Family feud: genetic arms race between parents benefits male offspring in a surprising way.

A gene that guides the maternal care that female mice give their newborn pups also directs how a male learns to reproduce efficiently, researchers report. This same gene pushes a fetus to draw nutrients from the mother, even at the expense of her health.

A genetic battle of the sexes rages between mammalian parents. The gene called Peg3 is one of about 100 genes where the copy contributed to the offspring by one parent is switched off, leaving the other parent's copy active. Peg3 and others where the father's gene is active promote fetal growth at the expense of the mother's health. In contrast, when the mother's gene rules, it limits the embryo's growth and preserves the mother's energy.

Scientists have known of this parental tug-of-war since the 1980s. But evidence is now accumulating that this picture of gender conflict might be too simple. "Rather than being just a story about conflict between males and females, there's also co-adaptation between males and females," says William T. Swaney of Columbia University in New York.

Scientists had assumed that the activity of Peg3 and other such genes would switch off after birth. But in the late 1990s, researchers discovered the role of Peg3 in the nurturing behavior of females.

Swaney and his colleagues recently looked for an influence of Peg3 on the sexual behaviors of male mice. The team compared normal mice with mice missing the gene.

Without Peg3, males didn't learn from sexual experience. After impregnating a female, normal mice came to prefer the scent of female urine over that of male urine, and they mounted females more quickly. But males missing Peg3 continued to behave like inexperienced mice.

"This is the first [study] to show effects [of Peg3] on the reproductive behavior of male offspring," comments Michael Baum of Boston University. "Previous work mostly looked at the effects on females."

The researchers also explored the brain changes that underlie these behaviors. They found that Peg3 is particularly influential in a region called the hypothalamus, which processes information about smell and directs sexual behaviors.

The hypothalamus became more active in response to female odors in the normal, sexually experienced males, but not in males lacking Peg3. The gene somehow enables the hypothatamus of adult mice to learn from sexual experience, the team reports online and in an upcoming Proceedings of the National Academy of Sciences.

The Peg3-driven maternal care that females give their newborn pups is also mediated by the hypothalamus. Because Peg3 and related genes affect fetal-brain growth, some researchers speculate that those genes boosted mammalian-brain size during evolution.