Prepared for the Past
Natural History, April 2001
Most of the time, evolution hands out its gifts sparingly. Natural selection generally produces animals that are well designed for the lives they lead—but not too well designed. Some hawks have eyes sharp enough to see a mouse up to 300 feet away, but not 1,000 miles away. After all, everything in life has a cost. An owl with eyes the size of beach balls might be able to see quite far, but it would have a hard time flying (let alone pumping enough blood to its eyes to keep them working properly). Some animals, however, do seem to be overdesigned. One of these is the American alligator (Alligator mississippiensis).
Recently, biologists Colleen Farmer and David Carrier, of the University of Utah, measured the breathing patterns of alligators. When an alligator is resting, it breathes intermittently, taking one breath per minute, much like other cold-blooded reptiles. To study alligators on the move, Farmer and Carrier trained them to do four-minute bouts on a treadmill. The scientists placed a mask over each alligator’s nose to measure how much air the animals inhaled and exhaled.
Your typical reptile—an iguana, for instance—can’t take deep breaths while walking. To exhale, an iguana uses some of its trunk muscles to squeeze its ribs, compressing the lungs and forcing out the air. As long as the animal is resting, this presents no problem. But when walking, it must use these same muscles to bend from side to side. The demands of walking and breathing thus come into conflict. Making matters worse, a walking iguana needs more oxygen than a motionless one. For iguanas and many other reptiles, the solution is to take quick, shallow breaths when they walk and never to walk very fast. (Some lizards can sprint, but they can’t keep it up for more than a few minutes.) Modern lizard skeletons look a lot like the oldest reptile fossils, suggesting that reptiles are still constrained the same way they were some 300 million years ago.
But not alligators, as Farmer and Carrier discovered. As soon as these animals started moving on the treadmill, they began to breathe deeply at regular intervals (about thirteen times per minute). Their walking breaths were four times deeper than their resting breaths—in fact, relative to their weight, alligators take in more air per breath than has been reported for any other exercising animal.
Alligators manage such an unreptilian feat because they don’t rely solely on rib muscles to breathe. When an exercising alligator exhales, its liver is pushed forward, ramming against its lungs and squeezing the air out. During an inhalation, a muscle attached to the pelvis pulls the liver back, creating negative pressure that draws air into the lungs.
Farmer and Carrier have discovered that the alligator’s liver-pump system is even more sophisticated than previously thought. Every time the animal breathes in, muscles rotate the front of its pelvis downward. This hip rotation stretches the abdomen, making room for the liver and for the muscles that retract it. When the alligator exhales, the front of the pelvis rotates back up and the abdomen shrinks, forcing the liver forward again. None of the muscles that help move the liver are involved in bending an alligator’s trunk from side to side, so there is no conflict between walking and breathing.
Impressive, yes, but given the gator’s lifestyle, this biomechanical design seems over the top. Alligators spend most of their time lying around, and their hunting style involves lurking in the water, motionless, and then suddenly ambushing their prey. A lion or a wolf may sometimes need to breathe deeply for long periods while running after prey, but putting a liver pump in an alligator is a bit like hooking a jet engine to a hang glider.
To explain this apparent excess, Farmer and Carrier point out that today’s alligators and crocodiles look very different from their ancestors. Those early reptiles, known as crocodylomorphs, date back about 240 million years and were the size and shape of a coyote with a heavy tail. They had long slender legs that fit under the body rather than sprawling out to the sides, and they ran on the balls of their feet. Farmer and Carrier speculate that the ancestors of crocodylomorphs evolved special breathing mechanisms for sustained locomotion on land—a liver pump complete with rotating hips, which would have allowed them to take the deep breaths needed to gallop. And indeed, the pelvis bones of fossil crocodylomorphs show features that could have played a role in driving air in and out of an animal’s lungs.
Perhaps 100 million years after the first crocodylomorphs trotted across the land, their descendants shifted to an amphibious way of life. Their legs grew short, and their tails became stout—an adaptation for swimming. Yet they retained the powerful hip-driven system for breathing. Whatever the reason, today’s alligators are biomechanically prepared for a way of life that vanished 140 million years ago.
Copyright 2001, Carl Zimmer. Reproduction or distribution is prohibited without permission.