been put forth blooded, use the same basic biochemical processes to produce energy. The chemical Virtually all animals, if examined reactions involved generally have a at the proper time, will appear to be particular optimal temperature. Hence the 'warm-blooded', their internal body sluggishness of cold-blooded animals in temperatures will be about the same. What cold environments or at night. In any long- is more important is the mechanism by term competitive situation, warm-blooded which the body temperature is maintained, animals will always win over their cold- and in this sense the terms endothermic blooded competitors, and this is what the and ectothermic are more appropriate; an history of mammal development ectothermic animal relies on heat from demonstrates. No large cold-blooded the environment to maintain body predators can develop against mammalian temperature, whereas an endothermic competition, instead they remain small and animal relies on heat generated within its occupy ecologically specialized positions own body by metabolic processes, and will where they can hide for most of the time therefore have a higher metabolic rate. and only need to hunt for food Once it was thought that all occasionally when safe to do so. There are, dinosaurs were cold-blooded - now many of course, attendant disadvantages to things suggest that at least some were endothermy, not the least of which is the warm-blooded, which would help to need for very much larger expenditure of explain why they became so plentiful and energy to maintain elevated metabolic dominant for so long. If we examine rates, and a commensurate increase in food today's fauna, we find no large land requirements. predators that are cold-blooded, except for crocodiles that occupy only one very Given that mammals have such an specific ecological niche. The same is true enormous advantage, what are we to make of the entire Cenozoic era - virtually all of the Mesozoic era, when for 140 million large predators were warm-blooded. years dinosaurs reigned supreme and few The reason is not difficult to find. mammals grew larger than a chicken. The position of top predator is a very Mammals and dinosaurs evolved together. competitive one. The ability to control Dinosaur ancestors (thecodonts) and body temperature and maintain it at a mammal ancestors (therapsids) were in direct competition in the late Triassic, with • Growth Rates the therapsids initially appearing to have Warm-blooded creatures grow the upper hand. However, by the end of much faster (5 - 10 times) than the Triassic the thecodonts were on top, cold-blooded ones do, and this is dinosaurs assumed the roles of top reflected most strongly in their predator and large herbivore, and all other bones. Whereas a crocodile under roles down to the very small, which they normal circumstances will only left to the mammals and other reptiles. grow about 30 cm per year, a How did they manage to take over young hadrosaur hatchling would in the first place, and then keep the reach its full size in about 4 years - mammals subservient for so long if a growth rate 5 times faster than a mammals had such a potent evolutionary crocodile. advantage in being warm-blooded? The logical answer, of course, is that dinosaurs • Predator / Prey Ratios had to be warm-blooded as well! For those The large top predators in any who believe that dinosaurs are just large system generally obtain most of reptiles this is an unacceptable view. their food intake from large Given that all we have left are lifeless herbivores. Warm-blooded pre- bones and footprints, is it possible to dators require about 10 times as produce evidence in support of the warm- much as an equal-sized cold blooded dinosaur hypothesis? blooded predator, so that the larger Surprisingly, perhaps, the answer the predator and the higher its is yes, although such evidence must be metabolic rate, the rarer it will be largely inferential . in any ecosystem. For modern examples Some of the evidence has already been such as the lion on the African presented above, in the comparison of savannah game parks, this ratio is dinosaurs with present day ecological about 1% or even less. For structures. Other evidence comes from Permian cold-blooded predators areas such as: such as Dimetrodon, the ratio is much higher, at 20%, equivalent • Bone Structure and Histology - to today's crocodiles and spiders. Hundreds of slices from bones of Prehistoric mammal predators all types of dinosaurs have now such as sabre-toothed tigers have a been examined and all show: ratio of about 3 - 5%. Woven bone, typical of rapid Tyrannosaurus ratios are almost bone growth. Which means dino- exactly the same as for sabre saurs had a high metabolic rate. tooths, and large dinosaur predators average about 3.5%. Vascular canals equivalent to Dinosaurs clearly fall into those of birds - Bone from warm- the same group as the unquestion- blooded creatures is also typified ably warm-blooded prehistoric by a much larger number of mammals and are much lower vascular canals and Haversian than cold-blooded predators of canals formed in rapidly forming today or the Permian period. bone. Cold-blooded animals The studies cited above are not always have far fewer such canals. considered flawless by all experts. Census counts based on income- Generally poor growth rings but plete fossil assemblages may be more obvious in teeth - Which is unrepresentative, and the assump- typical of mammals and birds. In tion that predator density is always all regards they resemble warm- limited by prey density is largely blooded rather than cold-blooded untested. animals

