The tale of how the peacock got his eyespots has taken a new turn.
His shimmering train of feathers tipped with eye-shaped spots ranks among the most cited examples of what Darwin called sexual selection. In this singles-bar approach to evolution, flashy plumage and other ornaments arise not because they enhance survival of the fittest but because they favor reproduction of the sexiest.
Basic principles aren’t in doubt for the peacock exemplar. Yet “everybody uses it without knowing much about how it works,” says Roslyn Dakin of Queen’s University in Kingston, Canada.
Watching how peafowl courtship played out in three clusters of free-roaming birds, she found that shimmying a train with especially high numbers of eyespots did not — repeat, did not — seem to improve a male’s chances of dazzling a female into mating.
That disconnect flies in the face of a series of classic experiments on peacock courtship, Dakin and coauthor Robert Montgomerie, also of Queen’s, acknowledge in an upcoming Animal Behaviour. They also report, however, additional work suggesting a new explanation for why peahens sometimes don’t appear to care about eyespot number.
Eyespots seemed a good predictor of a male’s chances of success in past studies of peafowl in England. A female cruising among males routinely picked the one who showed her the most eyespots, says a pioneer of peacock science, Marion Petrie of the University of Newcastle in England. Among her other eyespot studies, snipping 20 feather tips out of males’ trains ruined courtship success. Fastening the eyespots back into the train put the males back in the game.
“I suspect the eyespot measure must be a crude approximation for train elaboration,” Petrie says.
Feathers ruffled in 2008, however, when Japanese researchers studying feral peacocks reported finding no courtship advantage for eyespot number in seven years of data. Such displays may have become “obsolete” as a signal to peahens, the team proposed in Animal Behaviour.
Dakin and Montgomerie have now found similar results in two peafowl populations in Canada and one in the United States. Dakin repeated part of Petrie’s eyespot-snipping experiment. As predicted, males deprived of 20 of their eyespots — leaving fewer than 138 in the displays — managed to mate fewer times overall than fully eyespotted males.
When Dakin and Montgomerie pooled their eyespot data with other published reports, eyespot effects didn’t show up among the top 75 percent most successful maters of 102 peacocks. Yet the most eyespot-challenged birds, flashing numbers only in the 120s and 130s, rarely had any mating success at all. “It certainly looks like a threshold,” Dakin says, although she doesn’t have data on whether individual peahens have a response threshold.
The threshold idea certainly makes sense at first glance, says Adeline Loyau, a peacock researcher at the CNRS research station in Moulis, France. The struggle to understand the long-familiar peacock, she adds, “suggests that we are still far from unraveling the mechanisms of mate choice.”