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

The biologist and writer Julian Huxley, grandson of Darwin's great friend and supporter Thomas Henry Huxley, thought that the white plumage of the male ptarmigan might distract a predator away from the female. But when we tested that idea, pretending to be predators ourselves, the ptarmigan males led us toward the females rather than away. Of course, even though people have been serious ptarmigan predators for centuries, we might not be the best stand ins for natural predators.

Our close look at what was going on when individual males became dirty has, we think, shown why the transition to protective coloration is delayed. As do many other male birds, the male ptarmigan seems to gain mating advantages from conspicuous plumage. Perhaps females find such plumage more attractive, or perhaps it somehow helps a male prevail in competition with other males. Either way, more mating opportunities lead to greater reproductive success, or fitness. As a result, the trait of sporting bright plumage is passed on to the male offspring of bright-plumaged males.

Darwin distinguished this kind of natural selection by the term "sexual selection." In general, and more commonly, natural selection favors traits that enhance the survival of the individual in its environment, enabling it to reproduce. In the more specific case of sexual selection, selected traits directly enhance mating success (the male peacock's showy tail is a classic example). Such traits may actually reduce the individual's chances of long-term survival. Indeed, the mortality data suggest that among male ptarmigan, conspicuous plumage can lead to untimely death, but bright white plumage might still confer greater fitness, in the sense that males sporting white plumage father more offspring than less conspicuous birds do.

White plumage may be critical for attracting a mate, but even after pairing with a female during the breeding season, a male that keeps a clean profile may have an advantage. Adulterous matings appear to be common in ptarmigan, and so by remaining conspicuous the male may be able to better defend his territory against philandering neighbors and unmated males, or he may simply remain attractive to his mate when she is tempted by these intruders. So by staying white, a male could enhance his reproductive success. Once his mate's eggs are fertilized, though, he would have little to gain by maintaining his sexually alluring appearance.

What evidence can we offer that this account is correct? Within a given breeding season, the tinting of transformation from clean to dirty often varies dramatically among males, but it is tightly linked to the tinting of his mate's reproductive schedule: males get dirty when their mates are laying their eggs and will soon have no more eggs to fertilize. We have also observed a few males that obtained two mates in rapid succession. These polygamists got dirty later than monogamous males, remaining clean until their second female began laying.

We wondered what would happen if a female lost her eggs or nestlings to predators and began the nesting cycle anew. Would her now-dirty mate have a mating disadvantage? Although such re-nesting is extremely rare at our site, simply because the breeding season is so short, two intriguing cases suggest the answer. Within a day after their mates lost their nests to predators, two males cleaned up their act and went from "medium dirty" to immaculate. The reason, presumably, was that their mates became sexually receptive again. Thus plumage soiling not only provides instant camouflage but, unlike molting, is easily reversible.