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A Bird With a Big Air-Conditioning Bill
23 July 2009 (All day)
When it comes to keeping cool, toucans get top billing in the animal world. New research shows that the colorful bird uses its massive beak to rapidly radiate away heat, allowing it to chill out in tropical climates or when expending a lot of energy while flying. At its most efficient, the toucan is theoretically capable of jettisoning 100% of its overall body heat loss through its bill.
Birds don't sweat. Neither do elephants or rabbits. Instead, these creatures flush an uninsulated body part--such as a beak or an ear--with blood and let the heat dissipate into the air. Glenn Tattersall, an evolutionary physiologist at Brock University in Canada, wanted to find out just how much of a cooling effect the toucan's giant beak provided.
He and colleagues focused on the South American toco toucan (Ramphastos toco), which has the largest bill of any bird relative to its body size. (It can represent between 30% and 50% of the creature's overall body surface area.) The team then used infrared thermal scanners to record the bill's surface temperature while the bird was exposed to air ranging from 10° to 35°C--temperatures typical of the toucan's habitat--and also while flying. By comparing the temperature of the bill with the environmental temperature, Tattersall's team was able to gauge how much heat was being lost; the larger the difference, the more heat was escaping.
The bill radiated a great deal of heat at high temperatures and when the toucan flew, indicating that, like elephants and rabbits do with their ears, the toucans flush their bills with blood to cool down. At lower temperatures, the difference between air temperature and bill temperature dropped, meaning that the toucans were restricting blood flow to their bills. Based on its size, a toucan's bill can theoretically account for anywhere from 5% to 100% of the bird's body heat loss, the team reports tomorrow in Science. When the toucan is in flight, its bill is the most efficient heat-shedder ever reported, losing four times more heat than the bird produces while at rest. That's about four times more efficient than either elephants' ears or ducks' bills.
Juvenile toucans in the experiment didn't have as much control over how much blood flows into their bills. Why is unclear, but Tattersall says that "our best guess is that the control over the vessels takes time to develop, or the blood vessels in juveniles are much denser than in adults and are less capable of being controlled."
Gary Ritchison, an ornithologist at Eastern Kentucky University in Richmond, says that the study shows that toucans do use their bills to regulate body temperature. But that may not be the reason why toucans' bills evolved to be so large. "You'd have to expect that it's not the overriding explanation [for bill size]," he says. There are too many other considerations, such as defense and diet, to know for sure, he says.
Tattersall agrees but adds that biologists need to consider thermoregulation when discussing bird beak evolution. Most papers have tried to link bill size to the birds' diets. Even in birds with small bills, he says, there's evidence that "the bill is still a site of heat exchange and therefore under some selective forces in terms of thermoregulatory constraints."