For centuries, scientists have puzzled over why the toucan¹'s bill is so remarkably² large - but now one team thinks it might have an answer.

几个世纪以来，科学家被犀鸟巨大的喙所迷惑，但是现在一组团队认为他们找到了答案。

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As the surroundings heat up, the toucans dump body heat into their bills

Writing in the journal Science, the researchers say that the toucan uses its enormous beak⁴（鸟嘴） to stay cool.

They used infrared⁵ cameras（红外摄像机） to show the bird dumping heat from its body into its bill, helping⁶ it to regulate its body temperature.

The toucan has the largest bill of any bird, relative to body size.

It makes up about one-third of its total body length.

Hot secrets

The oversize appendage⁷（附加物） has received many different interpretations⁸（解释）: Charles Darwin thought it might be used to attract mates, more recent ideas centre on fruit peeling, nest predation（掠夺行为） and visual warnings.

To investigate further, a team of researchers from Brock University, Canada, and Universidade Estadual Paulista, Brazil, looked at the toco toucan (Ramphastos toco), which has the biggest bill of all the toucans.

Dr Glenn Tattersall, the lead author of the paper, from Brock University, Canada, said: "We used infrared thermal⁹ imaging technology to assess the surface temperature of the bill of toucans while they were encountering a range of air temperatures from 10C to 35C.

"This allowed us to measure the exact temperature of the bill."

The scientists found that as the surroundings got warmer, the toucan's bill would rapidly heat up, effectively acting¹⁰ as a radiator¹¹ to draw heat away from the bird's body, allowing it to stay cool.

Conversely, in cooler temperatures, little heat would radiate from the bill, letting the bird conserve¹² its warmth.

The effect was particularly evident at night: at sunset, the birds would, in a matter of minutes, pump out heat from their beaks¹³, lowering their body temperature as they slept.

The birds' bills have a network of blood vessels¹⁴ that can increase or restrict the flow of blood.

Dr Tattersall said: "By altering blood flow to the bill's surface, toucans can conserve body heat when it is cold or cope with heat stress by increasing blood flow.

"Essentially¹⁵, the large surface area of the bill, and the fact it is not insulated（绝缘）, means that the blood flowing through is able to release heat into the bill, thus cooling the bird.

"This blood-derived heat in the bill is then dissipated（浪费的） into the air."

Effective radiators¹⁶

Other animals also use parts of their bodies to regulate temperature, for example, elephants and rabbits radiate heat from their ears to cool down.

But the sheer size of the toucan's bill means that relatively¹⁷, it has one of the largest "thermal windows" in the animal kingdom.

The Science study also shows that the toucan is extremely effective at controlling its body heat with it: its bill can account for as much as 100% of body heat loss or as little as 5% if the blood flow is shut off.

Dr Tattersall explained: "Bird bills are not 'dead tissues', incapable¹⁸ of playing a role in heat balance, but are active contributors to thermoregulation（温度调节）."

He added: "Birds do not sweat, so must cope with other mechanisms¹⁹ to deal with elevated temperatures."

The researchers say that the finding that the toucan uses its beak to regulate body temperature does not rule its other suggested functions.

The team now plans to look at other birds to see how their bills are involved with temperature regulation.