Abstract
Ever since the first human saw a toucan, people have wondered why a bird would have such an awkwardlooking bill. Adding to the confusion is the fact that, once having grown the beak, the ingenious toucan puts it to use in a variety of ways, many of which are probably unrelated to its original function. We'd like to leap into the resulting fray with a suggestion of our own: we think the toucan evolved its bill as a cooling system.
The toucan's bill is a light, porous structure rich in blood vessels, internally similar to the smaller beaks of other softbills. Like all birds, the toucan has no sweat glands and hence must pant when overheated, using the evaporation of moisture within its open mouth as a cooling mechanism. But the toucan has a real problem regulating its body temperature; not only does it dwell in the hot, humid rain forests of Central and South America, where evaporation is a slow and difficult process, but its plumage consists of fine, almost fur-like feathers which can act like an insulating blanket. Perhaps the toucan's answer to the dilemma was the development of a long beak with broad internal surface area, allowing the blood circulating therein to cool more efficiently as the bird gaped.
There's little doubt that the bill is, at least sometimes, put to this purpose. During the record-breaking heat wave of New Orleans' summer of 1984, our pet emerald toucanet indulged in showers and baths up to five times a day. So closely does this bird link a wet bill with coolness that he has learned to ask for the shower mister by gaping as widely as he can, as if yawning. Once under the spray, he concentrates on catching the droplets within his rnandi bles by yawning and stretching vigorously.
But is there any evidence that this is the original reason for the bill's evolution? We believe so. In addition to the environmental factors already described, there is a rough correlation between habitat and biJJ size. The larger toucans of the genus Rampbastos live in the hotter jungle regions, while the smaller, shorter-billed toucanets generally inhabit the cooler mountain slopes. This trend reflects the need for greater heat loss (and hence larger bills) in the hotter, more humid sea-level rainforests. None of the alternate hypotheses addresses this observation.
Of course, competing theories abound. The most obvious was also the first discredited; like most birds, the toucan has a poor sense of smell.
Another dimly regarded possibility is that old standby, "protective coloration," which has been applied at one time or another to every unexplained bit of plumage in the class Aves. The first ornithologist to perform an in-depth field study on toucans dismissed this theory out of hand. In The Life History of the Toucan, Josselyn Van Tyne writes, " ... the actions of a toucan usually defeat any [such] purpose .... At the first sign of danger, the toucan almost invariably begins to bob about on its perch and croak loudly, advertising its presence to all within half a mile or more.'' Protective coloration is useless to an animal that can't sit still.
A more seriously considered idea is that the bill evolved for purposes of identification. In the field, some species can scarcely be distinguished from one another unless the bill is visible; the keel-biUed and the chestnut-mandibled (Swainson's) are such a pair. Yet the species identification thesis seems somehow inadequate. While it can hardly be argued that the toucans don't use this handy tool for recognition, we don't feel this theory tells us why the bill came about in the first place. After all, the fact that humans might use the shape of a nose to help recognize one another doesn't mean that Roman noses evolved for that purpose.
At any rate, Van Tyne's studies suggest that the bill developed before the markings. In thegenusRamphastos, for instance, we find that the most primitive species, the red-breasted toucan, has a dull green biJJ, while colorful patterns run riot on the beaks of more advanced species such as the keel-billed. Species recognition, in that case, is almost certainly a secondary function of the overgrown bill.