The orb-web spider Nephilengys malabarensis is into rough sex. So rough that, in order to avoid being eaten alive, the male will often voluntarily break off his whole sex organ, or palp, while it's still lodged in the female's abdomen, living out the rest of his life as an eunuch. Now a group of researchers think they know why evolution has allowed this dead-end dad to survive. They collected 25 pairs of spiders and introduced them to one another. After each pair had mated and the male's palp was left in the female, the researchers dissected the female and counted the sperm in her abdomen and the amount remaining in the embedded palp. That organ, they report online today in Biology Letters, continues to transfer sperm into the female long after the male has fled or been consumed. The longer it's embedded, the more sperm it transfers, and it's even more efficient when the male breaks it off himself to run away, rather than letting the female do it while eating him. So for the male, it's a sacrifice worth making.