Was Darwin wrong about the sexual allure of the peacock鈥檚 tail? A controversial study has found no evidence for the traditional view 鈥� practically enshrined in evolutionary lore 鈥� that peahens choose their partners depending on the quality of the peacocks鈥� 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 鈥� 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, Takahashi鈥檚 team observed 268 successful matings. But surprisingly, they found that females mated with poor-quality peacocks as often as with 鈥渇lashy鈥�, high-quality males.
They conclude that the peacock鈥檚 train is not the object of female sexual preference 鈥� contradicting Darwin鈥檚 .
Negative data
Behavioural ecologist at the University of Newcastle, UK, has dismissed the study.
鈥淎ll they have done is fail to find a relationship,鈥� she says. 鈥淭he 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.鈥�
Takahashi argues that it is the failure to find a relationship that makes her study so important. 鈥淯nfortunately because negative data have been seldom published, they are seldom discussed,鈥� she says.
Because it is 鈥渘egative data鈥�, Petrie says she doubts she would have been able to get this study published.
Hormonal factor
Takahashi points out that growth of the peacock鈥檚 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.
鈥淯ntil now, who cared that the peacock鈥檚 train was under oestrogen control?鈥� Takahashi says. 鈥淲e hope our paper will encourage others with [negative] data to publish.鈥�
But another peacock specialist, at the University Pierre and Marie Curie, Paris, echoes Petrie鈥檚 concerns.
鈥淢y major problem is that they didn鈥檛 consider the complexity of the signal,鈥� she says. 鈥淭hey only looked at the number of eyespots and train length as a sign of train elaboration. The number of eyespots didn鈥檛 correlate with mating, so they concluded that the train signals nothing.鈥�
鈥楥omplex issue鈥�
The peacock鈥檚 train is a highly elaborate structure, Loyau adds.
鈥淚t鈥檚 not just the number of eyespots 鈥� it鈥檚 the density of spots, it鈥檚 the arrangement of patterns, it鈥檚 colour 鈥� they didn鈥檛 talk about the colour at all.鈥�
Louise Barrett at the University of Lethbridge in Alberta, Canada, is an executive editor on Animal Behaviour, the journal that published Takahashi鈥檚 study.
鈥淚t is perfectly true that the Takahashi study didn鈥檛 consider colour,鈥� she says. 鈥淏ut they did consider many other aspects of tail elaboration and they failed to find any effect.鈥�
The arguments against Takahashi鈥檚 study wrongly suggest that the latest findings 鈥渢rump鈥� previous results, says Barrett. 鈥淩ather 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,鈥� she says.
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