How Male Widow Spiders Avoid Being Cannibalized During Sex?

For male widow spiders, mating is a notoriously dangerous activity. In these species, including the black widow and redback, the large females will often devour the smaller males during sex – hence the “widow” in their name. In some cases, the female catches the male while trying to escape. But often the male seems to welcome his fate and actively does somersaults on his partner’s teeth.

There is a reason for this suicidal gymnastics. In 1996, Maydianne Andrade discovered that sexual encounters between redback spiders last longer if the male allows himself to be cannibalized. By sacrificing his life, he can fill his partner with more sperm. The female stores that sperm in two storage organs and can control when to use those stored cells to fertilize her eggs. When she mates again, the second male’s sperm can displace that of the first now deceased lover. But Andrade showed that women who eat their first partner are more likely to reject another partner.

So by committing copulatory suicide, the men guarantee their future paternity. Besides, females are not common and many males die while looking for one; when the odds of finding more than one mate are slim, the cost of giving up your life to your first mate is low.

It seemed as if Andrade had discovered the redback’s strange behavior. One day her student Dani Baggio noticed something strange. She had housed young redback fish together and noted that the males often tried to mount immature females, making it very difficult to pull them apart. “I told her to keep a close eye on their activities and to get back to me if she sees this again,” Andrade says. “She did. And this time she said, I think they are mating. I told her to go away and isolate the females and see if they produced viable eggs. She did. And they did.”

That shouldn’t have been possible. At that stage of their juvenile life, the females’ genitals are well developed, but sealed. All the internal plumbing is there, but there is no opening. However, the males were clearly mating with them, and when they did, they almost never offered themselves as snacks.

Speaking at a conference about the discovery, Andrade learned that another student, Iara Sandomirsky, had seen the same behavior in another closely related spider — the brown widow. Together, the researchers showed that the males of both species can use their canine teeth to make openings in the females’ outer shells and access their genitals.

This is very different from the “traumatic insemination” process, which is practiced by bed bugs and some spiders. There, the males ignore the females’ genitals and inject them with sperm by inserting sharp penises right into their backs. These eerie acts obviously damage the females. In contrast, the immature redbacks and brown widows are not harmed by the premature advances of the males.

The males certainly benefit from it. When a person mates with an immature female, he is more likely to fill and close off both storage organs, preventing later suitors from moving his sperm. And since he doesn’t cannibalized, he gets a chance to find a second mate. This is not likely to happen for redbacks, but about 8 percent of males who mate for the first time with immature females are able to find a second mate. As for the brown widows, as many as 80 percent of males get a second chance to mate if their first mate is immature.

If that’s the case, why don’t the males do it? nothing but mate with immature partners? Possibly because that is difficult, says Andrade. It means finding females during a narrow window when their genitals are internally clear but not externally open. Adult females release pheromones into the air that attract males, but immature females don’t seem to. “My field data suggests that males sometimes stumble into the web of immature females while looking for adults,” Andrade says.

Yet this happens often enough that a third of female redbacks seem to mate before they reach full maturity. The same evolutionary pressure that led some men to commit suicide on a woman’s canines also led others to pursue sneakier and safer sex.

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