Spiders have some pretty strange mating habits, from sexual cannibalism to self-sacrifice.
Now, a new study suggests we can add another oddity to the list—destroying female genitalia.
Some male Cyclosa argenteoalba, a type of orb-web spider, maim their mates’ private parts after sex in what may be an attempt to prevent other males from copulating with her.
males of the spider species cyclosa argenteoalba maim their mates’ private parts after sex. [Photograph by Kensuke Nakata]
The study, published recently in Biology Letters, builds on a 2015 research that revealed the same behavior in a different kind of orb-weaving spider, Larinia jeskovi.
Sexual competition strategies—the contest to fertilize females and secure paternity—are common in spiders. Some male arachnids guard the female after mating, reduce her attractiveness to other males by transferring chemical substances, or even plug the female’s reproductive opening.
However, the orb-weaving spider’s genital destruction seems to be a newly discovered technique, says Kensuke Nakata, a zoologist at Kyoto Women’s University who conducted the recent research.
“My study and the Larinia jeskovi study both uncovered a new strategy for males to secure their paternity that is possibly widespread in orb-web spiders,” he says.
In the new study, Nakata collected 84 virgin females from bamboo forests in Osaka and Kyoto, Japan, and mated them with males in a fenced garden.
The results of the first experiment showed that males removed the female’s genital appendage, called a scape, in almost 90 percent of the sexual encounters.
Perhaps surprisingly, removing the scape does not hurt the female spider, which continues to respond to courtship from other males even after the scape is broken—most likely unaware of what happened to her.
Gabriele Uhl, a biologist at Germany's University of Greifswald and leader of the 2015 research, says that the new study is important from an evolutionary perspective. That’s because it shows that destroying genitalia evolved independently in two distantly related spiders.
“Since the phenomenon has been overlooked in spiders for decades, this may also be the case in other animals groups,” Uhl adds. “We suspect that other animal groups with external female genitalia may have also likewise evolved genital mutilation.”
According to Japan’s Nakata, genital destruction “is a very interesting phenomenon, since the female seems to have no effective way to resist the male.”
There are many other ways for male spiders to secure paternity, and usually females can counteract those—except in this case, he adds.
Without genitalia, there’s little the female spiders can do—other than plot their next evolutionary move.