July 28, 2009 | David F. Coppedge

Sexual Selection Discounted in Toucan Bill

Darwin thought that the large bill of the toucan might be an ornament produced by his theory of sexual selection.  A new study says, rather, that the bill serves as a heat radiator the bird uses to control body temperature.  National Geographic News summarized a paper in Science that explained the process.1  The authors studied the toco toucan, the species with the largest beak, and watched it with infrared cameras.  The bird is able to flush blood into tiny blood vessels in the beak to get rid of excess heat.  The authors said, “Our results indicate that the toucan’s bill is, relative to its size, one of the largest thermal windows in the animal kingdom, rivaling elephants’ ears in its ability to radiate body heat.”
    The authors quoted Darwin in their introduction.  In The Descent of Man, Darwin had speculated, “toucans may owe the enormous size of their beaks to sexual selection, for the sake of displaying the diversified and vivid stripes of colour with which these organs are ornamented.”  The discovery of a heat-transfer function for the large bill discredits sexual selection theory.  David Tyler commented on this shift in thinking from evolutionary principles to design principles at Access Research Network.  His article includes a photo of the handsome bird; the National Geographic article shows it glowing warm in infrared light.  For some reason, NG felt it worthwhile to speculate that dinosaurs used similar heat regulation methods.


1.  Tattersall, Andrade and Abe, “Heat Exchange from the Toucan Bill Reveals a Controllable Vascular Thermal Radiator,” Science, 24 July 2009: Vol. 325. no. 5939, pp. 468-470, DOI: 10.1126/science.1175553.

Poor Charlie can’t seem to get anything right these days.  His simplistic speculations are turning out to be, well, simplistic speculations (a euphemism for just-so stories).

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Categories: Birds

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