Saturday, July 25, 2009

The toucan bill as a thermal radiator: adaptation or exaptation?

Have you ever visited a rainforest in the New World, such as in Costa Rica or elsewhere in Latin America? If so, it is a great chance you have seen a toucan, those big fruit eating birds with HUGE bills (see picture above).

The function of such large bills has been quite unknown, but now a new study published in Science indicates a surprising novel function: the bill might be important as thermal radiator (bottom figure). Using a thermal image camera of similar kind as we have bought to our lab, the researchers were able to demonstrate that the many blood vessels in the bill helped to quickly cool down the bird and get rid of excess heat. The birds could actually cool down by ten degrees in only a few minutes (yellow areas are high temperature areas in the bottom picture).

This study is interesting for several reasons. First, it shows how new techniques can help to answer old questions and give new answers (sometimes quite surprising, as in this case). Second, it shows that one has to have an open mind when trying to understand the adaptive functions of traits and what traits are "for". Most people, including myself, probably thinks that the bill evolved "for" the specific purpose of efficiently picking fruit. This might very well be the case, but once the bill have reached a certain size, it can also secondarily get another function in the form of a thermoregulator.

In this context, perhaps the thermoregulatory function is simply an exaptation, i. e. not a true adaptation in the language of paleontologist Stephen Jay Gould? Or perhaps the bill was further exaggerated once it had reached its "optimum" size from the perspective of fruit picking, and only the extra length is a true adaptation in the context of thermoregulation? Perhaps future phylogenetic comparative studies involving other toucan species with different bill sizes and the thermal image camera can provide some answers to these fascinating questions?

(Thanx Maren for the tip!)


  1. thanks for sharing this great piece of news Erik (and Maren), this indeed fascinating and really intriguing... I hope you guys can achieve to uncover the same kind of biological wonder with the most recent acquisition of the Svensson Lab...

  2. Hi Erik et al.
    Thanks for posting your take on the article. I liked the comments you made, and tend to concur with your latter assessment re: exaptations (ala Gould). Although I was tempted to use Gould's terminology in the paper, I did avoid it in favour of mainly discussing the proximate use of the bill as a thermoregulatory organ. I don't believe we argued that the bill evolved for the purpose of being an exchanger of body heat, but that it is potentially useful as such a device. Since many other birds have similar capacities to lose heat (albeit from smaller surface areas!), the simple fact that arises from our study is that the bill could be a source of heat loss in many/all birds.

    I'd be happy to discuss this with you in the future and to hear more of your thoughts on such matters!

    Glenn Tattersall (lead author of the toucan bill study)