Meat-Eating Dinos Breathed Like Birds, Study Says

Brian Handwerk
for National Geographic News
July 29, 2005
Predatory dinosaurs may have had the same type of super-efficient respiratory systems found in modern birds, according to a recent study.

The high-powered oxygen pumps could have boosted dinosaur metabolisms, enabling meat-eaters, such as Velociraptor, T. rex, and others, to be active and effective hunters.

"It's fairly controversial just how active some groups of dinosaurs were and whether they actually had the capacity to be very active predators," said study coauthor Patrick O'Connor, a professor of gross and neuroanatomy at Ohio University in Athens. "We tackled the problem directly at the level of the lung."

"How much gas an organism exchanges is ultimately what dictates metabolism," he added. "In general terms, animals with higher metabolism were probably fairly active and energetic in their normal routine—much like we think of mammals and birds today."

Metabolism and Lifestyle

Modern birds have complex respiratory systems, with two lungs and up to nine airs sacs. Sometimes found in hollow bones, the air sacs boost birds' respiratory volume and efficiency. As a result, birds can extract oxygen from the air much more efficiently than mammals can.

Nearly a century ago, scientists noticed that some dinosaur bones were "pneumatized," or possessed holes like those that house avian air sacs.

"But since nobody knew exactly which air sacs invaded what bones in birds, earlier speculation always lacked a solid basis," said Leon Claessens, a Harvard University paleontologist and study coauthor.

The two researchers examined the air sac systems of some 200 modern birds and identified specific patterns showing which air sacs are found in which bones.

The avian blueprint allowed the duo to make a striking comparison between bird anatomy and the fossilized remains of the meat-eating dinosaur species Majungatholus atopus.

"These features are spot on," O'Connor said. "You can take the [ Majungatholus] bone and hold it up to any number of bird bones and see virtually identical anatomy. It's compelling evidence."

The pair recently published their study in the science journal Nature.

"The growing consensus of a lot of people interested in dinosaurs as animals is that understanding dinosaur respiration is really key to understanding their history," said Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park.

He noted that dinosaur metabolism, or energy production, is a product of two inputs: food and oxygen. "The classic work on dinosaur metabolism concentrates on the food end of it. … ," he said. "Previous work looked at adaptations that got [dinosaurs] more food, more sugar. But now people are beginning to appreciate the adaptations that got them more oxygen."

Adaptations that helped dinosaurs get more oxygen might have later helped them to acquire more food, and thus gain an evolutionary edge over their competitors.

"A predatory dinosaur with a higher metabolic rate than his prey might be able to outrun and catch its prey," Claessens, the study coauthor, explained. "A predatory dinosaur with a higher metabolic rate than his peers might be able to outrun them when trying to catch a single prey."

"I think that a better pulmonary system and a higher metabolic rate would have been an evolutionary advantage to most dinosaurs," he added.

The study of dinosaur respiratory systems could change long-held conceptions about dino behavior.

"They may not have been anything like any living reptiles that we know today or like the sluggish creatures that they're often made out to have been," O'Connor said.

Dino Descendents

Accumulating evidence suggests that living birds are the direct descendents of theropods, a group of meat-eating dinosaurs that walked on two legs.

They include Velociraptor, T. rex, and Majungatholus atopus, the subject of O'Connor and Claessens's study.

Other paleontologists, meanwhile, have recently uncovered fossil remains of feathered dinosaurs. Such insulating feathers would have been more common to high-metabolism, warm-blooded animals than to cold-blooded reptiles, which must absorb heat from their environment.

"It may be that the origin of a [feathered] fuzz on these guys occurred, because you had small-bodied, birdlike dinosaurs with a high metabolic rate," Holtz, the University of Maryland paleontologist, said. "Without a covering they'd be like today's Chihuahuas and shivering all the time."

Other recent fossil discoveries have uncovered dinos in birdlike sleeping postures. Among other reasons, dinosaurs may have slept that way to conserve heat.

Holtz noted that the new study of dino respiration syncs nicely with recent research into dinosaur growth rates.

"Dinosaurs had phenomenally fast growth rates compared to typical reptiles," he said. "They grew at rates comparable to modern marsupials, ground birds, and modern placental mammals."

"And in order to achieve those growth rates they had to have had a high metabolic rate. To have a high metabolism you've got to get oxygen into the lungs."

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