Dirty little secrets: "dress for success" is the key to the mating, game among Arctic ptarmigan

Natural History, June, 2004 by Bruce Lyon, Robert Montgomerie

Although conspicuous plumage appears important for mating success, we do not yet know which of Darwin's two main mechanisms of sexual selection are at work--female choice of mate, or competition among males. If the females are calling the shots, their choice may merely ensure that their own male offspring will also be attractive to females and, therefore, have higher fitness. But it's also possible that at the same time their offspring will be inheriting some strong survival skills. By "strutting his stuff" with clean, white plumage, a male could be advertising his ability to avoid predation despite being highly conspicuous. And such open risk-taking would be an honest signal, not a deception or a bluff, because staying alive while bearing such conspicuous plumage is proof of good survival skills.

The other possibility is that clean white plumage mainly helps keep male competitors at bay. Perhaps it functions as an aggressive signal between males, serving notice to would-be philanderers to keep away while the female is fertile. One way to distinguish between female choice and male-male competition would be to create dirty males experimentally, early in the season, and examine the consequences with respect to mate choice and interactions with other males. That experiment is much trickier than it sounds, however. On several occasions we actually tried to "dirty" some males with so called indelible marker pens, but we failed miserably: the males were just too good at keeping their feathers clean. Those experiments highlighted the importance of clean white plumage for these birds, but they have also taught us just how hard it can be to get at all of the rock ptarmigan's dirty little secrets.

BRUCE LYON is an associate professor of biology at the University of California, Santa Cruz, where he studies the evolution of social signals and reproductive strategies in birds that breed in open, treeless habitats. ROBERT MONTGOMERIE is a professor of biology and Killam Research Fellow at Queen's University in Kingston, Ontario, where he investigates sexual selection in a wide variety of animals, including humans.