The Evidence for
Endothermy in Dinosaurs
Dr. Bakker and others have presented numerous lines of evidence for dinosaurian endothermy — some reasonable, some not so reasonable. We shall list some of these and objectively mention some problems with each.
Top ten hypotheses
Given: Studies on modern animals seem to show that metabolic rate is proportional to maximum rate of locomotion. Hypothesis: Dinosaurs were built to move fairly quickly, so they must have had high metabolic rates. Problem: We do not know how fast dinosaurs really moved. See our dinosaur speeds page to see why.
Given: Ecological studies suggest that today's endotherms (mammals and birds) generally outcompete ectotherms in their area. Hypothesis: Dinosaurs evolved alongside mammals, so they must have been endotherms in order to compete with mammals adequately for the 170 million years that they coexisted. Problem: We don't know that the "Given" always applies; there have been few conclusive studies of endotherm-ectotherm competition. So the "Given" has not been conclusively established yet. Also, just because certain ecological patterns are evident today does not mean that they must have always applied.
Given: Dinosaurs and modern endotherms both had/have erect posture (weight-bearing limbs oriented directly below the body rather than sprawling out to the sides). Hypothesis: Dinosaurs must have been endotherms; an erect posture must be indicative of endothermy.Problem: Erect posture has not been shown to be necessary for endothermy, or vice versa. The two factors seem to be correlated, but cannot be assumed to always occur together. Some endotherms have a more sprawling posture (such as many small mammals); the earliest mammal relatives (the basal synapsids) certainly did not have an erect posture. Similarly, some modern ectotherms, chameleons for example, have an erect posture.
Given: Studies indicate that large brain size is correlated with endothermy. Hypothesis: Theropod and ornithopod dinosaurs had larger than normal brains (for "reptiles"), so they must have been endothermic. Problem: Large brain size may be correlated with endothermy, but it is correlated with many other features not necessarily related to endothermy. Not an empirically supported "if...then" relationship. Also, the data on dinosaur brain size is scant at best; being measured from the few complete skulls available. Many dinosaurs had quite typical reptile-sized brains.
Given: Ectotherms are not normally found at high latitudes (it's cold). Hypothesis: Dinosaur fossils are known from high latitudes, so they must have been endothermic to cope with the cold. Problem: The high latitudes (Alaska, Antarctica, etc.) that dinosaur fossils have been found at were not so cold in the Mesozoic; in fact they may have been almost sub-tropical. Even if they were somewhat cold, dinosaurs could have migrated there in the summer when it was warmer, and emmigrated during the cooler winter. Also, the climate in the Mesozoic was generally warmer and milder than today (we're just recovering from the last ice age, remember). Finally, today's snakes, lizards, turtles, the tuatara, and crocodiles do enjoy a surprisingly broad latitudinal distribution. However, UCMP field work on the North Slope of Alaska has uncovered the remains of many dinosaurs. During the Late Cretaceous when these dinosaurs were alive, the area was seasonally very cold; within the Arctic Circle. Remains of juvenile and hatchling dinosaurs suggest that these dinosaurs were not emmigrating. More work on this interesting case is needed.
Given: Ecological studies have shown that mammalian predator/prey ratios are lower than ectotherm predator/prey ratios (less mammalian predators can live off of a certain number of prey than ectothermic predators can). Hypothesis: Dinosaur predator/prey ratios were low, so they must have been endotherms. Problem: Many problems. The ecological studies taken as a "given" are not flawless, and no studies have been done with bird predator/prey studies (remember birds are dinosaurs!). Most importantly, dinosaur fossils are not the best source to get predator/prey ratios — many sampling errors are inevitable, since our knowledge of the fossil record is far from complete, and many animals were never fossilized. Theropods (the predators) are usually rare, but they have also been found in huge accumulations, and their thin bones probably didn't preserve as well as the herbivores' bones. The fact is, we don't have good evidence yet what the dinosaur predator/prey ratios were like.
Given: Some large dinosaurs had erect posture and a vertical distance between the heart and head to require a high blood pressure, like the giraffe. This had to have required a four-chambered heart to separate the high pressure blood going to the body from the low pressure blood going to the lungs. Hypothesis: A four-chambered heart is a characteristic of endotherms which require a high blood flow rate and a high blood pressure to the body. Having evolved endothermy and such a heart, the dinosaurs could evolve to larger size and upright posture. Problems: This hypothesis has never been refuted, except for the appearance of four-chambered hearts in ecothermic crocodiles.
Given: Dinosaurs were the ancestors of birds. Hypothesis: Since birds are endotherms, dinosaurs must have been endotherms. Problem: Yes, certainly true sometime during the evolution of birds. The problem is, where? Only one dinosaur was the ancestor to all birds, and we don't know which one that was for sure (although Archaeopteryx is assumed to be closely related). If the change to endothermy occurred just before the origin of birds, then no other dinosaurs would be endothermic. If it occurred earlier, where's the evidence? Where do we draw the line between ectothermic dinosaurs and endothermic ancestors to birds? A messy issue, but definitely some of the most compelling evidence for possibly endothermic non-avian dinosaurs.
Given: Endothermic animals grow faster than ectotherms. Hypothesis: Dinosaurs got really big, so they must have been endotherms to reach that size quickly. Problem: If dinosaurs had a long lifespan and were ectothermic, they would have had no problem reaching their huge sizes. But we can't really tell how long dinosaurs lived. We don't know how fast they grew, either, but some studies do show that they could grow quite fast. However, other studies have shown that typical ectothermic reptiles can grow quickly as well.
Given: Dinosaur bone is more similar to mammalian or avian (bird) bone in cross section than it is to typical ectothermic "reptilian" bone. Hypothesis: This bone structure similarity shows that dinosaurs were endothermic. Problem: Some of the best evidence for endothermy yet, but still has its faults. Bone structure does vary a lot throughout the life of an individual, at different sizes, and under different conditions. Bone that is similar in structure to bone of an endotherm has not been established to always be bone of an endotherm. Several researchers associated with the UCMP have been working on dinosaur bone histology (the thin-sectioning of bone to see its structure). The results are inconclusive so far, but have shown that many dinosaurs had rapid growth rates on a par with those of modern endotherms.
Return to the Introduction
Evidence for ectothermic dinosaurs
Summary of current hypotheses
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