When it comes to mating, wild animals make their own rules. From lionesses of East Africa that mate with many males before ovulating and committing their eggs, to male walruses that joust for several female partners, the animal kingdom is full of swingers.

In some human societies, sexual behavior akin to these animals would be shunned. Do these animals just not care what "society" thinks about their promiscuous behavior? Even the most domestic of animals, dogs, don't bat an eye before sniffing a fellow canine's butt or humping an owner's leg.

"Just about every animal is quite promiscuous," said Diana Fisher, a behavioral ecologist at the Australian National University.

Though such free animal love might appear light-hearted, survival and passing on genes are serious business in the animal world.

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Animal "personal ads" would reveal a conflict between males and females. Males want to mate with as many females as possible with the goal of fertilizing the most eggs. Females are a little more selective, preferring to hook up with the best males to fertilize their eggs.

One end result is polygyny—the most common mating strategy in the animal kingdom—in which males compete for access to a harem of breeding females. Sexual selection tends to favor adaptations that enhance reproductive success, including a large body size to boost success in pre-mating combat between males, and high sperm counts to up the chances of successful fertilization.

Rather than investing limited resources in inflating their bodies, females typically have a more conservative growth strategy and allocate more into the production and provisioning of offspring. By waiting on the sidelines during male-male jousting, the female can mate with the strongest male.

"Males fight it out and the best fighters get large harems of females," Tim Clutton-Brock, an animal ecologist at the University of Cambridge, told LiveScience. "If you just take the winner, you've got the best male. You don't need to sit back and choose carefully between males."

Human males must decide how much energy to put into weightlifting, for instance, to woo women, and how much they should focus on a career, which would benefit a family.

For polygynous horned beetles, the same trade-off exists between allocating resources to developing mate-winning weaponry in the form of horns and having more sperm to increase fertilization. In one study, when researchers cut off the beetles' horns, the pupa reacted by developing larger testes, supporting a theory known as resource allocation trade-off.

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 mouse-like 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 mouse-like 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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