Males of a tropical species of orb-web spider castrate themselves, either partly or fully, after mating.

It might seem a self-destructive habit, but in removing their genitals, a new study reports, the spiders are actually improving their reproductive success.

By becoming a half eunuch or full eunuch, the spider reduces its weight by 4 to 9 percent. It then stands guard by the female it has mated with. Its lighter weight allows it to better fend off other males.

“When invaders — intact males — come in, eunuchs fight and do much better than the intact males,” said Daiqin Li, a behavioral ecologist at the National University of Singapore and one of the study’s authors. By keeping away other males and preventing females from mating again, the eunuchs can reduce sperm competition and maximize their own reproductive success. The researchers found that half eunuchs had a 32 percent increase in their endurance levels and full eunuchs an 80 percent increase.