Since Darwin, the peacock exhibiting an elongated tailcomposed of ocelli has been considered a prime example of the strengthof sexual selection. Professor Marion Petrie's classical studies haveshown that females prefer males with a high number of ocelli. However,a remaining question concerning the role played by ocelli is howpeahens value their number. New research published today in Ethologydescribes that females may actually assess ocelli density.
AdelineLoyau, Michel Saint Jalme and Gabriele Sorci of the National Museum ofNatural History and the Laboratory of Evolutive Parasitology, Paris,have been studying sexual selection on free-ranging common peafowl toelucidate how females choose their mate. They took pictures ofdisplaying peacocks to count the number of ocelli, and captured them tomeasure tail length. They also made behavioural observations to valuemale displaying activity and male mating success.
"Preferredmales were those exhibiting the higher number of ocelli in the train,but surprisingly females seemed to prefer males with shorter tails.This was unpredictable because we also found that the longer the train,the more dominant the male," they say. The female preference for bothhigh number of ocelli and shorter tail gave the idea that females mayactually prefer the visual perception of a dense cluster of ocelli thana diluted number of ocelli over a large surface. "We calculated theocelli density and found that it did explain female choice."
Inthe peacock, the ocelli density of the train can only be assessed byfemales when males spread their trains during the courtship display."In this species, the expression of the ornament is modulated by theexpression of the behaviour. To be chosen, a peacock has to bebeautiful but also has to be able to show how beautiful he is. It isnot surprising that female preference is also driven by malebehaviour." Indeed, they showed that male success was determined byboth his ocelli density and his displaying activity. They investigatedthis preference further and demonstrated that these two cues providepeahens with information about male health. "In other words, it'sbeneficial for a female to mate with handsome and sportive mates," theyconclude with humour, "because these males are in better health."
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