Photograph courtesy Matjaž Kuntner

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Seen close up, stumps on both sides of its mandibles are all that remain of a male spider's genitals.

Photograph courtesy Matjaž Kuntner

"Castrated" Spiders Are Better Fighters, Study Says

With genitals gone, battling males "have nothing to lose."

Although emasculated spiders can no longer be lovers, they are better fighters, a new study says.

In many spider species, males have sex using two appendages known as pedipalps. But males will often lose one or both pedipalps during the act—behavior that might seem like a bad idea evolutionarily, since it renders the male sterile.

Scientists have proposed that such genital amputation plugs up a female to help ensure that other males don't successfully impregnate her.

To learn more about why male spiders become eunuchs, scientists examined the mating behaviors of the Southeast Asian orb web spider, Nephilengys malabarensis. (Related: "Largest Web-Spinning Spider Found.")

The scientists found that amputated genitals effectively plugged up a female spider 75 percent of the time. In addition, the team saw that males with missing genitals were far more aggressive and active in guarding females.

During contests between males, full eunuchs that had lost both pedipalps were better fighters than half eunuchs and intact males, the study showed. Overall, the eunuchs were more than three times more likely to attack, chase, and defeat any rivals.

The scientists think that, once a male has lost his means of procreating, he protects his investment further by becoming a superwarrior.

"We think, but are not sure, that the changes in behavior may be due to changing hormone levels" triggered by losing the pedipalps, said study co-author Matjaž Kuntner, a spider expert at the Slovenian Academy of Sciences and Arts in Ljubljana.

"But the ultimate reason for this behavior is that eunuchs, with no reproductive future, have nothing to lose and risk more in contests."

The study also observed that females of this species will frequently devour males after sex, Kuntner added. Males could thus be breaking off their genitals, he said, to help ensure their sperm still has a fighting chance even if they fall victim to cannibalism.

The eunuch-spider study was published online March 23 in the journal Animal Behaviour.