The trouble with small male spiders

For some spiders, the big guys have all the luck when it comes to mating. But that’s turning out to be not so much due to female choice as to old-fashioned mechanics.

Chad Johnson, now at the University of California, Davis, is trying to sort out the evolutionary pressures driving sexual cannibalism–the eating of one’s mate–in a North American fishing spider, Dolomedes triton.

“The females are huge, and the males are these wispy, long-legged things a third [the females’] size,” Johnson says. In large enclosures in his backyard, Johnson found that one in four males failed in courtship and became lunch instead.

Johnson tested an old notion that the females’ cannibalism counts as a form, albeit extreme, of rejecting suitors. Spider researchers often find that body size plays a role in mating. In general, the bigger the male, the better. However, when Johnson tallied females’ attacks, he found no evidence they were weeding out runts.

In the final stages of mating, big males did succeed more often, but Johnson attributes this to their long reach. From the typical mating position on a female’s back, a big male proved more able than a small one to stretch around her body and deposit sperm in the right place.