Sex is as complex and confusing for other animals as it is for humans: The thrill of the new puts a female cricket in the mood, but lizard ladies thrive on commitment. Female crickets mark their mates so they don't have sex with the same cricket twice, according to Tracie Ivy and her colleagues at Illinois State University. To keep track of her exploits, a female cricket leaves a chemical imprint on a partner during sex. If she reencounters a male bearing her particular mark—or the mark of her identical twin sister—she will often refuse to mate. The goal, Ivy says, is for the cricket mothers to get good genes, and "if they mate with the same male over and over, it's like buying a lot of lottery tickets with the same number." Painted dragon lizards don't need to be so picky, because a single lizard never produces young after a one-night stand. Tobias Uller and his colleagues are studying this unusual type of natural contraception at Göteborg University in Sweden, where caged female painted dragons line one wall of the lab and males line the other. The two sexes can see each other across the room, but if they are allowed to meet up only briefly, their trysts produce no offspring. Even when the female is ovulating, reproduction occurs only if the father sticks around. Uller is at a loss to explain why fertilization does not occur without constant male companionship: "There's something missing, and what that is I can't say."