Cold-blooded animals–such as fish, amphibians and reptiles–are ectothermic, which means they take on the temperature of their surroundings. This means when the external temperature is cool, they tend to be sluggish. Warm-blooded animals like mammals and birds are endothermic; they can generate heat within their bodies and maintain core body temperature regardless of their surroundings. These traits enable faster movement and increased brainpower. Cold-blooded animals do not have these advantages, but they are more economical and can survive longer on less food.
When scientists first started studying dinosaur remains, they assumed the prehistoric creatures were slow-moving and sluggish, like the lizards they resembled; they called them “tail-draggers.” Later, scientific opinion shifted in the other direction, arguing that smaller, faster predatory dinosaurs must have been warm-blooded in order to move quickly enough to hunt their prey. After the question of why dinosaurs went extinct 65 million years ago and how they are related to modern birds, the warm- versus cold-blooded debate can arguably be called “the last big one,” as John Grady, an ecologist at the University of New Mexico told BBC News.
Grady is the author of a new study, published this week in the journal Science, that suggests that dinosaurs may not have been cold-blooded or warm-blooded, but something in between. “They took a middle way–kind of like Goldilocks. And it seemed to work out very well for them,” Grady told NPR. He thinks the dinosaurs could generate body heat but not maintain a constant temperature, allowing them to move fast but also to grow bigger than other creatures roaming the planet around that time. This combination, dubbed “mesothermy,” might have been a useful adaptation made by the dinosaurs in order to survive in a world already populated by large, slow-moving reptiles.
To reach their conclusions, the team of researchers evaluated the metabolism of various dinosaurs using a formula based on their body mass (revealed by the size of their bones). They also came up with an estimate of the animals’ growth rate by measuring the growth rings in their bones, using a method similar to how a tree’s age can be determined by studying the rings in its trunk.
Out of 381 different species the researchers compared, 21 were dinosaurs–among them the predatory Tyrannosaurus and Allosaurus, the long-necked Apatosaurus, the duck-billed Tenontosaurus and the birdlike Troodon–and the others were a range of mammals and birds; sharks and other fish; and snakes, lizards and crocodiles. As one of the study’s co-authors, Brian Enquist of the University of Arizona, told Reuters:”Our results showed that dinosaurs had growth and metabolic rates that were actually not characteristic of warm-blooded or even cold-blooded organisms. They did not act like mammals or birds, nor did they act like reptiles or fish.”
While the muscles and nerves fire faster in warm-blooded animals, cold-blooded animals burn fuel more slowly and can get bigger than other mammals might be. According to Grady’s team’s findings, this intermediate metabolism may have enabled dinosaurs to grow bigger than any mammal could. For example, a mammal the size of T. rex would have to eat 24 hours a day, seven days a week to fuel its accelerated metabolism, and would probably have starved to death: “It is doubtful that a lion the size of T. rex could eat enough to survive,” Grady says.
Only a few modern-day animals combine warm- and cold-blooded characteristics. These include the echidna, a mammal that looks like an anteater and lays eggs, the leatherback turtle, the great white shark and the tuna.