Evidence For Dinosaurian Warm-Bloodedness

A very neat study came out today in the open source on-line journal PLos ONe. The matter of dinosaur metabolism has been long-debated. For a long time, dinosaurs were characterized as sluggish reptiles, cold-blooded and dim-witted. That thinking changed with the work of paleontologist John Ostrom in the sixties, and by the mid-nineties, pop culture had warmed to the idea of active, warm-blooded creatures. Just remember that signature moment in Jurassic Park when the raptor exhales and fogs up the window on the kitchen door.

This study, led by Washington University anthropologist Herman Pontzer, draws on the biomechanics of living animals to determine the metabolic cost of walking and running in bipedal dinosaurs. Why would an anthropologist have a unique insight into this issue? Pontzer has had previous experience in the energy cost of bipedalism and was the lead author of a 2008 study seeking to shed light on the evolutionary pressures that led to increasing bipedalism in hominids. As it turns out, the length of the leg - or distance of the hip from the ground - is a great way to determine how much energy a biped uses when walking about.

For this new paper, Pontzer and his colleagues analyzed dinosaurs ranging in size from tiny Microraptor up to giants like T. rex. In addition to the rather simple leg-length measurement, they also used a more complex method which figures up the amount of energy that would be needed to activate leg muscles for walking and running. For all dinosaurs, it was found that a slow run required more energy than cold-bloodedness can provide. To walk slowly, the results for the smaller dinosaurs were more ambiguous. But for the large ones, they still pointed clearly to warm-bloodedness. This doesn't necessarily mean that the dinosaurs possessed the kind of warm-bloodedness you and I do. It may very well be that there was a uniquely dinosaurian method of regulating temperature. But it certainly distances them from cold-blooded, sun-bathing reptiles even further.

Plateosaurus (big guy in the middle) by Luis Rey, from A Field Guide to Dinosaurs by Rey and Henry Gee

What's really interesting here is that this result doesn't just apply to the most bird-like dinosaurs, or the ones we might expect to have the most active lifestyle, such as T. rex. In fact, it held true for Plateosaurus, a familiar early ancestor of the giant sauropods from the Triassic era. this would indicate that warm-bloodedness, or something very much like it, was present down low on the dinosaur family tree. This issue is much like the bird origin issue: there's an ever-growing heap of evidence. It makes sense, really - consider that dinosaurs were the dominant form of large land-dwelling animals on the planet for nearly two-hundred million years. They occupied all kinds of niches in a great diversity of habitats. To be that widely successful, they had to have been very adaptable. Warm-bloodedness is a basic requirement for being able to take advantage of so much opportunity.

Check out Palaeoblog and Dinosaur Tracking for more info.