Relative to its body size, the toco toucan dons the largest bill of any bird. This magnificent hood ornament serves the toucan well. It functions as a refined feeding tool that enables the toucan to skin fruit and snare prey. It can be wielded as a warning flag to discourage rivals and ward off predators. And, as Charles Darwin noted, the enormity of the toucan's beak may serve as a bright beacon of virility to potential mates:
"...toucans may owe the enormous size of their beaks to sexual selection, for the sake of displaying the diversified and vivid stripes of colour, with which these organs are ornamented..." ~ from On the Origin of Species by Charles Darwin.
Now, a new study led by Glenn Tattersall of Brock University, Ontario, has revealed that the toucan's bill boasts yet another purpose. In the same way the oversized ears of an elephant enable it to dissipate excess body heat, the bill of a toucan provides a quick way to expel surplus warmth and maintain a constant, comfortable body temperature.
The maintenance of a constant, comfortable body temperature—a feat known as endothermy—is an adaptation particular to birds and mammals. Endothermy offers its possessors a unique freedom. By maintaining their own internal body temperature, endotherms (also known as "warm-blooded animals") can inhabit a wider range of environments than their cold-blooded cousins—the amphibians, reptiles, and invertebrates. While cold-blooded creatures are confined to habitats in which they neither over-heat nor over-cool, mammals and birds thrive in some of the harshest climates on the planet—from the icy polar regions to the balmy tropics.
Endotherms have a repertoire of adaptations that enable them to trap or release heat so their body temperature remains within an optimal range. Such tricks include dilation and contraction of blood vessels, sweating, panting, shivering. Some animals even have what's known as thermal radiators—structures that are specially designed to dump heat on cue into the surrounding environment. These thermal radiators—also referred to as biological radiators—are often enlarged, uninsulated structures, rich in blood vessels: like the ears of elephants or the bills of toucans.
It turns out that the toco toucan's massive bill is the among the most efficient thermal radiators in the animal kingdom, capable of dumping as much as 400% of the toucan's resting body heat. Compared to elephant's ears, which can offload a maximum of 91% of the animal's resting body heat, the degree-disposing power of the toucan's bill is astonishing.
In their experiments, Tattersall and his colleagues used thermal cameras to record the heat signatures of six toucans (4 adults, 2 juveniles) as they raised the temperatures in the bird's environment from 10 to 35 C. They recorded surface temperature at several key locations on each bird's body: the base and tip of the bill, the eye, and a patch of feathers on the back. They found that at temperatures below 16C, the surface temperature of the bird's back and its bill were both about the same—slightly above the temperature of the surrounding air. But above 16C, the base of the bill heated up and radiated excess body heat to quell overheating. When the ambient temperature climbed to 25C, the heat shedding capacity of the bill shifted into overdrive. The blood vessels in the tip of the bill became dilated, heating to 4 degrees above higher than ambient temperature.
Refs:
Tattersall, G., Andrade, D., & Abe, A. (2009). Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator Science, 325 (5939), 468-470 DOI: 10.1126/science.1175553
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