Toothy valves control crocodile hearts.

Inside their hearts, crocodiles have toothlike gear cogs. Researchers now have figured out what makes those teeth clench.

Unlike more standard heart valves, which swing open or closed as blood pressure changes, the cogs interlock to reroute blood in response to changing hormone levels, say Craig E. Franklin of the University of Queensland in Australia and Michael Axelsson of the University of Goteborg in Sweden. "To our knowledge, this is the first report of an actively controlled intra-cardiac vane in a vertebrate," they claim in the Aug. 24 NATURE.

"What we've shown here is an evolutionary novelty," Franklin says. "The crocodile heart is the most complex and the most bizarre in terms of its plumbing."

Crocodilians--alligators, crocodiles, caimans, and gavials--run on four-chambered hearts, as do mammals and birds. Other reptiles power their circulatory systems with three-chambered hearts.

Most vertebrates control cardiac blood flow with what Franklin calls passive valves. Thin leaves of tissue simply flap open when pressed by surging blood and then snap shut when the surge subsides. In contrast, an active valve like the one he proposes for crocodiles works more like automatic doors triggered by a motion sensor to slide open.

Since the 19th century, biologists have known that crocodilians have cogged teeth made of connective tissue. They're located just inside the valve flap of the right ventricle. To figure out what controls the valve, Franklin and Axelsson monitored its activity in a beating heart from an estuarine crocodile, Crocodylus porosus. They treated the heart with sotalol, a compound that blocks one of the receptors for fight-or-flight hormones. This blockage of the beta-adrenoceptor mimics the low-adrenaline state of a relaxed croc.

Under these circumstances, the cogs interlocked, shunting blood that had just come from the body back for another tour instead of sending it to the lungs to reload oxygen.

The researchers next reversed the relaxation effect by administering adrenaline and related compounds that overwhelmed sotalol's binding to the receptor. The cogs responded by opening and permitting blood to flow to the lungs.

That finding contradicts long-held assumptions, explains James W. Hicks, who studies reptile cardiology at the University of California, Irvine. Since the 1960s, biologists have known that crocs, turtles, and some other creatures can shunt the blood away from the lungs. However, experiments had suggested that passive mechanisms outside the heart are at work. In turtles, for example, when lung blood vessels constrict, blood taking a path of less resistance washes back to the body.

In the new work on crocodiles' valves, "it's the active component within the heart that, to me, makes it exciting," Hicks says.

There's still no solid explanation of why certain animals shunt their blood in the first place, Hicks observes. One conjecture is that the tactic might somehow enable diving animals to stretch their oxygen supply, but mathematical models cast doubt on whether the divers would get very much benefit. Hicks finds that surgery to prevent alligators from shunting blood makes no difference in their first-year growth.

Says Hicks: "What we really need to do is find when shunting occurs in the wild."