How Eunuch Spiders 'Won' at Sex

Being a eunuch may not exactly sound like a badge of honor, but for some 30 spider species, missing your genitals means that you survived sex to tell the tale. What's more, no other male will ever get your girl.

That's at least according to a study recently published in the journal Behavior Ecology and Sociobiology, which details how, in the exceptionally brutal world of the spider bedroom, self-emasculation not only helps a male survive, but also makes his "wife" a loyal lady spider.

How does this work? Let's just say that spider marriage is not your standard exchange of vows.

Near the end of copulation, when a male spider has already spent his sperm, he will voluntarily chew off his own genitals, leaving them in the female like a cork.

Researcher Matjaž Kuntner, from the Slovenian Academy of Sciences and Arts, spent a great deal of time observing this disturbing behavior in spiders, finding that it helps them become better fighters than other males, when defending their pregnant mates.

That was two years ago, when he described in the Proceedings of the Royal Society B how fully-eunuch Nephilengys malabarensis spiders boasted 80 percent more endurance, when compared to whole males and even partial eunuchs.

This, he argued, supports something called the "gloves off" hypothesis, where eunuchs could have enhanced endurance thanks to a loss in total bodyweight. In most spiders, their sperm-producing organs take up a stunning tenth of their mass.

However, he also described how about 75 percent of N. malabarensis still fall victim to cannibalism when their far larger mate grows hungry after sex. So why are the survivors willing to mutilate themselves for an ungrateful mate? (Scroll to read on...)

"It is an extreme form of monogamy," Kuntner recently told New Scientist. "Males put all their eggs in one basket and focus on a single female."

He explained that most male spiders only have enough sperm for one shot at making babies. After that, their genitals are practically useless.

However, there is still one use left. When pleased with the sexual performance of their partners, many female spiders will secrete fluids to help males craft what is called a genital plug - a post-sex "chastity belt" that will prevent other males from "contributing" to a claimed female.

However, some spiders do not reward their males in this way. In those cases, males may take the initiative, using their own detached genitals as the plug.

Kunter adds that this strategy may have developed because males would never survive to see a second sexual encounter anyways (more often than not, they're lunch). In that way, one-shot sperm and self-emasculation may have developed to ensure, above all else, that their offspring are born.

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