Cannibals have better babies: female spiders who dine on guys have more, tough young.

One's company and two's--lunch.

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

About a third of female spiders studied in their natural habitats ate at least one suitor, Moya-Larano 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 the young earlier than females that experimenters deprived of a male meal, Moya-Larano says. That head start on laying eggs let the spider-lings 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.

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Just how sexual cannibalism has evolved remains a puzzle, says Moya-Larano, as until now the benefit of eating a potential mate has been hard to pin down.

Many of the experiments have taken place in the lab, but Moya-Larano and his colleagues spent two field seasons studying spiders in natural habitats as well as in the lab.

Sparse food could favor the evolution of cannibalism in the Lycosa tarantula spiders, Moya-Larano 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 typically weigh several grams, about the same size as females, and make a substantial dinner. In some species males are tiny compared with 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-Larano says. What the new study does confirm is that sexual cannibalism, with benefits, isn't some artifact of lab tests in this species.