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.

(Related pictures: "'Torture' Phalluses Give Beetles Breeding Boost.")

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