Whoa: Polygamous Wolf Spiders Have a Natural Form of Birth Control

Females have figured out how to get rid of unwanted sperm, allowing them to reap the benefits of multiple mates

Male wolf spiders may have eight eyes, but they still can't tell whether the female they're mating with is dead or alive. (© Mofeed Abu Shalwa / Smithsonian.com Photo Contest)
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Humans aren’t the only ones who sometimes find themselves in need of Plan B: Some female wolf spiders have developed a natural strategy for killing off unwanted sperm after a male has deposited it in their body. But in the arachnids’ case, it’s so they can enjoy the evolutionary benefits of having multiple mating partners.

Natural birth control isn’t the only remarkable thing about wolf spider sex. Their coitus, which lasts an average of 100 minutes, involves vibrating legs, twin sexual organs and sometimes even prolonged threesomes. Now researchers have confirmed that females of a species found in South America are capable of reducing the amount of male sperm inside their bodies after—and even during—these marathon mating sessions.

Females of the nickel-sized species Schizocosa malitiosa are both sweet and scary, carrying an army of up to 200 spiderling babies on their backs until the young ones are old enough to set out on their own. Venomous to small insects but harmless to humans, they are one of more than 2,000 species of wolf spiders that live in almost every part of the world. This particular species is native to Uruguay, Argentina and Bolivia.

During intercourse, males penetrate the females’ twin sexual organs with their pedipalps—a pair of body parts next to their fangs that look like furry boxing gloves, and are used to inject sperm—for upwards of an hour and a half, according to Maria Albo, a researcher at the Biological Research Institute Clemente Estable and the lead author of a study recently published in the journal Ethology.

Females are able to store that sperm for about a month within their bodies before fertilizing a clutch of around 200 eggs. During that month, they can mate with multiple partners, meaning that the eggs generally hatch with a collection of different fathers among them. Researchers have always wondered whether the females had a way to get rid of some of this sperm after mating, similar to methods used by species including scorpionflies and guppies.

To find out, they had to enter the realm of the truly macabre. “We induced matings with dead females because we wanted to disentangle male and female control of the sperm,” explains Albo. By using dead females, researchers could count the amount of sperm the males had inserted and compare it to the amount they found in live females which had a chance to reduce the fluid.

Fortunately for the researchers (though not the research spiders), playing dead is a typical wolf spider come-on. The female equivalent of flirting, Albo says, is basically staying still and allowing the male to approach. In fact, other research has found that some wolf spider males are so undiscerning that they will actually mate not only with dead bodies, but with anesthetized females of other species.

Albo and her coauthor took advantage of the males' undiscerning ways, taking frozen, freshly dead females and letting them heat back up to room temperature to trick the males into thinking the object of their affection was alive and ready to mate. After the deed, they counted the sperm deposited in the dead female.  

It turned out that, in nature, a substantial amounts of the sperm was missing. In fact, females usually reduce the sperm down to a mere 17 percent of what was originally inserted a day after mating. Researchers still aren't sure what mechanism the females use to get rid of such large quantities of sperm, but it’s likely either by absorbing and then digesting the sperm as a source of nutrition, ejecting it from their bodies or somehow killing it off inside them.

Ann Rypstra, a zoology professor at the Miami University in Ohio who runs a spider lab but was not involved in this research, says that Albo’s paper shows some of the best direct evidence for this phenomenon. “It’s really interesting data that shows pretty clearly that the females can select sperm and get rid of a large proportion of what’s transferred,” she says.

Albo's results show that the males and females might have evolved in conflict with each other. Possessive males trying to pass on their genes would prefer that females use their sperm—and only their sperm—to fertilize their egg clutch. To give them a boost against the threat of their sperm being usurped, research has found that males often insert sperm surrounded by mysterious droplets that might make the females less sexually receptive to other males.

This kind of male behavior puts a cramp on the female's free-loving ways: After all, she has an evolutionary interest in mating with as many males as possible to ensure a genetically diverse clutch of babies. Which is why females—or their bodies, at least—undermine their mating partners by reducing their sperm. For wolf spiders, it seems, sex is a game of deceit on both sides.

About Joshua Rapp Learn
Joshua Rapp Learn

Joshua Rapp Learn is a D.C.-based journalist who writes about science, culture and the environment. He has crossed the Sahara Desert, floated down the Amazon River and explored in more than 50 countries.

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