Ladies! When you started stepping out with your beau, did he make an effort to maintain a basic standard of personal hygiene and appearance? And does he, now that your affair has matured, sport 10-year-old T-shirts and running shoes that should be buried in concrete?

Take heart: some male birds adopt a similar approach to relationships. While a-wooing, the plumage of the male rock ptarmigan (Lagopus mutus) is dazzling white. But once this bird has settled down with a mate, he wastes no time in getting as muddy as possible, researchers now report.

For the ptarmigan this behaviour has more to do with self-preservation than sloth. Plumage-soiling is a way for males to camouflage themselves when the danger of attracting predators with their conspicuous garb outweighs the benefits of attracting females, say Robert Montgomerie, of Queen's University, and colleagues.1

This is the first time that a bird has been found to get grimy in the name of disguise. "We can spot a white male from a kilometre away," says Montgomerie. "When they're dirty, we walk right by them."

In the winter, rock ptarmigans' white feathers blend in with their snowy habitat on the Canadian tundra. As the snow melts in early June, females turn a mottled brown, and so stay camouflaged. Males hang on to their old outfit for several weeks after the snows have melted - their cryptic winter plumage doubles as vivid breeding plumage.

Male rock ptarmigans pay a heavy price for their finery. Their death rate to predators is one of the highest known in birds: in some places, about a quarter of males are eaten in June and July.

So white plumage tells females that a male can avoid predators, and is a good mate. Once a female is incubating her clutch, and can no longer be fertilized, her mate sheds his attention-grabbing appearance with the aid of mud and dirty water. "They look pretty bedraggled," comments Montgomerie.

Males that mate with more than one female, and so have an extended breeding schedule, stay clean longer than monogamous males. So, too, do bachelors, who, instead of defending a territory, roam around trying to mate sneakily with paired females.

"The timing fits precisely with the idea that the male remains conspicuous for as long as he is trying to attract females," says behavioural ecologist Malte Andersson of the University of Gothenberg, Sweden. This would not be the case, for example, if the male's white plumage was a signal to predators that he was so healthy he was not worth chasing, Andersson points out.

Rolling in the mud is quicker than changing feathers, taking at most a day. Moulting lasts 2-3 weeks. It is also more flexible - when a female lost her clutch, and became sexually receptive again, her grubby mate rapidly spruced himself up.

Males finally moult into cryptic summer plumage in July, when the year's breeding opportunities are past.