Sex differences in cognition and behavior--such as increased aggression in males--are usually thought to involve hormones, which can "masculinize" or "feminize" a brain temporarily or permanently. But now, a mouse study shows that some sex-linked genes don't need hormones to shape male and female behavior.
In the 1990s, scientists learned how to breed mice whose genes and hormones function independently. Normally, females have ovaries and two X chromosomes, whereas males have testes and one X and one Y chromosome. By knocking out the testes-determining gene, called Sry, on the Y chromosome, researchers made XY mice that churn out estrogen; and by adding Sry to females, they produce XX mice that manufacture male hormones. Previously, researchers led by behavioral geneticist Emilie Rissman of the University of Virginia, Charlottesville, used this mouse model to show that in aggression and nurturing behaviors--which are typically attributed to hormones--genes unrelated to hormone production also played an independent role.
A team led by neuroscientist Jane Taylor of Yale University was interested in habit-forming behaviors in which gender differences also have been documented. She and her colleagues trained these mice, as well as normal male and female mice, to poke their noses through one of three holes in order to obtain a food pellet. Then, some of the mice were subjected to "conditioned taste aversion"--after eating the food, they were injected with a chemical that made them sick. Ordinarily, mice will quickly learn to avoid the food, but they will still eat it if they have developed an automatic habit. That happened more often for the XX mice regardless of whether they produced male or female hormones, the team reports online 21 October in Nature Neuroscience. Thus, they say, the sex difference must have something to do with genes that are not involved in the production of sex hormones.
Neurobiologist Lawrence Cahill of the University of California, Irvine, says that the study "relates very well to established sex differences in the acquisition of addictive habits." For example, women progress from casual drug-taking to a drug habit faster than men do--a phenomenon some have attributed to hormones. Taylor says that the work also implies that women can be good multitaskers--by quickly forming habits that leave their higher brain functions free for other chores.