Bird's distinct bill offers a big chill: study identifies new function for toucan's sizable front end.

Toco toucans have a huge cooling bill. But it's the good kind.

While overheated people crank up their air conditioning, toucans increase the blood flow to their supersized, uninsulated bills, report Glenn Tattersall of Brock University in St. Catharines, Canada, and his colleagues. Extra heat radiating from the bill keeps the bird comfortable, the researchers report in the July 24 Science.

This study adds an "entirely plausible" twist to a long-standing debate, says Winston Lancaster of California State University in Sacramento. "I suspect that few people have ever seen a toucan without wondering 'Why the beak?'"

But documenting cooling powers for the toucan bill doesn't mean other ideas for its function are wrong, says coauthor Denis Andrade of the Silo Paulo State University's campus in Rio Claro, Brazil. "The bill has many functions," he says.

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To test the cooling properties of bills, the researchers focused on the toucans with the largest bill, the toco toucan (Bamphastos toco). The bill of an adult bird can account for up to half of its body surface area.

Researchers put birds one at a time into a temperature-controlled chamber. Over the course of six hours, the chamber warmed 10 degrees Celsius. Thermal imaging showed the researchers how various parts of the birds' bodies changed temperature during the warm-up.

As the temperature increased, the birds' bills warmed too, a change that the researchers interpret as a sign that the birds were flooding their beaks' blood vessels with extra blood. Yet the unfeathered skin around the birds' eyes, an indicator of core body temperature, stayed about the same.

Toucans also spent a night in the chamber, and researchers monitored them during sleep. When the birds settled down, their bills warmed up, suggesting they were dumping heat. Animals typically cool down as they fall asleep.