Male Widow Spiders Prefer Younger Ladies—So They Don’t Get Eaten

Female widow spiders have a bit of a reputation. In species like the western and southern black widows of North America, they kill and devours the male soon after doing the deed. But for two particular species—the Australian redback spider and the brown widow—the male gives them a helping hand. In mid-coitus, he flips over and offers his body to be eaten, all for the chance that she'll bear his children. (And you thought paying for an expensive date was uncool.)

It's one of the most extreme, one-sided mating patterns in nature. And there’s not much the male can do about it. By almost all accounts, the female, who weighs up to 100 times more, has the upper hand. Or does she?

Now, researchers have found that the male has some tricks of his own up his eight sleeves. His main strategy: Get ‘em young. Male redbacks and brown widows often go for females who have yet to reach full maturity—and who tend not to eat their mates. This way, the male gets to be the first to inseminate her, increasing his chances at fertilization. Better yet, he lives to mate again.

"From the male perspective, this is genius," says Jonathan Pruitt, a behavioral ecologist at the University of California at Santa Barbara. "These females are not all that aggressive at this stage. So basically, the males don't have to be sexually cannibalized, and don't have to worry about having females that are too choosy. You basically have females with no resistance to copulation at all. You bite her open, mate with her, and wander away."

That's right, the male has to bite her open—her exoskeleton, anyway. During the female spider's last stage before maturity, she is sexually mature and her genitalia is fully developed. But she has yet to molt, meaning her body is still encased in the exoskeleton of her youth. "It's almost as if you have an adult female inside a wetsuit," says Maydianne Andrade, an evolutionary biologist at the University of Toronto Scarborough who led the new study, published in the Royal Society's journal Biology Letters.

To access the female spider's genitalia, the male must first make a slit in her exoskeleton with his fangs. Then, he inserts a whip-like extension, called an embolus, into the female's reproductive organs and deposits his sperm. The female stores the sperm and, within days after reaching maturity, can lay the fertilized eggs as if she had mated as an adult.

For the male, mating is a daunting task. After becoming an adult, he leaves his home web and searches for an elusive female. According to Andrade, about 86 percent of all males die without ever finding a mate. (Sadly, there is no spider Tinder.)

If a male does encounter a female hanging out in her web, he spends more than two hours courting her. To let her know he's interested, he shakes her web. If she deems his romantic vibes worthy, he goes in to deposit his sperm. In the middle of the act, though, he does a somersault, and offers his body to be eaten. "While they're mating, the male dangles the main part of its body over the female's mouth," Andrade says. 

If he survives, he does it all again; the spiders have two sex organs each. After dismounting, he goes back—often already partially digested—courts her, and inserts his second embolus into her second sperm sac. This time, though, she finishes her meal.

Why would evolution favor such a brutal mating strategy? The traditional explanation is that it’s so hard to find a female, that he’s pressured to go all in on the first mate. As long as he manages to pass on his genetic material, his job is done. If he has to sacrifice himself, then so be it.

But an observation by Daniela Biaggio, one of Andrade's graduate students, challenged that neat understanding. In earlier, separate experiments, Biaggio noticed the male redbacks were mating with immature females. Meanwhile, Iara Sandomirsky, a graduate student at the Ben-Gurion University of the Negev in Israel and a coauthor of the new study, saw brown widows doing the same thing. That prompted the researchers to study this curious behavior more closely.

They found that in such situations, not only did the males live, they expended less energy in courtship. Males also had better success with insemination, depositing their sperm in both sacs more often. The brown widow was better at leaving behind his embolus, which can break off during copulation and plug the sperm sac, preventing rivals from depositing their sperm. Plus, most of the spiders didn't do their self-sacrificial somersault move.

Researchers have long wondered why male redbacks and brown widows produce sperm throughout their adult lives, if they ostensibly mate only once. This new research may provide an answer: Mating with immature females allows them to have more mates, which gives them reason to always have sperm at the ready. The idea that males can hook up more than once is a big departure from previous assumptions, says Eileen Hebets, an evolutionary biologist at the University of Nebraska in Lincoln. "It's going to cause us to rethink a little bit how we understand mating systems and evolution—especially in these extreme behaviors," she says.

The male spiders' behavior also provides bug researchers with a practical research tip. Usually, bug dissection in the lab is messy. "If you pierce the cuticle of a spider, often times they pop like little balloons and a bunch of their insides spill out, or they bleed quite profusely," Pruitt says. But these males can apparently slice open an immature female without harm. "That's a pretty phenomenal maneuver," he says. Finding a way to mimic this surgical move could allow biologists to probe bug physiology and anatomy more effectively, without spilling as many bug guts.

Some other male spiders, such as orb-weaving spiders, are known to pounce on the female just after she molts, when she's defenseless. But Andrade says this is the first time anyone has seen male spiders mate with a female before she molts. The researchers still have much to learn about this new mating tactic. For instance, while an immature female may indeed be less aggressive, it still isn't clear why she doesn't eat the male, Andrade says. What’s in this mating technique for the ladies, if they don't get to finish it off with a snack?

"Definitely up to now, the story of how mating works in these species has been one where the physically dominant female has all the cards," Andrade says. "This is one situation where we don't yet know who's holding the cards.”

Marcus Woo | | READ MORE

Marcus Woo is a freelance science writer based in the San Francisco Bay Area. He has written for Wired, National Geographic, BBC Earth, BBC Future, New Scientist and Discover.