For male widow spiders, mating is an infamously dangerous activity. In these species, which include the black widow and redback, the large females will often devour the smaller males during sex—hence the “widow” in their names. In some cases, the female catches the male while he’s trying to escape. But often, the male seems to welcome his fate, actively somersaulting onto his partner’s fangs.

There’s a reason for these suicidal gymnastics. In 1996, Maydianne Andrade found that sexual encounters between redback spiders are longer if the male allows himself to be cannibalized. By sacrificing his life, he can fill his partner with more sperm. The female keeps that sperm in two storage organs, and can control when she uses those stockpiled cells to fertilize her eggs. If she mates again, the second male’s sperm might displace those of the first now deceased suitor. But Andrade showed that females that eat their first mate are more likely to reject a subsequent one.

So, by committing “copulatory suicide,” the males guarantee their future fatherhood. Besides, females aren’t common and many males die while searching for one; when the odds of finding more than one partner are low, the costs of giving up your life to your first mate are low.

It seemed like Andrade had figured out the redback’s weird behavior. Then, one day, her student Dani Baggio noticed something strange. She had housed young redbacks together, and noticed that the males would often try to mount immature females, making it very hard to pull them apart. “I told her to watch their activity closely and come back to me if she saw this again,” says Andrade. “She did. And this time she said: I think they are mating. I told her to go away and isolate the females and watch to see if they produced viable eggs. She did. And they did.”

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

View Images An adult male redback enters a web. In 1996, Maydianne Andrade found that sexual encounters between redback spiders are longer if the male allows himself to be cannibalized. Photograph by MCB Andrade 2003

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

View Images The male brown widow (pictured) also uses their fangs to create openings in females' outer shells and access their genitals. Photograph by MCB Andrade 2014



This is very different to the process of “traumatic insemination,” which is practiced by bedbugs and some spiders. There, the males ignore the females’ genitals and inject them with sperm by stabbing sharp penises straight into their backs. These grisly acts clearly damage the females. By contrast, the immature redbacks and brown widows aren’t harmed by the males’ premature advances.

The males certainly benefit. When one mates with an immature female, he is more likely to fill both storage organs and plug them, preventing later suitors from displacing his sperm. And since he doesn’t get cannibalized, he gets a shot at finding a second mate. The odds of doing so aren’t great for redbacks, but even so, around 8 percent of males who first mate with immature females can find a second partner. As for the brown widows, a whopping 80 percent of males get a second shot at mating if their first partner is immature.

View Images A female brown widow tends to her web. Male spiders may stumble onto the webs of immature females while looking for adults. Photogrpah by MCB Andrade

If that’s the case, why don’t the males only mate with immature partners? Possibly because doing so is tricky, says Andrade. It means finding females during a narrow window when their genitals are internally ready but not externally open. Adult females release airborne pheromones that attract males, but immature females don’t seem to. “My field data suggests that males sometimes stumble into the webs of immature females while searching for adults,” says Andrade.