Being a bitch is somehow costly in biological terms. Females 'going' from one male to another are more exposed to injury, sexually transmitted diseases and predators, not to mention the wasted energy.

Still, polyandry (females mating with several males) is widespread in nature. In 1 % of the birds, polyandry is the rule: nandu, cassowaries, some shore birds (like phalaropes), lily-trotters or buttonquails. Many amphibian and reptile females, too, are highly promiscuous, but so are social spiders and insects, like beetles attacking stored food.

Now a study made at the universities of Leeds and Exeter has found that polyandry offers higher genetic health for the female's grandchildren, a genetic solution for species that either accidentally inbreed, or start populations from small colonies.

A mathematical model found a significant genetic advantage of polyandry for species where inbreeding is widespread. "Not only is mating dangerous enough for females, but most species covered by our model should be able to get all the sperm they need for a lifetime's reproduction from a single mating, so it has puzzled scientists as to why this type of behavior would be so common. You would think that their time would be better spent foraging for food, or at least avoiding being eaten by predators," said Dr. Stephen Cornell (Leeds).

Some species are only apparently polyandrous, due to 'paternity biasing', as the female, even if mating with more males, will use just the sperm from the 'preferred' male to fertilize her eggs, or will take care only of the offspring fathered by her chosen mate. But many species cannot fertilize preferentially their eggs, being genetically polyandrous.

Because the female's offspring are only 50 % siblings in the case of the polyandrous females, their higher diversity in inbreeding conditions means their offspring are less prone to genetic defects (as the brothers and sisters will mate between them, so that the "grandchildren" will be less likely to receive a double dose of harmful genes).

"It's common for animals to breed with siblings if no other suitable mates can be found. For example, female bruchid beetles - a major pest in Africa that infest stores of dried beans - move on after mating to inhabit a new patch of food, and in the new colony inbreeding may be inevitable until new beetles arrive." said Cornell.

"It's well known that male animals mate with multiple females to sire as many descendants as possible. But females are playing a very different game. They are normally expected to focus on quality, rather than quantity of mates. It's all about weighing up the costs of additional mates and in many species the benefits outweigh the risks in ensuring the genetic line is continued.", said Dr. Tom Tregenza.