The toucan's enlarged bill may not just be for attracting mates or handling food, as biologists have speculated. It also may be able to exchange heat with its environment, enabling the bird to adjust its body temperature as its surroundings change.
With the largest beak relative to body size of all birds, the toucan has long fascinated researchers, including Charles Darwin, who speculated that the beak's size was used to display colors to the opposite sex, giving bigger-billed birds a reproductive edge.
Accounting for 30% to 50% of the body's surface area and about one-third of its length, the colorful bill has many blood vessels and is not insulated. These factors, contend the authors of a new study, make the beak well-suited to regulate body temperature.
In the study, published in Friday's issue of the journal Science, researchers placed four adult and two juvenile toucans at separate times in a chamber, changing the air temperature in increments from 50 to 95 degrees Fahrenheit. Infrared thermal imaging technology was used to determine surface temperature of the birds' bills. By comparing the temperatures of the bill and the chamber, they calculated heat loss from the bill.
The study's conclusions: As the birds' surroundings heat up, blood flow increases to their bills. There, heat carried by the blood is radiated into the air, cooling the body. At lower temperatures, blood flow to the bill decreases. Less heat is lost, and the toucan is able to keep warm.
If the findings hold up, they would place the toucan's bill in a similar category as rabbit and elephant ears, which can radiate body heat.
The bill can account for 100% of total body heat loss, or as low as 5% if the bird shuts off blood flow to the bill entirely, said study coauthor Glenn Tattersall, an associate professor of biological sciences at Brock University in Canada.
"Bird bills are not 'dead tissues' incapable of playing a role in heat balance, but are active contributors to thermoregulation. Birds do not sweat, so they must cope with other mechanisms to deal with elevated temperatures," added Tattersall.
But questions about the bill remain. "We can see heat radiated by the bill, but that doesn't prove what the purpose of the bill is," said John Pepper, assistant professor of ecology and evolutionary biology at the University of Arizona, who was not involved in the study.
One biologist was skeptical of the paper's conclusions. Said Bernd Heinrich, professor emeritus at the University of Vermont: "The sample size is four adults and two juveniles. It's very few."
The researchers also did not control for the toucans' activity, said Heinrich, who has studied insect thermoregulation and bird behavior. "Are the birds in flight? Are they sitting? Behavior varies with temperature very much. As a consequence, heat loss is confounded by activity."