Relative to its body size, the huge beak of the toco toucan is the largest of any bird. It allows the toucan to eat both fruit and small animals, and display to both mates and rivals. Darwin himself speculated that it acts as a billboard, shaped by sexual selection to display bright colours that could be attractive to potential mates. But the toucan's bill has another function that has only been discovered. Like the ears of an elephant, the toucan's bill is a radiator.
It certainly has all the characteristics of a biological radiator. It's big and has a surface area that's 25-40 times larger than normal for a bird of its size; in fact, the bill accounts for 30-50% of the bird's total surface area. It also lacks any insulating layer of fat, feathers or fur, and beneath its horny exterior, lies a rich network of blood vessels.
These vessels are the means through which the toucan exchanges heat. When it's warm, it widens the vessels and allows the heat of its blood to radiate into the atmosphere. When it's too cold, it limits the loss of heat by shutting down the blood flow to its bill.
Glenn Tattersall from Brock University, Canada, discovered this ability by using a thermal camera to film six resting toucans. As the temperature of their room changed, Tattersall recorded the infrared radiation given off by the bills. The surface temperature of the bird's back feathers was only slightly higher than that of the ambient air. The bill was the same at low temperatures, but it started radiating heat into the atmosphere at anything above 16C.
Between 16 and 25C, the part of the bill closest to the face starts dumping heat and dilating blood vessels take its temperature to around 6 degrees higher than the surrounding air. At 25C or higher, the front end of the bill chips in too and becomes up to 4 degrees hotter than ambient temperature.
It's clear that the bill can be used to shed heat, but it does so to a record-breaking level. At 21C, in still air, the beak radiates at least 25% as much heat as the toucan's metabolism actually produces. With an air speed of 12 miles per hour, it can jettison as much as 400% - four times as much as its body generates. This astonishing range is the largest known for any animal. For contrast, the elephant's ears can only lose around 9-91% of the heat it produces at rest.
The fact that the bill is most efficient when air flows around it surely helps the toucan in flight, a time when it requires huge amounts of energy and produces around 10-12 times as much heat as it does normally. But at rest, such an effective radiator puts the toucan in danger of inadvertently losing too much heat when it gets too cold, even if its blood vessels are shut off.
To prevent that from happening, they tuck their giant bills underneath their wings and bend their tails over their backs. Not only do they shut off the valves of their radiator, but they cover it in a feathery duvet. If it gets too hot, the birds can even regulate their bill temperature in their sleep, dumping heat very quickly and briefly when they became too warm.
Adult toucans can adjust its bill so finely that it can account for no more than 5% of the heat lost from its body. For very short periods of time, it can even account for virtually all of the vanishing heat. But toucans only gain this ability when they start growing. In their first four or five weeks of life, their bills are a fairly normal size for their body and only later does it reach its disproportionate bulk.
Tattersall found that two juvenile toucans, whose bills were actively growing. Youngsters lack this fine control and they're particularly bad at shutting off their radiators, when it gets too cold. It's not for lack of need either - the naked hatchlings don't gain protective plumes until 4 weeks of age, and aren't brooded by adults. They shiver at even balmy temperatures of 26-27C, so the ability to regulate their body temperature would certainly come in useful.
The trouble is that they simply can't do it. To actually grow the bill in the first place, they need to deliver a steady, roaring torrent of blood to it. Tattersall thinks that they just can't get away with shutting off that supply, even if it means staying losing too much heat.
Reference: Scienceto be published soon at 10.1126/science.1175553
All images by Glenn Tattersall
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