Meanwhile, over at the University of Wisconsin, researchers have been spending three hours a day tickling baby rats on the belly. As fun as that sounds, they were actually doing it in the name of science, to prove that a mother's behavior towards her infant can have a strong effect on the child's gender expression. Young male rats are usually the object of more maternal grooming and licking, which apparently helps their genitalia develop. But when the Wisconsin scientists applied the same intensive touching regimen to a group of girl-rats, not only did the number of estrogen receptors in the stroked females decrease, but their actual DNA changed--permanently, the researchers suspect. This is a groundbreaking finding--as _The New Scientist_ puts it, it's "the first time that epigenetic influences outside of the womb have been linked to sex differences in mammals." In other words, it's not just the genes and the hormones anymore!