The infamous T-Rex may have been a cold-blooded killer, but new evidence suggests he probably had warm blood.
Paleontologists have debated the issue of dinosaur metabolism for decades: Did those ancient, lumbering beasts rely mostly on the sun’s warmth to regulate their body temperature, like today’s reptiles and amphibians, or could they generate their own body heat like mammals and birds? Respected scientists have come down on both sides of the issue, and there's a host of arguments to support each theory.
Now, using a biomechanical model that predicts the energy cost of walking and running based on the size of an animal's leg bones, researchers have shown that the biggest dinos couldn't have gotten around without a warm-blooded metabolism.
"Using studies of living animals, we can figure out the relationship between limb design and the amount of muscle an animal needs to support its body weight as it walks and runs," said anthropologist Herman Pontzer of Washington University in St. Louis, who co-authored the paper published Thursday in PLoS ONE. "The size of muscle is very good predictor of how much energy you need, because to turn on muscle, you need oxygen."
Because warm-blooded animals have much greater aerobic capacity than their cold-blooded counterparts, finding bigger muscles and higher energy demands in dinosaurs would favor the warm-blooded hypothesis. Indeed, when Pontzer and colleagues looked at anatomical models of 14 different species of extinct dinosaurs, they were surprised to find that even at a slow walk, most dinos needed more energy than a cold-blooded metabolism could provide.
Of course, drawing conclusions about extinct dinosaurs from a model based on modern-day animals involves making some assumptions. Pontzer acknowledges that it's possible dinosaurs had a physiology completely unlike anything alive today, a cold-blooded metabolism that provided enough energy to meet the needs of these fast, muscular creatures.
"That's a limitation of this analysis," he said, "and maybe a limitation of any similar analysis that uses what we see in the world today. But it just seems to us more likely that they're warm-blooded than that they have some bizarre form of physiology that we have no record of today."
The new research fits well with a previous study on dinosaur cardiovascular anatomy, based on a CT scan of a 66-million year old dinosaur fossil with a preserved heart. Imaging revealed a four-chambered, double-pump heart with a single aorta — essentially, the heart of a warm-blooded mammal or bird, not a cold-blooded reptile. But other anatomical studies have led to different conclusions: A survey of dinosaur noses, for example, found that dinos lacked special bones in their nose, called turbinates, that protect against water loss during rapid breathing and are found in 99 percent of warm-blooded animals.
Paleontologist John Ruben of Oregon State University, who led the turbinate study, questions much of the recent evidence on dinosaur metabolism, and he rejects the idea that higher energy demands mean dinos couldn't have been cold-blooded.
Regardless of whether animals are warm-blooded or cold-blooded, and whether they're upright or not, "the net cost of all locomotion is roughly similar in all extant terrestrial vertebrates," Ruben wrote in an e-mail to Wired.com. To accept the paper's conclusions, he says you'd have to accept the idea that the net cost of walking and running in dinosaurs increased as they became larger — and not all paleontologists agree with that assumption.
Pontzer, whose primary research involves the biomechanics of ancient human locomotion, says the argument over dinosaur metabolism isn't going to end soon. "Our data, as far as the method goes, are pretty clear that the big guys are probably warm-blooded," he said. "But it won't be the last word, surely. These big debates will, and perhaps should, be debated for awhile."
Image: Figure 1 from "Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs." Pontzer H, Allen V, Hutchinson JR, PLoS ONE 4(11), 2009.
See Also:
- Four-Winged Fossil Bridges Bird-Dinosaur Gap
- Want Your Own Dinosaur? Place Your Bids
- Dinosaurs Were Warm-Blooded
- Paleontologists Find Dinosaur Protofeather
- Massive Volcanic Eruptions Could Have Killed Off the Dinosaurs ...
- Reptiles Needed More Than Feathers to Fly
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