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Wednesday, February 16, 2011

Practice Makes Perfect: Spiders Rehearse Sex

Would it surprise you to learn that some animals routinely engage in what can best be described as heavy petting?  Indeed male and female spiders of the species Anelosimus studiosus often get to third base without scoring a proverbial home run.  This is because males reach sexual maturity more quickly than females and begin the process of finding a mate before the fair sex is capable of reproduction. So like many human teenagers over the ages, they just make out instead. Or if you are a scientist: they pseudocopulate ("pseudo" means "false" and "copulate" means .. well, you know what it means).

Here is how teen angst works for spiders: sexually mature males leave the nest to find a mate, scrap it up with other suitors, and remain with the contested female if the male is tough enough to drive away the competition.  The only problem is that the female is not yet old enough to mate.  Rather than pass the time until she sexually matures with platonic affection, the male begins drumming on her web in a particularly seductive way and she responds by offering her body in a pose that leaves little doubt as to her intentions.  The male climbs beneath her and assumes the appropriate position but doesn't put his you-know-what into her you-know-what. (It's called pseudocopulation for a reason!)  The pair repeat the ritual until at last the female reaches maturity and actual reproduction ensues.

Jonathan Pruitt and Susan Riechert, of the University of Tennessee, wondered what all the making out was about.  After all, it seems like rather a lot of wasted energy at first glance.  Pruitt and Riechert suspected that the spiders might in fact be rehearsing for copulation -- that by practicing spiders might be better at closing the deal when the time finally arrives.  As the authors discuss in a forthcoming article in the journal Animal Behaviour, being good at copulation might be particularly important for a male, who needs to be quick about it before a stronger competitor comes along to drive him off.  If she gets cozy with subsequent males, his sperm are more likely to do the fertilizing if he manages to inseminate her first.

There is also the rather unpleasant possibility that if he isn't quick enough the female will eat him.  And you thought dating humans was tough!

Pruitt and Riechert found that "nonconceptive" sexual experience resulted in speedier mating in sexually mature pairs and that spiders with multiple experiences were even quicker to mate.  This would seem to confer the previously described advantages to males, but what do females get from these quickies?  Females with nonconceptive sexual experiences produced heavier egg masses. Pruitt and Riechert suggest that females may invest more heavily in offspring sired by high-quality males.

How can male spiders demonstrate their quality? Possibly through the stamina it takes to copulate after so much pseudocopulatory foreplay.

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