The Toco Toucan (Ramphastos toco) is the largest species in the toucan family but not only that, it has the largest bill relative to body size of all birds. As with most things in Zoology the function of the bill has been hotly debated, even Darwin himself weighed in with an explanation of his own. He thought that the exaggerated size of the bill may have been due to sexual selection. Seems a little extreme though, especially when you consider all the adaptations for flight birds already exhibit (see here for the basics). Why then would natural selection begin to select individuals with heavy large beaks, surely the extra matings acquired due to the size of your “birdhood” would be offset by your reduced capacity for flight. Right?

Well that depends, what if there were other benefits to having this huge bill. Like for example thermoregulation. Like other enlarged body parts used for thermoregulation, like for example the enlarged ears of many desert dwelling mammals, the bill of the toucan is highly vascularised (supplied with blood vessels) and it seems the toucan has the ability to control the amount of blood flowing to the blood vessels around the bill.

Thermoregulation is somewhat of a hot topic (if you’ll pardon the pun), recently there has been some suggestion that the plates and spikes of the Stegosaurus may have been candidates for thermoregulatory function, where they had been previously thought to have been for defence. Similarly and somewhat more bizarrelythe long neck of many animals both extant and extinct has been discussed as a possible means of thermoregulaltion in this wonderful article by Wilkinson and Ruxton (2011).

So the long and the short of it? As with many aspects of the animal kingdom, without wanting to blunt Occam’s razor the simplest answer may not always be correct.

Author

Keith McMahon: mcmahok[at]tcd.ie

Photo credit

http://www.sciencemag.org/content/325/5939/468.full.pdf