Female spiders who eat would-be suitors produce more babies, and those babies are stronger and bigger, than spiders who stick to more mundane fare, researchers reported on Tuesday.
And the merciless mother spiders waited until they had mated with another — ensuring they would hatch spiderlings — before consuming their new beaux, the researchers found.
They said their study is the first "natural" experiment to prove correct the old folklore about spiders, and said it also shows why such behavior might be beneficial.
"Now we know that, at least in one species, sexual cannibalism benefiting females occurs in nature," Dr. Jordi Moya-Larano of the Estacion Experimental de Zonas Aridas in Spain, who led the study, said in a statement.
The Mediterranean tarantulas in the study did not eat their mates, but instead ate males before courtship — and usually after the females had already mated with another male, the researchers found.
Some other studies have suggested that males may sacrifice themselves for the sake of their offspring, but this study showed that, at least in this species of spider, the males are purely unlucky victims and only the babies benefit.
Some studies had also suggested that studying spiders in the lab produced skewed results, perhaps because the creatures were stressed or perhaps because they could not obtain all their needed prey or nutrients.
So the researchers set up a field experiment in which they watched the spiders, sometimes snatching the males from the jaws of females before they were devoured.
"At natural rates of encounter with males, approximately a third of L. tarantula females cannibalized the male," they wrote in their report, published in the Public Library of Science journal PLoS ONE.
"The rate of sexual cannibalism increased with male availability, and females were more likely to kill and consume an approaching male if they had previously mated with another male," they added.
"We show that females benefit from feeding on a male by breeding earlier, producing 30 percent more offspring per egg sac, and producing progeny of higher body condition. Offspring of sexually cannibalistic females dispersed earlier and were larger later in the season than spiderlings of non-cannibalistic females."
One theory had also held that females who ate males were simply more aggressive and perhaps better hunters — but when the males were saved just in time, those females did not produce superior broods, suggesting that the male meals were an important source of nutrition.