What makes an ideal man? For some women, it's a charming personality; others just want to see a nice set of abs. Things aren't quite so complicated in the rest of the animal kingdom. In most species, every female prizes the same trait in a male, whether it be bright plumage or a pretty song. So researchers have been surprised to discover that female yellowthroats don't always agree on what turns them on--a finding that may offer a window onto speciation.
Male yellowthroats sport large black masks and bright yellow bibs. Vibrant colors result from pigments called carotenoids, which are also antioxidants and thus a sign of health. So it was little surprise when biologist Corey Freeman-Gallant of Skidmore College in Saratoga Springs, New York, and colleagues found in 2001 that local female yellowthroats preferred males with the most vivid yellow bibs. But in the same year, biologist Peter Dunn of the University of Wisconsin, Milwaukee, published something different about his local population of yellowthroats: Females seemed to be targeting the size of males' black masks to determine whether they were worth a fling. That didn't make sense, because the black masks are generated from melanin, which has no connection to health. "I was taken aback," says Dunn.
To confirm the findings, Dunn and colleagues brought yellowthroats from the New York state and the Wisconsin populations back to aviaries near Skidmore College. They spied on the females from behind a blind a few meters away as the birds were presented with multiple bachelors, some with big masks, some with bright yellow throats. Dunn measured the amount of time females spent ogling the various males and confirms in a future issue of the Journal of Avian Biology that both he and Freeman-Gallant were correct. New York state yellowthroats want males with large yellow bibs, and Wisconsin yellowthroats prefer males with big black masks.
That still leaves unanswered the question of why females would be drawn to the masks. It's possible that in Wisconsin, masks are an even better indicator of fitness than bibs are, says Dunn, but further research is needed.
Regardless of the reason, the fact that female preferences differ at all is very unusual, notes evolutionary biologist Michael Webster of Washington State University in Pullman. Variation in female preferences for male traits has long been proposed as an evolutionary force that could lead to speciation, he says, and such a divergence may be under way in yellowthroats. "The first step is to show that female mating preferences do vary geographically, and this study is doing that," says Webster.