If you’re like me, you’ll have asked yourself many times, “Jerry, why do toucans have such ridiculously big bills?” (See Figs. 1 and 2.) The first answer that might strike you is that the bill — like bodies, plumage ornaments, and other traits in birds — was driven to extreme size by sexual selection. But that won’t wash because male and female toucans have identical-sized bills, and if the male’s bill is brightly colored, so is the female’s. (There are several dozen species of toucans in five genera, all Central or South American.)
The next most obvious hypothesis is diet: maybe toucans eat a type of food that requires large bills to handle. But that doesn’t seem likely, either. Toucans are frugivores (fruit eaters; Fig. 3), and there’s no obvious reason why they need such a big bill to handle fruit. Indeed, there are many frugivorous birds, like parrots, and none of the others have such hypertrophied beaks.
A paper in today’s Science gives a clue: the bill is a radiator. You might suspect this because the bill is full of small blood vessels and is uninsulated. But can the birds control the flow of blood to the bill as needed?
The authors used thermal-imaging video cameras to record the temperature of the birds’ bills and bodies in rooms adjusted to different temperatures ranging from 10 degrees C to 34 degrees C (the species was the toco toucan, Ramphastos toco, which has the largest bill of all toucans). What the authors found, and what you can see in the movies below, is that the toucan can adjust blood flow to the bill depending on ambient temperature. When the room heats up, the surface of the bill heats up rapidly, allowing body heat to be dumped. The reverse happens at cooler temperatures.
When birds are flying — a time when they produce 10 to 12 times more metabolic heat than when they are resting — the bills can heat up by as much as 6 degrees Centigrade. And when the birds get ready to sleep, a time when their body temperature is reduced (this saves metabolic energy), the surface of the bill transiently heats up, allowing them to dump heat (see movie #1 below). There are also, as you can see in movie #2, transient changes in bill temperature during sleep, presumably to regulate body temperature (like many birds, the toucan tucks its bill under its feathers while asleep, presumably also to buffer heat loss).
Movie 1. Body heat moves to the bill right before the bird goes to sleep (note bill glowing bright orange, while body stays darker; temperature scale to right). “Heat dumping to the bill during entry into sleep. Thermal imaging video demonstrating transient movement of body heat to the bill during initiation of sleep in a toco toucan. Time-lapsed data obtained at 10-s intervals. Total frames = 724, total length = 2 hours.”
Movie 2. “Sleep-state transitions witnessed as changes in bill temperature. Thermal imaging video of transient changes in bill temperature that occur during sleep while the bill is tucked between the wings. Time-lapsed data obtained at 10-s intervals. Total frames = 724, total length = 2.7 hours..”
Now none of this answers the question of why the beaks are often brightly colored. That probably has the same answer to the question of why some other non-dimorphic birds, like parrots, are also brightly colored. There are lots of theories (ease of recognizing members of your own species is one), but, in short, we don’t know why. And we also don’t know why toucans, but no other species, have beaks this large. Why do toucans need to thermoregulate more than other species? A final question — one that probably can’t be answered — is this: did natural selection increase bill size because that increase directly helped with thermoregulation, or is the thermoregulatory function an exaptation, a beneficial byproduct of a feature selected for some other reason?
Fig. 1. The ridiculously large bill of the toucan. This is a keel-billed toucan, Ramphastos sulfuratus.
Fig. 2. The toco toucan, subject of this study. Is that a banana in your mouth or are you glad to see me?
Fig. 3. Toucan Sam
G. J. Tattersall, D. V. Andrade, A. S. Abe. 2009. Heat exchange from the toucan bill reveals a controllable vascular thermal radiator. Science 325:468-470.