In response the Eliot Spitzer scandal, Natalie Angier has a great piece in the New York Times about jealousy, cheating spouses and paying for sex in the animal kingdom this week. In it, she cites a University of Washington psychology professor as saying the only species that never strays is a kind of parasitic flatworm whose pair bonds fuse into one body and stay that way until death.
My favorite, though, was a study published in Animal Behavior by European researchers: The great grey male shrike gives his mate all sorts of edible gifts impaled on sticks. But when he wants to get it on with another female, he offers her even bigger gifts. The bigger the gift, the more likely she was to acquiesce.
As for jealousy, even insects seem to suffer from it. Angier writes about jealous female scarab beetles rolling their mates into balls of dung when they attempt to stray.