Male spiders amputate organs, run faster.

Tiny male spiders of a species common to the southeastern United States routinely remove one of their two oversize external sex organs. It's an extreme act, but one that apparently enables them to run faster and longer, a potential advantage for winning mates, researchers say.

As is typical in spiders, a male of Tidarren sisyphoides develops two protrusions with hollow tips, or pedipalps, on the front of his body for delivering sperm, explains Duncan J. Irschick of Tulane University in New Orleans. The male spider grows to only about one-hundredth the size of a female, yet one pedipalp accounts for some 10 percent of his body mass.

Although these spiders haven't been studied in depth, researchers had noticed that within hours after a young male's penultimate molt prior to mating, he amputates one of his pedipalps. He does this by attaching a strand of web silk to one pedipalp, tightens the silk thread by turning in circles, and then pushing at the pedipalp with his legs.

A potential benefit of such a practice became apparent in video recordings of the spiders made by Margarita Ramos, who worked with Irschick and Terry Christenson at Tulane. Ramos filmed 16 males sprinting along a strand of spider silk and found that their maximum speed increased 44 percent after losing a pedipalp. When Ramos chased young spiders around a sheet of paper in an endurance test, single-pedipalp males ran nearly three times as far as did young males that still had both organs.

Irschick rates the spiders as an "extreme example" of a species that "got stuck in a massive evolutionary conflict and had to evolve a behavior to get out." As males shrank and females enlarged during the course of T. sisyphoides' evolution, the species probably couldn't afford to reduce pedipalp size too much, says Irschick.

Ramos and her advisors report their findings in the April 6 Proceedings of the National Academy of Sciences.