Dinosaurs were likely warm-blooded, according to recent research that challenges the long-held belief that they were cold-blooded reptiles. Studies suggest that many, if not most, dinosaurs had high metabolic rates similar to modern birds, allowing them to dominate their ecosystems for millions of years.

Dinosaur wearing a scarf

This warm-bloodedness would have given them significant advantages, such as greater mobility and the ability to thrive in various climates, contributing to their success as the ruling terrestrial animals of the Mesozoic era.

Understanding Thermoregulation

The terms ‘warm-blooded’ and ‘cold-blooded’ are used for simplicity throughout this essay. In fact, these terms are something of an over-simplification. Virtually all animals, if examined at the proper time, will appear to be ‘warm-blooded’—i.e., their internal body temperatures will be about the same.

What’s more important is the mechanism by which body temperature is maintained (thermoregulation). The terms endothermic and ectothermic are relevant here. An ectothermic animal relies on heat from the outside i.e. the environment, to maintain body temperature, whereas an endothermic animal relies on heat generated within its own body by metabolic processes, and will therefore have a higher metabolic rate. There are even more precise technical terms for processes intermediate between full endothermy and full ectothermy which will not be discussed here.

Shift in Thinking About Dinosaurs

Once it was thought that all dinosaurs were cold-blooded. However, recent studies, including a significant one published in 2022, suggest that most dinosaurs were, in fact, warm-blooded. This idea helps explain why dinosaurs became so plentiful and dominant for so long. If we examine today’s fauna, we find no large land predators that are cold-blooded, except for crocodiles, which occupy a specific ecological niche and are primarily water dwellers. The same is true of the entire Cenozoic era. Virtually all large predators were warm-blooded.

Advantage of Warm-Bloodedness

The reason is not difficult to find. The position of top predator is highly competitive. The ability to control body temperature and maintain it at a constant value (i.e., warm-blooded) is a large advantage. It allows the animal to hunt at any time of the day (or night) or in any season, and to operate at maximum efficiency.

All creatures, whether warm- or cold-blooded, use the same basic biochemical processes to produce energy, with the same enzymes and substrates. The chemical reactions involved generally have a particular optimal temperature. For every drop of 10 degrees C, the process will be twice as slow, explaining the sluggishness of cold-blooded animals in cold environments or at night.

Evolutionary theory suggests that, in any long-term competitive situation, warm-blooded animals will generally outcompete their cold-blooded rivals, as history shows with mammalian development. No large cold-blooded predators developed against mammalian competition, remaining small and occupying specialized ecological positions.

Drawbacks of Endothermy

There are, of course, disadvantages to endothermy, particularly the need for much larger energy expenditure to maintain elevated metabolic rates and, consequently, increased food requirements.

Mesozoic Puzzle

Given that mammals have such an enormous advantage, what are we to make of the Mesozoic era when, for 140 million years, dinosaurs reigned supreme and few mammals grew larger than a chicken? Mammals and dinosaurs evolved together. Dinosaur ancestors (thecodonts, particularly ornithosuchians) and mammal ancestors (therapsids, particularly cynodonts) were in direct competition in the late Triassic, with the therapsids initially appearing to have the upper hand. However, by the end of the Triassic, the thecodonts were on top, dinosaurs assumed the roles of top predator and large herbivore, and occupied all other roles down to the very small, which they left to the mammals and other reptiles.

How did they manage to take over in the first place and then keep mammals subservient for so long if mammals had such a potent evolutionary advantage in being warm-blooded? The logical answer is that dinosaurs must have been warm-blooded as well, a view now supported by recent research.

Evidence Supporting Warm-Blooded Dinosaurs

Given that all we have left are lifeless bones and footprints, is it possible to produce evidence supporting the warm-blooded dinosaur hypothesis? Surprisingly, perhaps, the answer is yes, although such evidence must be largely inferential.

Some of the evidence has already been presented above, in the comparison of dinosaurs with present-day ecological structures. Other evidence comes from areas such as:

  • Bone structure and histology
  • Growth rates
  • Predator/prey ratios
  • Speed and agility
  • Rate of evolution
  • Similarities with birds
  • Parental care
  • Bone isotope composition
  • Insulation
  • Arctic faunas

All these lines of evidence and comparisons between dinosaurs, mammals, and cold-blooded reptiles support the idea that dinosaurs were warm-blooded or at least had a much higher metabolic rate than conventional cold-blooded creatures. Recent studies further suggest that warm-bloodedness may have been common among most dinosaurs, challenging the older notion that only birds among the dinosaurs were endothermic. However, some counterarguments have been presented, such as:

Debate Continues

The question is still debated, but the balance of evidence now heavily favors the idea that most dinosaurs were at least partially warm-blooded. Research indicates that some dinosaurs, like the groups containing Triceratops and Stegosaurus, may have evolved from warm-blooded ancestors but later lost that ability. In this scenario, dinosaurs would simply be one group in a line of succession of warm-blooded animals.

What does seem clear is that dinosaur physiology was complex and varied among different species. We can never know the answer by direct measurement, and as long as we rely on analogy, we may never reach a complete consensus on this topic.

Endothermy is not an either/or proposition, as the large number of potential physiological mechanisms involved make a wide range of alternatives possible. The large size range of dinosaurs alone probably means that they did not all share a common physiology, nor use the same strategies and mechanisms to reach a particular physiological state.