Dave Steinberg hasn't seen his roommate in months. "Giles never leaves his room anymore," says Dave. "Ever since he started dating Betty, he just stays inside his bedroom with her. He doesn't eat, he doesn't work, he doesn't even go to the washroom. It's eery. I'm surprised he hasn't died of starvation yet."

Every now and then, Dave can hear the two having sex. "Sure, Giles and Betty get in on sometimes, but it's not like they're always having sex in there. They screw once or twice a week, max. The rest of the time, I think Betty just sort of sits on top of Giles, so that he can't go anywhere. She traps him under her thighs, like some sort of paranoid animal."

Sitting on top of your sex partners to prevent them from straying is a common evolutionary strategy according to biologist Jan Werzing. "It's called mate guarding," says Jan. "The male stick insect, for example, will sit on top of a female it's mated with until it dies of starvation, it's withered corpse acting like a gruesome chastity belt. Male wasp spiders snap off their penises inside their partners in order to prevent them from having sex with other spiders. Mate guarding is everywhere in the animal kindgom. "

Mate guarding is a common way for animals to prevent competitors from reproducing with their partners. "Male stick insects would rather die than let their lovers have babies with other male stick insects," says Jan. "Betty is engaging in a similar strategy with Giles. By sitting on top of him like a giant sack of concrete, she guarantees that her boyfriend will never sleep with another woman. It might seem insane, but it makes perfect evolutionary sense."

Dave doesn't care if it makes evolutionary sense. "Giles and I used to party all the time. We'd go out drinking, and singing, and dancing. Now, he just sort of lies there, in his room. Sometimes, I swear I can hear him moaning for help. If he doesn't come out of his room in the next few days, i'm going to break into it and toss Betty out. She can go find another man to sit on -- I want my roommate back."