Alligators On A Treadmill Hint How Dinosaurs Breathed; Rotating Bone In Pelvis Helps Gators Walk And Breathe Simultaneously

The researchers discovered that alligators, unlike lizards, are able to walk and breathe at the same time by using a rocking pubic bone - part of the pelvis - to help them inhale and exhale.

Along with previous findings that birds use their pelvic bones to help them breathe, the alligator studies suggest that a similar way of breathing may have given dinosaurs and prehistoric flying reptiles called pterosaurs the endurance to lead an active lifestyle.

"Years ago, dinosaurs were thought to be very clumsy, cold-blooded, slow and stupid animals," said Colleen Farmer, a research assistant professor of biology at the University of Utah. But the new findings and other data in recent years indicate "these animals weren't sluggish, cold-blooded and unintelligent. "They could have sustained vigorous exercise for prolonged periods of time and been quite active animals much like birds and mammals are today."

David Carrier, an associate professor of biology, added: "If you can find ways to run and breathe simultaneously, then you can be active for extended periods of time."

The discovery of the pelvis' role in breathing in alligators - and other researchers' similar discovery in birds - "illustrates how little we know about animals which were thought to be well understood," Carrier said. "This basic mechanism of breathing was not realized before. It just wasn't understood."

Farmer was working as a postdoctoral researcher at the University of California, Irvine, in 1998 when she traveled to Utah to study alligators with Carrier. She joined the Utah faculty in January 2000.

The biologists obtained five young American alligators from Florida game officials who cull local gator populations. The gators - as large as 3 feet long and 6 pounds - lived in large fiberglass tubs and were fed mice, rats, goldfish, smelt and eggs.

It took several months of training before the alligators walked on a treadmill at 1 mph for three to four continuous minutes, as required for the research.

"If you handle them gently, they become tame and are very easy to handle," Farmer said.

While walking, the alligators wore masks so the researchers could measure the oxygen they consumed, carbon dioxide exhaled, and how often and how deeply the animals inhaled and exhaled. Monitors also were attached to determine which muscles the alligators used when breathing and walking, and to measure pressure in the abdomen.

The discovery that alligators use the pelvis to help them breathe resulted from research initially aimed at learning how blood flows from the hind legs through the abdominal cavity to the heart during exercise - a characteristic that gives mammals the endurance to escape predators, migrate and hunt for food. Farmer said impairment of such blood flow can contribute to heart attacks and cause problems for pregnant women.

Farmer and Carrier wanted to study how legs-to-heart blood flow evolved. They soon realized that lizards do not breathe when they run because the muscles that move the ribs during breathing also help the trunk bend from side to side during running. "So they run, they stop, they breathe, then they run again," Farmer said. Blood flow from the legs to the heart also was impaired in lizards.

Carrier said that when people and other mammals inhale, ribs lift upward and outward, expanding the chest volume. At the same time, the diaphragm - which separates the chest and abdominal cavities - contracts and moves down, also increasing chest volume. So mammals breathe by a combination of chest and diaphragm movements.

Alligators lack a diaphragm, but have a diaphragmaticus muscle that serves a similar function. When a gator inhales, the muscle pulls the liver back toward the pelvis and tail. The liver, in turn, is attached to the lungs. So when the liver is pulled backward, it helps an alligator's chest and lungs expand during inhalation. The liver slams forward into the lungs when the animal exhales.

Scientists have known since the 1940s that the liver acts as a breathing pump in alligators, Carrier said. It also was known for years that gators - unlike humans and other mammals - have a hinge-like joint in their pelvis that allows the pubic bone to move rather than remain fixed.

Farmer and Carrier discovered that certain muscles pull the pubic bone downward when an alligator inhales, expanding the abdominal cavity to make room for the liver and the muscle that pulls it backwards. The extra room means blood can flow unimpeded from an alligator's tail back to the heart, giving the animal more endurance than if blood flow was impeded.

"We stumbled into a mechanism that was actually quite surprising - that is, the pelvic girdle was playing a role in lung ventilation," Carrier said.

Other researchers discovered a decade ago that in pigeons and certain other birds, the pelvis is connected to the spine by a hinge-like joint, helping the birds breathe. The pelvis rocks away from the breastbone during inhalation, and squeezes toward it when the pigeon exhales, Farmer said.

The discovery of a similar mechanism in alligators raised a question: Was the use of movable pelvic bones in breathing a trait that both birds and alligators share with their prehistoric relatives? Birds descended from dinosaurs; alligators are the closest living relatives of dinosaurs, Farmer said.

She and Carrier propose that dinosaurs, pterosaurs and their ancestors breathed with the help of a muscle that ran from the rocking pubic bone to belly ribs or other bones, pulling them to help expand the abdomen and aid breathing. Farmer said the architecture of dinosaur pelvic bones "supports the idea."

"The bones are very long, they're very thin," she said. "They appear to be quite weak. They don't appear to have been able to support a lot of forces that you would expect from locomotion of large animals. And so we think maybe they played a role in breathing."

Alligators and dinosaurs descended from a common ancestor, yet gators rarely use the endurance they inherited with their pelvic bone structure. Instead, alligators are "sit-and-wait predators" that lurk quietly until they pounce on prey that approaches too close.

"Our hypothesis is dinosaurs were much more active and they [alligators] have reverted back to a sit-and-wait lifestyle," Farmer said.

The biologists found the alligators took more frequent, deeper breaths while walking, compared with lizards, which must walk more slowly and intermittently, taking frequent, shallow breaths.

Carrier said he and Farmer study breathing in modern and extinct creatures because "understanding the history of how animals evolved is very important to understanding why modern animals are built the way they are."