The bills of toco toucans, like this one in Brazil's Pantanal Conservation Area, make up a third of their body length and have been the object of considerable scientific speculation. (Courtesy of Thiago Filadelpho)A toucan's enormous bill works like a built-in cooling tower, helping it beat the heat in the steamy rainforest, Canadian and Brazilian researchers have found.

"Biologists all the way back to Darwin and even before have debated the purpose of the enormous beak," said Glenn Tattersall, a biologist at Brock University in St. Catharines, Ont., in an interview with Science. The journal published the results of his study in its July 24 issue.

The toco toucan's bill represents up to a third of its length and 50 per cent of its body surface area, making it the largest beak of any bird relative to body size.

Some biologists had speculated that having a giant bill might be useful for peeling fruit, attracting mates or warding off other toucans.

While researchers can't answer why the giant beak originally evolved, Tattersall and his Brazilian co-authors did find that the beak is extremely efficient at removing heat from the bird's body, which cannot sweat to keep cool as many mammals do.

As the bird fell asleep, its bill was 10 or 15 degrees warmer than the surrounding air. (Courtesy of Glenn Tattersall)The researchers filmed toco toucans with heat-sensing cameras as they were exposed to different temperatures and found the bill's surface temperature changed quickly as the air warmed or cooled.

As the birds fell asleep, their beaks were 10 or 15 degrees warmer than the air around them. Tattersall noted that most warm-blooded animals lower their temperature during sleep, including toucans.

"In order to do that, they have to dump he body heat somewhere, and we're proposing they use the bill to dump that body heat," he said. In that way, it works similar to a radiator or cooling tower.

The beaks cooled down within minutes, showing that they are very efficient at releasing heat. Further measurements found that a toucan's bill routinely accounted for 30 to 60 per cent of the total heat loss from its body, and that the birds' physiology could greatly vary the proportion of heat dissipated through their beaks. Young toucans, which don't grow feathers until they are four weeks old, weren't very good at reducing heat loss from their beaks, however, and shivered when the temperature was as high as 26 or 27 degrees.

Other research in the past has found that elephants use their ears to radiate heat and keep cool the way toucans use their bills.