The blue-footed booby is an extraordinary-looking bird. It has fairly dull plumage but strikingly coloured blue legs and feet. What could be the evolutionary benefit of such a conspicuous feature? Both sexes have blue feet so they don鈥檛 seem to be for impressing potential mates.
鈥� Although not obvious at first sight, during courtship blue-footed boobies (Sula nebouxii) are dimorphic in terms of their feet: male feet are brighter and more of a greenish-blue, while the females have duller feet that are bluish.
The birds exhibit their feet to prospective partners in a series of courtship displays. These include a kind of ritualised strutting around that allows them to show off their feet, plus stylised or 鈥渟alute鈥� landings which serve the same purpose.
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I am a member of a research group that studies the sexual behaviour of the blue-footed booby. In one experiment, we altered the colour of the courting males鈥� feet and recorded the females鈥� response.
Females paired to males with duller feet were less enthusiastic about courtship and less likely to copulate compared with females paired to males with normal, brightly coloured feet. Similarly, when we altered the females鈥� feet to a dull blue, males became less interested in courting them. Birds in poor health often have dull blue feet.
鈥淔emale boobies paired to males with duller feet were less enthusiastic about courtship鈥�
What鈥檚 more, females whose mates had dull blue feet produced smaller eggs, and their chicks had a poorer immune response compared with normal females. This may sound surprising, but it is in accordance with theoretical expectations.
All this suggests that males are probably under strong selection pressure to maintain greenish-blue feet during courtship. This will ensure not only that they copulate successfully but also that their mates will lay big, healthy eggs. Overall, our results suggest that foot colour is a trait maintained by mutual male and female preferences.
Roxana Torres, Institute of Ecology, National Autonomous University of Mexico
鈥� Both male and female Sula nebouxii have blue feet, but it is the male that presents his feet prominently in courtship. This, in effect, is a way of saying that he is of the same species as the female.
I cannot offer any specific reason why the blue-footed booby has blue feet, but I would point out that foot colour does seem to be significant in the genus 鈥� there is an equally striking red-footed booby, Sula sula. This suggests that as members of the genus evolved, they adapted to different ecological niches which, in turn, meant that there was an advantage in the birds splitting into different 鈥渢ribes鈥� which could only mate with their own kind.
鈥淔oot colour does seem to be significant in boobies 鈥� there is an equally striking red-footed booby鈥�
This is as an example of what is called , where one species evolves into two within a shared territory. In contrast, occurs because populations become isolated from each other. For sympatric evolution to succeed, it is essential that some sort of difference between the species arises so that a bird can distinguish between a bird of a related species and one of its own kind.
Guy Cox, Associate Professor, Australian Centre for Microscopy & Microanalysis, University of Sydney, Australia