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Spider males good for mating, food

For female Mediterranean tarantulas, eating a male means having tougher and more baby spiders

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7:29pm, October 21, 2008
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One’s company and two’s — lunch.

In nature, female Mediterranean tarantulas rarely eat the first male they mate with, reports Jordi Moya-Lara±o of the Arid Zone Experimental Station in Almería, Spain. The suitor that shows up next, though, faces triple the risk of becoming a meal instead of a mate.

About a third of the females studied in their natural habitats ate at least one suitor, Moya-Lara±o and his colleagues report online October 22 in PLoS ONE. And dining instead of mating worked out well for the females.

Females that ate a male had more young and had them earlier than females that experimenters deprived of their male meal, Moya-Lara±o says. That headstart on laying eggs let the spiderlings hatch earlier and grow bigger and tougher than offspring of noncannibals. When researchers staged fights between the offspring of cannibals and noncannibals, the cannibals’ youngsters won.

“This is the first comprehensive study of fitness consequences of sexual cannibalism in nature [under field conditions],” says Mike Maxwell of National University in La Jolla, Calif., a behavioral ecologist who studies sexual cannibalism in praying mantises.

Just how sexual cannibalism has evolved remains a puzzle, says Moya-Lara±o, as the benefit of eating a potential mate has been hard to pin down, especially with studies done in natural settings. Many of the experiments have taken place in the lab, but Moya-Lara±o and his colleagues spent two field seasons studying spiders in the natural habitats as well as doing lab experiments.

Sparse food could favor the evolution of cannibalism in the Lycosa tarantula spiders, Moya-Lara±o says. They live in arid places, and male spiders amount to what he calls “high-quality prey,” a substantial nutritional boost.

As it turns out, the female Mediterranean tarantulas do reap the benefit, he notes. Males end up providing prenatal nutrition for some other male’s offspring.

“Good data” on female benefits, says Chad Johnson of Arizona State University in Phoenix. “But it needs to be acknowledged that this is a species with minimal sexual size dimorphism.”

The males can reach 5 grams in weight, about the same size as females and therefore a substantial dinner. In other species, males are tiny compared to females, and yet the females still eat their suitors.

“It’s really the systems with more extreme sex size dimorphism that are difficult to explain,” Johnson says.

He and other researchers have suggested that if spiders don’t get a good lunch or other benefit from sexual cannibalism, maybe killing suitors is just a spillover of a trait that does have value. Among fishing spiders, Johnson and his colleagues linked a female tendency to cannibalize suitors before mating to extra ferocity in hunting.

Mediterranean tarantulas might have some spillover effect too. The experiments don’t rule it out, Moya-Lara±o says. What the new study does confirm is that sexual cannibalism, with benefits, isn’t some artifact of lab tests in this species.

Moya-Lara±o and his collaborators monitored a large population of spiders in patches at Spain’s Cabo de Gata-Níjar Natural Park. “You’d turn on your headlamp and see hundreds of spider eyes shining all around you,” he says.

For one test, the researchers dug trenches around the edges of experimental plots where females had set up territories. To test whether the density of males affected cannibalism, and to also maintain a natural flow of prey, the team spent one season sorting through the creatures that tumbled into the trenches, allowing in some but not others. “It was a lot of work,” Moya-Lara±o says. The system did show that more males meant more cannibalism.

Males show some signs of evolving ways to minimize chances. Female spiders hunt mostly at night, and Moya-Lara±o says males mostly come courting during the day.

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