Animal swingers play the mating game

Females in charge
For females, the drawbacks of sex with lots of partners include an increased probability of inbreeding, higher chances of predation, more risk of catching disease and physical injury or exhaustion from the frequent sex.

Even so, in some species, females "wear the pants." Called polyandry, by mating with multiple males, a mom can produce healthier offspring, and in some species, ensure devotion and help in child-raising by many fathers.

In a study of a mouselike marsupial, scientists found that by sleeping around, females had a better chance of finding males with good-quality sperm and high sperm counts [video]. "So that means that females that mate with lots of males get more of their offspring sired by the good-quality males that increase the baby survival," Fisher told LiveScience, referring to the mouselike marsupials.

In a lab study, researchers found that female guppies mating with four different males gave birth to 73 percent more young than their monogamous sisters. Plus, the young were more skilled at swimming in tandem with another fish and jetting out of trouble.

But strict polyandry, where one female guards a group of male mates, is rare. "The benefit for the female is quite clear, because if you have four husbands and you lay them each a clutch of eggs and they sit on it, you're doing much better than if you have one husband and you lay one clutch of eggs," Clutton-Brock said.  "What's harder to explain is what the benefit to the male is."

Deciding factors
As with humans, animal mating often relies on a careful dance in which males and females develop ways of getting what they want without going too far and ending up empty handed.

The female Australian hanging fly will allow males that provide larger nuptial gifts to copulate longer, and in turn transfer more sperm, skewing paternity. Males providing insufficient gifts get flung off by the female before insemination is complete.

A study of 14 water-strider species found that species in which males had flat stomachs and powerful forelimbs for clutching lovers mated more frequently than species where females were in charge — those with spines protruding from their back ends. When the females had the upper hand, mating occurred about twice a day compared with 20 times a day in the male-dominating species.

Find out how 10 animals have developed polygamous behaviors, many of which will cause a blush or two, in this LiveScience special: Top 10 Swingers of the Animal Kingdom.

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