It’s a spider-eat-spider world.

It’s a spider-eat-spider world.

We’ve known for years that female black widow spiders and other arachnids eat males during mating.

Now, new research shows that males of a type of ground spider known as Micaria sociabilis also eat females, and scientists are trying to figure out what motivates this behavior.

A male M. sociabilis attacks his mate. Photograph courtesy Lenka Sentenská

More than just a first date from hell, sexual cannibalism happens when one member of a species kills and eats a member of the opposite sex immediately before, after, or during mating.

This behavior is most common in arachnids like the black widow, as well as other invertebrates like insects, gastropods, and copepods. Most commonly, the female eats the male—but occasionally, the reverse is true. Male sexual cannibalism has been observed in another species of spider, Allocosa brasiliensis, and in crustaceans, but previously researchers had no idea what factors drove this behavior. (Watch a video of a male spider attacking a female one.)

Ultimate Sacrifice

On the surface, this seems like a pretty weird way to pass along your genes to the next generation. To scientists, however, sexual cannibalism can make a lot of sense. When prey is scarce and males are abundant, males become extremely valuable as a food source to females, noted study co-author Lenka Sentenská, a biologist at Masaryk University in Brno, Czech Republic.

Females invest much more energy into egg production than males do in sperm production, which tends to make them pickier about who they mate with. (See “Male Spiders Give ‘Back Rubs’ to Seduce Their Mates.”)

As well, not all males seem to fight being cannibalized, said Sentenská, whose study appeared recently in the journal Behavioral Ecology and Sociobiology.

“Males of some species voluntarily sacrifice themselves to a female, because such behavior enables them to copulate longer and pass more sperm. Males who are trying to escape usually copulate for a shorter time. Sometimes, this behavior is also viewed as an extreme paternal investment of the male, who sacrifices his body to provide nutrients to his future offspring via the female,” she said.

Deadly Dinner Date

Researchers have traditionally focused on female sexual cannibalism, both because of its relative frequency and perhaps because of humanity’s attraction to the femme fatale.

But in some species, such as M. sociabilis, females are also in danger of being eaten by males. Sentenská and her colleague Stano Pekár examined what factors drove male sexual cannibalism in this particular species. (Watch a video of the world’s largest spider.)

These small brown spiders commonly live in trees in Central Europe. M. sociabilis has two generations of offspring each year—one in spring and one in early summer. Whereas females born in the springtime frequently survive to see the arrival of the second generation in June or July, male numbers drop precipitously in May. This creates some significant factors that the authors believe could lead to sexual cannibalism.

Males can choose among females of different size and quality—for instance, by mating with older females from the spring generation or with virgin females from the summer generation. (See pictures of spiders up close.)

To test whether these factors actually affect whether males ate females, Sentenská and Pekár paired adult male M. sociabilis with females of various ages and sizes. They found that larger males were more aggressive and much more likely to attack a female regardless of her age or size. Overall, however, males of any size were more likely to attack and eat females from the older generation.

Picky Males

Between 44 and 52 percent of the time, the males and females mated. In 20 percent of all of these trials, the male attacked and ate the female during copulation. However, the frequency of cannibalism varied depending on time of year. (See “‘Castrated’ Spiders Are Better Fighters, Study Says.”)

“Males are usually viewed as nondiscriminatory machines eager to impress and mate with every female encountered. Our study shows that males can be choosy and that they can present their choice in a quite extreme way—by killing unpreferred females. Moreover, despite the rarity of such behavior, this strategy seems to be advantageous for them,” Sentenská said.

These results might help explain male sexual cannibalism in other species, the researchers concluded.

However, Sentenská quipped, “I definitely would not encourage readers to try this strategy in their personal life.”