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
More on this topic:
Return to the Introduction
Evidence for ectothermic dinosaurs
Summary of current hypotheses
Back to DinoBuzz
Learn more about the Dinosauria