SALT LAKE CITY — When pairs of young comb-footed spiders engage in an arachnid version of heavy petting, the males gain experience that appears to pay off later.
A male spider that repeatedly courts and mock-mates with a not-quite-mature female ends up reaping benefits later, said Jonathan Pruitt of the University of California, Davis. Speaking January 4 at the annual meeting of the Society for Integrative and Comparative Biology, he proposed that such seemingly pointless spider encounters, which can’t produce offspring, may resemble other young animals’ racing and wrestling by providing practice for life’s future tasks.
“I thought it would sound silly if I called my talk ‘Spider Sex Play,’” Pruitt said, “but that’s essentially what it is.” And he ranked it as the first example of any kind of play behavior demonstrated in spiders.
Among the Anelosimus studiosus spiders, which live and spin webs along rivers and under bridges from Maine to Patagonia, females don’t develop an opening to their reproductive tract until their final molt. Males mature faster and hang around not-quite-mature females, often going through most of the mating routine.
During almost-sex, the male doesn’t load his sex organs with sperm but performs a courtship display by drumming the female’s web with his legs and sex organs. If she assumes a cooperative posture, he approximates a mating position too. He then taps her body where the reproductive tract will eventually open.
Sexual behavior even at this stage brings some risks, such as a chance that the male spider will be killed by the potentially cannibalistic female. So sex that can’t possibly produce offspring remains puzzling, Pruitt said. To test the idea that such encounters might be more than wishful mistakes, he and Susan Riechert of the University of Tennessee, Knoxville set up some young spider pairs for near-matings but kept other maturing individuals isolated.
When all the spiders finally developed, the researchers observed real matings. If either spider partner had participated at least once in mock sex, a pair tended to reach the point of real mating faster than two inexperienced spiders did. The brisk proceeding wasn’t a matter of knowing a particular partner though, Pruitt said. Even pretend-sex with a different individual tended to hasten the process.
Speed should benefit the male by reducing the opportunity for some intruder to dash in and displace him, Pruitt said. Timing matters because the first male will father most of the eggs in a particular egg case.
And experienced females typically invested more in those egg cases as measured by weight, the researchers found.
The test didn’t address whether females might find some benefit too, but Pruitt speculated that they might. For example, if mock-mating turns out to be a sign of a superior male, then a female that rushes to consummate a real pairing reduces her chances of being distracted by some inferior interloper.
Spider shenanigans reminded meeting attendee Ned Place of Cornell University of chipmunks and other species of small mammals in which males emerge first from hibernation. These eager males are already waiting when the first females get moving again. Mock-mating in the spiders might confer a similar advantage by increasing the chances that a male won’t miss an opportunity to mate.
