Spider play sex isn't all fun and games

They say practice makes perfect, and that's a lesson comb-footed spiders have apparently taken to heart: young spiders play at sex before sexual maturity. The behaviour has a downside for young males, though, who may later be too tired for the real thing.

The aptly named Anelosimus studiosus is the only spider known to play in this way. An adult male hangs out on the web of a juvenile female, waiting for her to mature sexually. While he waits he courts her, going so far as to "copulate" with her, even though her genital tract is still sealed, making it impossible to fertilise her eggs.

"This pays off for both," says Jonathan Pruitt of the University of Pittsburgh in Pennsylvania. Females that have played at sex when they were young produce larger egg sacs. And if either the male or female have played at sex, courtship goes faster.

Pruitt has found that play sex has its disadvantages ? at least for the males. He put males and young females together for four hours, then added a second male to the web. Males that had had more play sex were more likely to be driven off by the new male, leaving them without a mate.

Males must balance the benefits of play sex against the risk of being bested by another male, says Pruitt. If they are the only male in a colony, they can practice sex with each of the females. If other males are lurking, the risk of being chased off may be too great.

"The best strategy then is to let someone else do it for you, and then supplant them," he adds.

We know very little about invertebrate play, says Elisabetta Palagi of the University of Pisa in Italy. In 2006 Palagi found evidence that juvenile female paper wasps play at being dominant, months before they actually start a new nest. "Play is studied much more in vertebrates, particularly social mammals," she says.

Journal reference: Ethology, DOI: 10.1111/j.1439-0310.2011.01980.x

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