Pop quiz, hotshot: you’re walking through the woods. It’s a chilly day autumn day. Birds are chirping and a light breeze licks the treetops. You hear a rustling ahead to the right, but before you can even wonder what it is a tyrannosaurs bursts through the brush and stares you down from 30 metres away. What do you do? What do you do?

To answer this question you first need to contemplate a feature of dinosaur anatomy, which has been puzzling paleobiologists for as long as there have been paleobiologists: were dinosaurs warm- or cold-blooded?

According to a paper published in the journal Public Library of Science in November, there is strong evidence to support the theory that bipedal dinosaurs, weighing more than 20 kg, were probably warm-blooded.

Warm-blooded versus cold-blooded

The term “cold-blooded” is a bit of a misnomer in that it suggests that animals, such as reptiles, amphibians and fish, actually have cold blood. This is not the case. To reflect this, scientists often refer to cold-blooded animals as ectothermic, and warm-blooded animals as endothermic, or “outside-heated” and “inside-heated” respectively.

These terms reflect the primary difference between ecto- and endothermic animals, in that ectothermic animals rely on interactions with their environment to thermoregulate (maintain their body temperature), while endothermic animals — such as birds and mammals — thermoregulate independent of their environment, through the heat generated via their cellular metabolism. Both systems have their advantages and disadvantages.

Endothermic animals can live and remain active in a larger range of climates. This is demonstrated by the fact that endothermic animals can be found across our planet, from north to south poles. Endothermic animals are also able to maintain a higher level of activity than their ectothermic cousins. However endothermic animals pay the price for this form of metabolism by having to consume more food more often.

Ectothermic animals are more restricted by their environments, forced to either inhabit warmer climates, or hibernate when temperatures become too cold for them to function. They also have much slower metabolisms, meaning they don’t have to eat as often as endotherms, typically growing slower and living longer. But their slow metabolism also means that ectotherms are generally incapable of maintaining a high level of activity for a long time, limiting them to short bursts of speed.

Were dinosaurs endo or ectotherms?

Until the 1960s, there was little doubt that dinosaurs were — like their reptile cousins — ectothermic, relying on the sun’s rays and the environmental temperature to maintain their body heat. This theory was supported by research into the biology of modern large reptiles, such as crocodiles, komodo dragons and Galapagos turtles. The mass of these large reptiles allows them to maintain their body temperature through internal homeothermy, slowing the rate at which they cool thanks to a relatively small skin-surface area-to-volume ratio, when compared to smaller reptiles.

However, as our understanding of dinosaur ecology and physiological similarity to birds has developed, researchers started to question their ectothermic assumptions.

Without being able to dissect a living dinosaur, let alone take its temperature, modern scientists have had to rely on more indirect methods for answering the metabolism debate. This led to the teaming up of Herman Pontzer, an anthropologist who studies how our early human ancestors moved, and John Hutchinson, an expert on dinosaur locomotion. In an interview with Sciencedaily.com, Pontzer says that when he realized that his research — which looks at the energy costs of running and walking — could be applied to dinosaurs, he immediately contacted Hutchinson and suggested that they collaborate.

The pair developed two methods for determining the amount of energy a bipedal dinosaur would expend through a variety of movements, such as walking and running.

The first measured the distance from the ground to the animal’s hip joint, also known as “hip height.” From this measurement Pontzer and Hutchinson could make educated guesses on how the animal moved and how much energy that movement expended based on comparisons with modern animals.

The second method measured the volume of muscle activated while walking and running. From this number, the pair could make an estimate about how much energy would be consumed by the muscle. By comparing this theoretical rate of energy consumption in bipedal dinosaurs to what we know about modern ecto- and endothermic animals, the pair concluded that, in all but the smallest bipedal dinosaurs (those under 20 kg), even the simple act of walking would have been next to impossible for a cold-blooded dinosaur. Furthermore, a slow run would have exceeded an ectothermic dinosaur’s ability to expend energy by 100-500 per cent.

Couple that with the knowledge from the fossil record that larger bipedal dinosaurs, unlike modern reptiles, grew relatively quickly, and it is starting to look less and less likely that dinosaurs were exclusively ectothermic, if ectothermic at all.

Pontzer and Hutchinson do point out, though, that smaller dinosaurs fell below the energy use threshold for an ectothermic animal, but only just. Furthermore, since their study only looked at bipedal animals, due to the difficulty of adapting their methods to four legged animals, they could not extend their conclusions to all dinosaur species.

So, back to your tyrannosaurus issue. If you believe that he’s cold-blooded, according to modern thinking, he’s probably barely able to walk, let alone run, so don’t worry. If, however, this article has swayed your opinion, you better tear like heck out of there, because an endothermic T-rex is a fast T-rex, and he needs to eat a lot to maintain that body temperature of his!