Animal swingers play the mating game

By Jeanna Bryner

updated 3:39 a.m. ET Nov. 16, 2006

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 lighthearted, 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.

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