Talk about tough love—some male spiders lop off parts of females' genitalia to prevent her from mating again, a new study says.

The behavior, which guarantees that the male will father all of her offspring, is the first to suggest that males evolve behaviors to maim external parts of the female genitalia.

Published November 5 in Current Biology, the discovery also adds further nuance to the theory of sexual selection, which holds that males and females within a species compete for opportunities to mate—even if it kills them.

“All the time, we’re discovering [such] new, astonishing adaptations,” says Jutta Schneider, a biologist at the University of Hamburg who wasn’t involved in the study but has collaborated with some of its authors. “This competition has enormous power.”

Spiders in particular get freaky in the pursuit of sexual success, deploying everything from cannibalism to self-castration to land a mate. (See "'Castrated' Spiders Are Better Fighters, Study Says.")

The Amazing Peacock Spider Male peacock spiders are known for their brightly colored backs and mating dances. Their vibrant colors are a double-edged sword: While they're useful for attracting females, they can also draw predators' attention.

But researchers haven’t closely studied how males mess with the structure of the female genitalia—an idea that biologist Gabriele Uhl and her colleagues at Germany's University of Greifswald stumbled into after examining female specimens of Larinia jeskovi, a species of orb-weaving spider native to Siberia and eastern Europe.

The team observed that after mating, many females were missing their scapus, a bicycle seat-shaped knob that sits above the genitalia. But why?

Caught in the Act

To find out, the researchers caught wild L. jeskovi and allowed them to mate under careful observation in the lab. When a male successfully mounted a virgin female, researchers froze the pair with blasts of liquid nitrogen, allowing the team to microscopically scan the spiders’ interlocked genitalia.

Freezing spiders in the act proved challenging; their hookups last mere seconds. “We had to be super quick and super lucky,” Uhl says. (Read about how some male spiders seduce using back rubs.)

A male spider delivers its sperm via pedipalps, a pair of leg-like appendages near its mouth that latch onto the female's scapus from above and below.

The scans revealed that the L. jeskovi pedipalp grasps and twists the scapus as the male dismounts, snipping it off as if with scissors. Without this crucial handle, other males can’t grasp the female at all, preventing her from having another sexual partner.

It’s a twist on the typical arachnid battle of the sexes. Many female spiders have sex with multiple males but fertilize their eggs with only one suitor’s sperm. This competition has prompted some species’ males to take drastic action, such as castrating themselves to plug the females’ reproductive tract.

In this case, however, “males have found a very clever means to prevent females from remating without mutilating themselves,” says Uhl. (Also see "7 Bug and Spider Myths Squashed.")

View Images (A) A female Larinia jeskovi in her web. The arrow points to her external genitalia. (B and C) Microscopic images show the female genitalia with the scapus (Sc) intact (B) or missing (C). Arrows point to the female's copulatory openings. Courtesy of Current Biology. Photograph by Gabriele Uhl

“It’s great, brilliant,” says Scott Pitnick, a biologist at Syracuse University, who praised the scientists for looking at the structure of the female's genitalia and how it changed after mating.

“It’s sort of shocking how often people fail to go after the structure-function relationships," Pitnick says.

Harm or Help?

The researchers suspect that the phenomenon isn’t unique to Larinia jeskovi, either. Females in some 80 spider species have a scapus that’d be vulnerable to similar male meddling. (Also see pictures: "'Torture' Phalluses Give Beetles Breeding Boost.")

But it’s unclear if the female is actually harmed—the team won't know until it studies their life expectancy and overall fertility.

“We’re still struggling a bit with this question,” says Uhl. “Maybe she only has costs, and that this is something the male forces onto her.”

But it's possible having a single-use love handle may even benefit the female. Female spiders can store viable sperm for years, so having only one sexual partner might not hamper their fertility. (See "Flirty Female Spiders Use Silk to Capture a Male's Interest.")

And preventing other males from mating might be an efficient way to “make them go away," Pitnick says, which has another, non-sexual benefit for the female: She won't have to share her hard-earned meals with random suitors.