Rodent of the Week: Having a bunch of sisters might make you less sexy

Pity the poor boy--in the United States at least--wedged in the midst of a bevy of sisters. When he’s little, he’ll certainly be dressed and made up like a doll. He will doubtless spend time cooling his heels at the bathroom door. He’ll likely pine for the attention of his sisters’ girlfriends. But a new study confirms what many such boys learned long ago: that as a sexual prospect, the kid brother is chopped liver.

If he’s a rat, at least.

“Does family play a role in shaping the adult phenotype?” asks psychobiologist David Crews and colleagues in a study published this week in the journal Psychological Science. Specifically, Crews and his co-authors wondered what is it about family that most strongly influences the later sexual behavior of a male rat? Is it the prenatal environment--the warm pool of hormones in which he is bathed as a developing fetus? Is it the gender mix in which his formative years are spent? Is his sexual confidence a function of how well his mother loved him as an infant?

There’s evidence that all three factors play a role in shaping adult sexuality. (A child’s attachment to his mother has long been the cornerstone of theories about adult mental health, and burgeoning research on “fetal programming” has shown how powerfully our time in the womb can shape us--as does a new book, Origins: How the Nine Months Before Birth Shape the Rest of Our Lives). But Crews and colleagues found that the gender composition of the “family” in which a male rat matures is the most powerful influence on his adult sexual behavior. They did so with an ingenious experiment.

First, they bred 88 litters of rats, all to be born on the same day. On the rats’ birthday, researchers recorded the gender composition of each litter born (to gauge the effects of prenatal environment on later sexual behavior). Then, to assess the influence of family composition, they reshuffled half of the baby rats into new families that differed in gender composition from the ones in which they developed in the womb. Some were assigned to litters with equal numbers of brothers and sisters; some went to a litter where boys outnumbered girls three-to-one; and some went to a litter where girls outnumbered boys three-to-one. To test the influence of mother-love on an adult rat’s sexuality, researchers took measures of their mothers’ attentiveness to the rat pups throughout the infant stage.

After weaning, the males were all given the chance to mix it up with some receptive females--sort of a speed dating for rats--to see which ones moved in quickly and which held back, and to gauge how attractive they were to females. When they compared the rats that had been born in male-dominated litters to those who had been incubated with lots of sisters, the researchers saw no difference in the rats’ sexual behavior. They saw no discernible differences between those nurtured by attentive mothers and those whose rat mommies were more detached.

But they did see that male rats who grew up in female-dominated litters mounted females less frequently. They managed to penetrate the female rats and ejaculate just as often. But researchers quickly surmised that male rats were skipping the foreplay and getting quickly down to business because they were less attractive to females. They could only score, it would seem, by moving fast.

How could they tell? When female rats want to attract a male, they perform a come-hither move called the “dart-hop”--the rat equivalent of a hair flip (they also wiggle their ears, a move that “drives males nuts,” according to Crews). With males reared in male-dominated or equally divided litters, females performed far more dart-hops and ear-wiggles than they did with males that were reared in a female-dominated litter.

Why are these rats less attracted--and attractive-- to females? Crews and his colleagues suggest that the lack of mystery may sap a male’s sexual interest in females when he has grown up in a household full of sisters. Afterall, the authors note, “novelty can enhance the expression of copulatory behavior in rats.” When a female sighting is a ho-hum affair, the male may not emit the scents or sounds that arouse interest in a female.

For humans, Crews is a little less specific about the take-home message. “It tells you that families are important” in shaping personalities, he says. How many brothers and sisters you have, and the interactions among those individuals--these factors may not determine personality. But they can be influential.

-- Melissa Healy/Los Angeles Times