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.