Growing up with lots of sisters makes a man less sexy. For rats,
anyway. A new study published in Psychological Science, a journal of
the Association for Psychological Science, finds that the sex ratio
of a male rat's family when he's growing up influences both his own
sexual behavior and how female rats respond to him.
David Crews, a psychobiologist at the
University of Texas at Austin, is interested in how early life
affects behavior later. This is an area that has received a lot of
attention, such as research showing that the position of a fetus in
the uterus matters. For example, a female fetus that spends the
pregnancy sandwiched between two brothers grows up to be more
masculinized, because she's been exposed to their hormones.
Other researchers have found that sex ratio of the litter itself
affects adult behavior. But Crews wanted to separate the effects of
life before and after birth. "Life is a continuous process: you're a
fetus, then you're born into a family. Each one of these periods can
be important," he says—and they don't necessarily have the same
effects.
When rat pups were born, the researchers
counted the number of males and females in each litter to determine
the sex ratio in the womb. Then they reassembled litters in three
ways: so the litters were balanced between males and females,
strongly male-biased, or strongly female-biased. Then they observed
the mother's behaviors toward their pups and, once the males grew
up, tested them to see how they behaved with sexy female rats.
The researchers found no effects of the
sex ratio in the uterus. But they did find differences in behavior
based on the kind of litter in which the males grew up.
When males who were raised with a lot of sisters were presented with
receptive female rats, they spent less time mounting them than did
male rats that were raised in male-based litters or in balanced
families. But they penetrated the female rats and ejaculated just as
much as did the other males. This means "the males are more
efficient at mating," Crews says.
The males may be compensating for the
fact that they're less attractive to females. You can tell this by
watching the females—if they want to mate with a male, they'll do a
move called a dart-hop, says Crews, and "they wiggle their ears. It
drives males nuts." The females did this less when they were with a
male rat that had grown up in a female-biased litter. Crews carried
out the study with Cynthia B. de Medeiros, Stephanie L. Rees,
Maheleth Llinas, and Alison S. Fleming of the University of Toronto
at Mississauga.
These were rats, but the results have
implications for humans, too, Crews says. "It tells you that
families are important—how many brothers and sisters you have, and
the interaction among those individuals." Families are particularly
important in shaping personalities, he says. The environment where
you were raised "doesn't determine personality, but it helps to
shape it."
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