The University of Chicago public relations folk have conjured up an unnerving mental image to illustrate a scientific discovery: a 130-pound woman eating a 30-pound hamburger.

Impossible? Well, maybe, but alligators and crocodiles often achieve such feats of gastronomical excess, though they're sort of sloppy about it. A croc may tear into an unlucky water buffalo and quickly gobble meat that would weigh the equivalent of 23 percent of the reptile's body mass.

"But what do they do with all of that food?" the University of Chicago asks in a news release. "If they do not digest their meal quickly, crocodilians risk death from within, or, if they are young, by predators."

The question of how they manage to quickly digest enormous feasts has puzzled biologists for centuries, but now C.G. Farmer, an assistant professor in the University of Utah biology department, and colleagues at the U. have found the answer. It involves an extra heart valve that an alligator, cayman or crocodile can open or close at will.

"The Right-to-Left Shunt of Crocodilians Serves Digestion" article appearing in the March-April issue of the journal Physiological and Biochemical Zoology provides the answers. The journal is published by the University of Chicago; Farmer is the article's lead author.

Bird and mammal species have a single aorta, which carries blood from the heart's left ventricle throughout its body, Farmer said in a telephone interview. Blood in the right ventricle is pumped into a pulmonary artery that carries it into the lungs. There, oxygen moves into the blood supply and carbon dioxide goes into the lungs, to be breathed out.

But crocodilians have a special valve in the pulmonary artery, she said.

"It is the only known neurologically controlled valve in a vertebrate," Farmer said. In other words, these creatures can open or close the valve at will; it's not automatic, like the heart valves humans have.

"When the animals want to, they can shut down blood flow into the lungs," she said. "This blood instead is injected into a second aorta that they have."

That second aorta moves blood to the stomach and gastrointestinal system. "It's a direct shot," Farmer said.

She and her fellow scientists suspected the sudden infusion of blood dramatically aids digestion, allowing these predators to secrete gastric acid at a high rate.

To test the theory, they surgically tied off the extra aorta in some juvenile American alligators, so blood could not be shunted through this system. That made their heart function like mammals' with the arterial blood flowing to the lungs. Compared with other alligators that had not had the restriction, gastric acid was produced at a significantly reduced rate, she said.

The crocodilians' special design allows them to secrete acid at about 10 times the rate of other animals, helping them digest bone faster.

"The question that remains is, why is this so important in reptiles?" Farmer wondered. "Is it because they eat bigger meals, less frequently?"

Other authors are T.J. Uriona and M. Steenblik of the U.'s biology department; D.B. Olsen, Utah Artificial Heart Institute, and K. Sanders of the University of Utah Health Sciences Center.

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