• Speed and Agility efficient system for providing The life style of the smaller, agile oxygen and removing waste gases. dinosaurs also supports the warm- Most dinosaurs also show such air blooded hypothesis. Whereas sacs in the backbone, and may modern, cold-blooded reptiles are well have had body sacs also. 'sit and wait' hunters, predatory It has been suggested (as argued dinosaurs were active in pursuing above), that sustained activity and and attacking their prey. Such obligatory bipedalism, as exhibit- activity requires a high metabolic ted by dinosaurs and birds, re- rate. quires an endothermic meta- All theropods and many bolism. other dinosaur types were bipedal, Some authorities suggest an obligation which requires more that for animals of the size and metabolic energy than a spraw- apparent vigorous life-style of ling, four-legged posture. Some dinosaurs, sufficient heat will be commentators go so far as to say generated by the maintainence of that bipedalism cannot be attained hich activity levels to make the without some form of endothermy. animals effective endotherms, re- Using the footprint calcu- gardless of the presence or lations on prehistoric mammals absence of any specific mechan- we get speeds the same as present isms. day mammals, whereas the cold- blooded reptiles and amphibians • Rate of Evolution of the Cretaceous Period are much The length of time that any one slower (3 - 6 kph). Dinosaurs and species, genus, family etc lasts thecodonts, on the other hand, varies greatly. Cold-blooded appear to have been just as fast as animals are generally less mammals, a conclusion supported susceptible to famine and drought by their fossil skeletons. Their and come under much less limbs were built for speed and evolutionary pressure. Hence their prolonged exercise. average species lifetimes are The ability to be fast and relatively long - crocodiles and agile for an extended period re- turtles have changed little from quires a high metabolic rate and their origins to today, with thus a large heart and efficient average species lifetimes of 30 lungs. Such organs do not, of million years. The late Permian course, fossilize, but most cold-blooded reptiles Dimetrodon dinosaur skeletons have a much (carnivore) and Edaphosaurus wider body space in the chest (herbivore) lasted for 20 million region when compared with cold- years. Cold-blooded families may blooded reptiles and could easily be unchanged for as long as 55 have accomodated large hearts and million years. lungs. The hadrosaurs and horned In contrast, warm-blooded dinosaurs do not show such en- animals are aggressive competi- largement, but neither do some tors that reproduce rapidly and birds - they compensate by having diversify to occupy as many a series of air sacs throughout the ecological niches as possible. The vertebrae and in body spaces that increased evolutionary pressure are connected to the lungs. Air thus applied leads to much faster flows continually in one direction turnover times. Mammals may instead of breathing in and out, generate 5 or 6 new genera every and the blood flows in the 10 million years, and families may opposite direction at gas exchange only survive for 25 million years areas, leading to an extremely (half as long as cold-blooded animals). Dinosaur species changed as rapidly as every 5 -6 spent considerable period of time million years (horned dinosaurs), in the nest after hatching (badly and families lasted for the same fragmented eggshells indicative of length of time as mammal repeated trampling, juveniles of families. different ages within the nest, As well as shortened ex- poorly developed limb bone joints istence times, warm-blooded ani- indicating limited locomotion of mals also produce more species the young) to support the theory per genus and more genera per that the parent hadrosaurs were family than cold-blooded creatur- involved in foraging to provide es. Dinosaurs averaged 3 -4 spec- food for their rapidly growing, ies per genus and 12 genera per nest-bound young, and that this family, as did mammals. The behaviour was strong evidence hadrosaurs and horned dinosaurs against them being cold-blooded. produced 7 and 5/6 new genera respectively over 10 million years, • Bone Isotopes comparable to mammals and in The oxygen isotope composition contrast to, for example, the giant of the phosphate part of vertebrate turtles, that have produced only 1 bone is related to ingested water new genus over the past 5 million and to the body temperature at years. which the bone forms. It is in equilibrium with the individual's • Similarities with Birds body water, which in turn is in a There are