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Keeping Out the Competition

25 June 2003 (All day)
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Danger zone. Male Argiope aurantia spiders make the ultimate sacrifice after mating with the larger female.

Some female spiders are notorious for devouring their mates during or after sex. Males, as you might expect, don't go gently. Now scientists report that males of one species seem to keel over intentionally. This death wish could be a strategy to plug female genitals with their corpses and block any competitors from mating, a radical and previously untested mating strategy.

In some other spider species with deadly couplings, males vigorously try to escape from cannibalistic females. This makes sense from an evolutionary standpoint: Surviving males could try to keep sowing their genetic oats by mating with other females. But a few spiders, such as male Australian redback spiders, practically somersault into female jaws.

Even given the bizarre antics in other species, evolutionary biologists Matthias Foellmer and Daphne Fairbairn at the University of California, Riverside, were surprised at what they found, during an unrelated study, in the garden-dwelling spider Argiope aurantia. The genitals of male spiders are a pair of extremities called pedipalps, found behind the fangs. In field and lab experiments, the researchers found that while some males attempted escape after inserting just one palp, all males that achieved both insertions died within 15 minutes of mating, even if the females didn't chow down on them. Other males often tried to pull out their dead rivals, but succeeded in only three out of 11 observed cases, the researchers report online 25 June in the Proceedings of the Royal Society: Biology Letters. Foellmer says this suggests that such sacrifices provide an evolutionary advantage, by safeguarding paternity.

There are still many secrets regarding male sacrifice during mating, says behavioral ecologist Jutta Schneider of the University of Bonn in Germany. If the second insertion affects paternity and sperm competition, she says, that would help explain why males are willing to go to such lengths.

Related sites
Argiope aurantia
Jutta Schneider's home page

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