There’s a bizarre phenomenon going on in the spider world: some males are severing off their own sperm sacs after sex. Why? According to new research from the Slovenian Academy of Sciences and Arts, it makes males better at playing bodyguard. The work has been published in Behavioral Ecology and Sociobiology.

You know an animal is dedicated to sex when they are willing to chop off their own genitals to keep rivals off the scene. While this may sound like a strange way to improve their chances of reproductive success, male genital mutilation is actually common in monogynous/bigynous spider species, where the male can only mate with one or two females throughout his entire lifetime, although the female can have many mates.

By breaking off bits of their sperm transferring organs (palps) during or after sex, the males effectively plug up the female so that sperm from rival males can’t get in. A more extreme form of this, known as the “eunuch phenomenon,” involves completely severing one or both palps, resulting in half or full eunuchs. The former have another chance to mate, but the latter are effectively rendered sterile.

As New Scientist points out, there is also another reason for engaging in emasculation—it makes the males better at fighting off the competition. Since several males can fertilize the same batch of eggs in a female, keeping rival males away after the deed has been done is the only way to guarantee that the offspring all carry his genes. And those hefty sperm sacs, which can make up around a tenth of their total body weight, are nothing but a drag, so discarding them likely makes the male a more agile fighter. Why have a red bull when you can chop off your own genitals?

Indeed, evolutionary biologist Matjaž Kunter and colleagues found that one particular species of spider which severs its genitals during mating does so to become a better bodyguard. But what about those that chop off their sperm sacs after sex? No one had investigated this behavior before, so Kunter endeavored to find out why these animals might be doing this.

They looked at male coin spiders (Herennia multipuncta), which have a sex life not envied by many. Not only do they have to work with a female four times their size, they also have to avoid being eaten by said female after they copulate.

For the study, they allowed a group of males to mate with females, and then compared the behavior of the resulting eunuchs with virgin spiders. They found that those lacking one or both palps were much more aggressive towards competitors than those who never mated, and also stayed much closer to the females. Furthermore, they remained active for longer when poked with a paintbrush, possibly because they don’t weigh as much. The results therefore seem to support the “better-fighter” hypothesis, whereby emasculation results in enhanced aggressiveness.

Of course, it’s possible that the sex itself was responsible, rather than the emasculation behavior, since virgin males don’t have much motivation to protect females. But Kunter believes the emasculation at least plays a part, and since the spiders only have one chance, they will do whatever is needed to ensure success.

[via New Scientist and Behavioral Ecology and Sociobiology]