A curl-crested aracari (Pteroglossus beauharnaesii) from the Tambopata Research Center. Why do the aracaris and the rest of the toucans have such big bills? At first glance, it might seem to be the result of sexual selection, the same force that drives many other tropical birds to exuberant coloration and ornamentation. However, in toucans, both males and females are essentially identical–they’re not sexually dimorphic–ruling out this possibility. A recent study in the journal Science suggested that the toucan’s bill performs quite an unexpected function–it acts as a radiator! By controlling blood flow to the bill, the toucan can control its body temperature, much in the same way an elephant uses its big ears to cool off. When the toucan warms up, say, after a long flight that increases metabolic heat by a factor of 10, it can send blood to the bill to dump that excess heat as the bill cools. The bill can also store heat, which explains why toucans sleep with their bills tucked under their feathers. The coolest thing about these findings might be how they were made. The researchers used thermal imaging cameras to measure blood flow to the bill, and changes in temperature, in a variety of rooms adjusted to different temperatures. Now, the biggest question that remains is whether the bill evolved first to regulate heat, or if it evolved for another reason, and the radiator function was a secondary benefit. That question will be much more difficult to answer.
Two more aracaris, part of a flock of 8 observed foraging through the Tambopata rainforest canopy. The bills of the two individuals shown here are different; however, it’s difficult to know if these differences are due to normal variation between individuals or some other factor, such as differences between adult and juvenile coloration. Whatever the case, male and female bills do not differ significantly among toucans, and there is no way to separate males from females apart from examining the sexual organs via endoscope; this provides strong evidence that the toucans’ large and colorful bills evolved that way for reasons unrelated to sex or sexual selection.