The question of whether dinosaurs were warm-blooded or cold-blooded is one of the most enduring in palaeontology. Did they generate their own body heat like today's mammals; was their temperature more influenced by their environment like today's reptiles; or did they use a mixture of both strategies? Scientists have put forward a slew of arguments for all of these alternatives, but Herman Pontzer from Washington University has a new take on things which suggests that many dinosaurs were indeed warm-blooded.
Based on our knowledge of living animals, Pontzer worked out the energy that 14 dinosaur species would have used while walking or running. His model reveals that these ancient reptiles would have needed more energy than a cold-blooded physiology could supply. Their metabolic demands were within the range of modern warm-blooded animals like mammals and birds, which can keep up their physical activity for far more time than their cold-blooded peers.
While both warm-blooded and cold-blooded animals can be equally active over short bursts, warm-blooded ones have the advantage in the long run with their higher capacity for aerobic exercise. This aerobic capacity is signified by a measurement called VO2max, which is often measured by getting animals to run on a treadmill. Obviously, that's not feasible if the animal in question has been dead for 65 million years before the invention of the treadmill, but Pontzer had a solution. In earlier work, he showed that you can predict with 98% accuracy how much energy an animal needs to run or walk by looking at how high their hips were from the ground.
Pontzer looked at the hip heights of 13 species of dinosaur including Tyrannosaurus, Velociraptor and Archaeopteryx, as well as a closely related non-dinosaur called Marasuchus, and used these values to calculate crude estimates of their aerobic capacity. He focused on species that walked on two legs, since the way they distributed their weight is clearer-cut than four-legged relatives like Diplodocus or Triceratops.
His figures showed that the aerobic capacity of his dinosaurs, especially the larger ones, were consistently over the maximum values for living reptiles, from alligators to iguanas. Even while walking, the energy demands of the largest dinosaurs, particularly the large meat-eaters, would have far exceeded anything that cold-blooded animals could have coped with.
For more accurate estimates, Pontzer also used a mathematical model to calculate the size of the dinosaurs' walking muscles and from these, their aerobic capacity. Again, he came to the same conclusion. The largest species simply wouldn't have been able to function with cold-blooded metabolisms, and the smaller ones like Velociraptor could have walked but not run. Only Archaeopteryx, the smallest of the baker's dozen, had VO2max values that approached the range of living cold-blooded animals.
There are a couple of alternative interpretations. It's possible the larger dinosaurs were cold-blooded but had adaptations that granted them greater aerobic capacities than modern reptiles can achieve, although Pontzer thinks this unlikely. It's also possible that the dinosaurs didn't go in for sustained bursts of speed and, instead, relied on sprints, as many monitor lizards use to run down prey. But that would saddle the largest species with unfeasibly long recovery periods, when they could barely function.
Pontzer says that the relationships between his 13 species support the idea that all dinosaurs powered their runs with a warm-blooded metabolism. If he takes a conservative view of his estimates, the alternative explanation is that warm-bloodedness evolved at least three times - in the early sauropods, in the tetanurans (including birds and most of the carnivores) and in modern birds - and was lost once in between among the small predatory coelurosaurs like Velociraptor. That reconstruction is not only messy, but it contradicts evidence from bones and primitive feathers suggesting that the coelurosaurs were warm-blooded.
Nonetheless, there's a risk that by looking exclusively at two-legged dinosaurs, Pontzer has biased his dataset to species that were perhaps most likely to be warm-blooded anyway. Groups like the massive sauropods and the diverse ornithischians are represented only by three of their earliest members - Plateosaurus, Heterodontosaurus and Lesothosaurus.
It's still possible that the largest of the plant-eaters had a different physiology altogether including "inertial homeothermy", where they maintain a constant temperature simply because their gargantuan bulks lose heat very slowly.
Pontzer's new study far from settles the debate about dinosaur physiology, but it adds another piece of evidence into the mix. This debate isn't just an academic fancy - it's critical for understanding how the dinosaurs lived, evolved, and ultimately died.
Reference: Pontzer, H., Allen, V., & Hutchinson, J. (2009). Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007783