LEAPIN' LIZARDS--UC IRVINE RESEARCH SHOWS SOME REPTILES BREATHING EASIER
Can Lizards on Treadmills Shed Light on How We Breathe?
Irvine, Calif. -- They are the bionic beasts of lizardom, the Michael Jordans of the reptile realm.
Speedy reptiles known as monitor lizards have an anatomical advantage most other lizards can't claim, according to research done at UC Irvine. Monitor lizards can breathe while they run by pumping air through sacs in their throats, researchers report in the June 4 issue of Science. Other types of lizards can't breathe and run at the same time.
Mammals are believed to have evolved from reptilian ancestors. The study suggests that the evolution of increased, sustainable aerobic capacity in animals required the development of muscles that could power the lungs to move air in and out quickly, independently of whether an animal was running.
Studies of the locomotor and ventilatory systems of lizards also are important in providing a framework for ultimately understanding how humans breathe and the primary functions of human trunk muscles.
Tomasz Owerkowicz, a graduate student at Harvard University, and Beth Brainard, an assistant professor at the University of Massachusetts, observed that monitor lizards were pumping air through their throats. They came to UCI to better understand how the pumping contributes to breathing, and turned to James Hicks, UCI professor of ecology and evolutionary biology, and UCI postdoctoral fellow Colleen Farmer for help. Hicks is a leading expert in the heart-and-lung systems of vertebrates, especially reptiles, and researchers from across the country and world come to Hicks' lab to perform studies on animals from rattlesnakes to alligators.
Lizards scamper ahead by twisting their bodies from side to side as they move their feet forward. Muscles in a lizard's trunk make it possible to move forward this way. Those same muscles also control how a lizard rotates its ribs and expands its lungs to breathe.
Dave Carrier, an associate professor at the University of Utah, observed in earlier studies that lizards cannot use the same muscles for two actions--running and breathing--at the same time. So, most lizards cannot run far without getting out of breath. After a burst of speed, they must rest to recover the normal level of oxygen in their blood. But monitor lizards can run quickly and for some distance, and their blood oxygen levels remain high and constant while they're moving. This inconsistency has puzzled scientists.
The new study in Science shows that monitor lizards use a "gular pump"--a rapid pumping of sacs in the throat--to push air into the lungs while they run. It is a backup system that keeps air entering the lungs, even when the lungs cannot expand the ordinary way.
"The monitor lizard, the aerobic king of the lizard world, has circumvented the basic constraint of its breathing and walking apparatus by having an accessory ventilatory pump," Hicks said. "This would be analogous to the evolution of the diaphragm in mammals, which ventilates the lung independently of locomotion."
To do their study, the researchers compared the 12-inch long monitor lizards to another lizard, the green iguana. They put three iguanas and six monitor lizards through their paces on a treadmill. Each lizard ran at the maximum speed it could steadily scamper on the treadmill for one to three minutes, and the scientists shot x-ray videos of the creatures.
Next, they examined the videos frame by frame for evidence of gular pumping. (See pictures of the lizards at http://www.communications.uci.edu/99releases/lizard.html). They then measured the flow of air through the lizards' nostrils and mouths and determined that the monitor lizards used about three gular pumps per breath when running. But iguanas did not use gular pumping at all.
When the biologists blocked the monitor lizards' gular pumping, the monitor lizards were found to have the same constraints on their breathing as iguanas.
Gular pumping has previously been shown to play a role in defensive behavior, the sense of smell and many lizards' abilities to regulate their body temperature. Preliminary videos of six other species show that gular pumping for breathing is present in some species that move around a lot and is absent in other species that are less active.
Monitor lizards are among the oldest living lizards, and include the fierce Komodo dragon, which can grow to weigh as much as 300 pounds. They are characterized by long, forked tongues, powerful tails and strong, quick legs. They can engulf and swallow large prey such as birds and reptiles whole.
Besides their unique mode of breathing, monitor lizards also differ from most other lizards in that their hearts are functionally divided into four chambers, similar to those of mammals (most lizards' hearts allow blood to slosh back and forth, instead of moving efficiently in and out of the heart). They also have a larger surface area inside their lungs than other lizards, giving them more space in which oxygen can move into the blood stream and carbon dioxide can move out, contributing further to monitor lizards' stamina in the wild.
"Monitor lizards have a metabolism that overlaps some mammals like hedgehogs," Hicks said.
Lizards, snakes, turtles and similar creatures are studied closely in Hicks' lab. Hicks and Farmer, a postdoctoral student whose work is supported by the National Institutes of Health, will study the cardiovascular patterns of alligators this spring. Other upcoming projects at the UCI lab include studies of the shaker muscle in rattlesnakes, oxygen transport and blood chemistry of monitor lizards during digestion, and the drastically lowered metabolic rates in water turtles that come from long stints underwater without breathing. For more information on Hicks' lab, see http://compphys.bio.uci.edu/hicks/hicks.htm.
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Contact: Alicia Di Rado, (949) 824-6455
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