many similarities uniform state throughout the body. between birds and mammals (erect Therefore, by measuring the posture, efficient hearts, variation in the phosphate oxygen intelligence, bone-structure, food isotope composition the average requirements per size, feathers, temperature variation both within and caring for their young). a bone and between bones can be It is now agreed that birds are calculated for fossil bones. probably direct descendants of The results of such an dinosaurs, and may have evolved experiment on well-preserved from them as early as the Jurassic Tyrannosaurus bone suggest that period. As birds are undoubtedly its body temperature was warm-blooded it is not unreason- maintained within 4 degrees C. able to suppose that their ancestors Such a degree of temperature were either already warm-blooded uniformity is consistent with a also, or developed the capability relatively high metabolic rate over the next 100 million years. similar to known warm-blooded animals. • Parental Care In 1991 it was suggested that the • Insulation question of dinosaur temperature The question of insulation arises regulation might be addressed with regards to body temperature from the point of view of control because of the perceived parenting behaviour. This argu- problems insulation would cause ment starts from the premise that for an animal whose temperature post natal care of the young, such control depends on the external as foraging for extra food, is an environment. It would seem energy expenditure that cold- counter-productive for a cold- blooded animals simply cannot blooded animal to be insulated, afford. and there appear to be no extant Using the hadrosaur examples of such a situation, nesting grounds as an example, we whereas all known warm-blooded cite the evidence that the young animals have some form of insulation. Thus the discovery of ians and theropods. If these were an insulated dinosaur would also ectotherms, the question provide strong evidence for their becomes “how did they manage to status as warm-blooded animals. survive so well when other The problem with such a ectotherms did not?” discovery is the selectivity of the One postulated explanat- fossilization process, which tends ion is that they migrated during not to leave evidence of skin and the coldest times, but the presence associated features such as hair or of neonates and many juveniles feathers, even when they were suggests that this did not happen, present. There is therefore and indeed, migration is not enormous interest whenever such generally considered a viable a discovery is a possibility. option for ectotherms due to The discovery of the metabolic restrictions on the long- Chinese Sinosauropteryx in 1997 term activity levels needed for again raised the possibility. The migration. A more likely ex- initial reports were quite definite planation for the presence of that this animal, an undoubted dinosaurs in this environment is theropod dinosaur, had feathers, that they were endotherms and and caused considerable excite- thus able to cope better with the ment. Subsequent investigations extremes of temperature, possibly by a number of Chinese and by hibernating, or by some Western experts have not been adaptation against freezing. quite so definite. Some detractors have gone so far as to suggest that All these lines of evidence and the hair-like structures have comparisons between dinosaurs, mammals nothing at all to do with hair or and cold-blooded reptiles support the idea feathers, while others consider the that dinosaurs were warm-blooded, or at hair-like, apparently hollow very least had a much higher metabolic structures as good candidates for rate than conventional cold-blooded ‘proto-feathers’. Further exami- creatures. Not surprisingly, some counter nations, including chemical arguments have been presented, such as: analysis, are underway and a definitive answer may still be • Gigantothermy some way off. One argument against warm- blooded dinosaurs suggests that • Arctic Faunas large dinosaurs such as the sauro- Ectotherms, by prediction and by pods would not need to be warm- observation of extant examples, do blooded, as their size would pre- not do well in extreme cold. Late vent temperature fluctuations. Cretaceous faunas are now known Heat production is related from a number of locations in to body mass, while heat loss is Alaska and other Arctic areas that related to body area. As an animal were laid down when mean annual gets larger, its body area decreases temperatures ranged between 2 relative to its body mass, so heat and 8 degrees C. These deposits loss decreases and it becomes are notable for the almost com- more efficient at maintaining body plete absence of ectotherms such temperature. This theory, termed as the crocodyliform champso- 'gigantothermy', together with the saurs that are normally abundant possible aid of plates, spikes, frills in comparable North American or nasal cavities used as heat faunas from warmer climates. exchangers, proposes that large However, these deposits dinosaurs living for the most part contain large numbers of in a warm environment could