Was Darwin wrong about the sexual allure of the peacockās tail? A controversial study has found no evidence for the traditional view ā practically enshrined in evolutionary lore ā that peahens choose their partners depending on the quality of the peacocksā tails.
Mariko Takahashi and at the University of Tokyo in Japan studied peacocks and peahens in Izu Cactus Park, Shizuoka, from 1995 to 2001.
They judged tail quality in two ways ā first by simply measuring tail length, and secondly by taking photos of each male during the tail-fanning display ritual and counting the number of eyespots. Next they examined whether females chose mates with the best-quality tails.
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During the seven years of observation, Takahashiās team observed 268 successful matings. But surprisingly, they found that females mated with poor-quality peacocks as often as with āflashyā, high-quality males.
They conclude that the peacockās train is not the object of female sexual preference ā contradicting Darwinās .
Negative data
Behavioural ecologist at the University of Newcastle, UK, has dismissed the study.
āAll they have done is fail to find a relationship,ā she says. āThe authors seem to ignore the fact that three previous independent studies have found relationships between mating success and train morphology. Rather than consider what is unusual about their study, they conclude that peahens in general do not prefer males with elaborate trains.ā
Takahashi argues that it is the failure to find a relationship that makes her study so important. āUnfortunately because negative data have been seldom published, they are seldom discussed,ā she says.
Because it is ānegative dataā, Petrie says she doubts she would have been able to get this study published.
Hormonal factor
Takahashi points out that growth of the peacockās train is dependent on the absence of oestrogen rather than the presence of testosterone. She says this undermines the assumption that the train is a sexual signal.
āUntil now, who cared that the peacockās train was under oestrogen control?ā Takahashi says. āWe hope our paper will encourage others with [negative] data to publish.ā
But another peacock specialist, at the University Pierre and Marie Curie, Paris, echoes Petrieās concerns.
āMy major problem is that they didnāt consider the complexity of the signal,ā she says. āThey only looked at the number of eyespots and train length as a sign of train elaboration. The number of eyespots didnāt correlate with mating, so they concluded that the train signals nothing.ā
āComplex issueā
The peacockās train is a highly elaborate structure, Loyau adds.
āItās not just the number of eyespots ā itās the density of spots, itās the arrangement of patterns, itās colour ā they didnāt talk about the colour at all.ā
Louise Barrett at the University of Lethbridge in Alberta, Canada, is an executive editor on Animal Behaviour, the journal that published Takahashiās study.
āIt is perfectly true that the Takahashi study didnāt consider colour,ā she says. āBut they did consider many other aspects of tail elaboration and they failed to find any effect.ā
The arguments against Takahashiās study wrongly suggest that the latest findings ātrumpā previous results, says Barrett. āRather it illustrates that the story is more interesting and complex than we thought. One should never be too complacent and think that a problem has been solved,ā she says.
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