in dinosaurs - hadrosaurs, ceratop- fact have found the body tempera- ture of a fully warm-blooded reduced and they would have animal thermally stressful and required much less food on a disadvantageous. weight basis than smaller animals, There are a number of just as an elephant only requires counter-arguments. Although gi- about one twentieth as much gantothermy provides for in- relative food as a rabbit. creased efficiency of temperature regulation, it is still far less • Lung Structure efficient than true warm-blood- The debate about dinosaur lung edness (6 - 8 degrees variation structure and its application to the instead of 1 -2 degrees), and question of thermoregulation warm-blooded animals would still stems from an analysis of be expected to win easily in any Sinosauropteryx. evolutionary competition. It begins with a detailed Gigantothermy also does morphological comparison of lung not address the problem that all structure in reptiles and birds, dinosaurs arose initially from suggesting that the simpler, relatively small ancestors, cer- bellows-like reptilian/crocodilian tainly too small for gigantothermy lung is probably incapable of to have any impact. If these small supporting the increased metabolic ancestors had developed warm- rates necessary for active bloodedness and thus competed endotherms. While birds have a effectively with the mammals for similar septate lung, modifications domination, why would evolution leading to a series of connected air produce giant descendants that had sacs throughout the thorax and lost the ability? abdomen and a uni-directional airflow increase oxygen transport • Rate of food supply efficiency sufficiently to allow a Another argument was raised very high metabolic rate. It also against warm-blooded dinosaurs maintains that the processes by and applied particularly to the which the lungs are powered differ large herbivorous sauropods significantly in reptiles and birds. centres around the ability of the It then turns its attention animal to provide sufficient food to dinosaurs, noting that although to provide the extra energy these probably had the same required to maintain body septate lungs as reptiles and birds, temperature. they did not have the necessary This argument tends to skeletal and muscular mechanisms rely on observations of the teeth necessary to provide a bird-like, and jaw. In some dinosaurs eg high efficiency air circulation and diplodocids, they had small heads oxygen-extraction system. Further and no grinding teeth, and so (it is arguements on the shape and claimed), could not possibly function of the pelvis of primitive consume enough food. Some birds such as Archaeopteryx and modern large flightless birds have dinosaurs leads it to conclude not exactly the same problems, but are only that with reptilian style lungs unquestionably not only alive but dinosaurs could not have been also warm-blooded. They cope by endotherms, but also that it makes modifications to food processing it much less likely that birds are after it has been swallowed. descended from dinosaurs. Another fact to be con- This analysis has come sidered is that the enormous size under considerable criticism, as of the sauropods gave them a very yet unpublished, by various small body area to body mass experts. In particular they point ratio. Heat loss was thus greatly out that the crocodilian system requires a mobile pelvis, and that physiology, nor use the same strategies the pelves of theropods are simply and mechanisms to reach a particular not able to substitute. physiological state.

Summary Hemildaskrá The question is yet to be decided, but on http://www.isgs.uiuc.edu/dinos/de_4/5c51 balance the likely outcome seems to be d90.htm heavily weighted in favour of at least Were dinosaurs warm blooded? partially warm-blooded dinosaurs. The (Skoðað: 1.12.2003) tests for endothermy have also been applied to the mammal ancestors (the R. Bakker 1972: Anatomical and therapsids - cynodonts, dicynodonts) and Ecological Evidence of Endothermy in thecodont dinosaur ancestors. In both Dinosaurs. NATURE 238; 636 - 658 cases the results suggest that they were more like warm-blooded than cold- G. Paul 1991: The Many Myths, Some Old, blooded animals, and that endothermy Some New, of Dinosaurology.MODERN may have developed in both lines at an GEOLOGY 16; 69 - 99 early period (late Permian?). In this scenario, dinosaurs would simply be one M.Brett-Surman and J.Farlow 1997: group in a line of succession of warm- Dinosaurian Physiology: The Case for blooded animals. ‘Intermediate” Dinosaurs. THE What does seem clear is that COMPLETE DINOSAUR 449 – 473. dinosaur physiology was different, and may even have been different in different types of dinosaurs. We can never know the answer by direct measurement, and as long as we are forced to rely on analogy we will probably never reach a concensus on this controversial topic. Endothermy is not an either/or proposition, as the large number of potential physiological mechanisms involved makes a wide range of alternatives possible. The large size range of dinosaurs alone probably means that they did not all share a common