Twin fates: sharing the womb with a brother may influence a girl's development

Science News, May 10, 2008 by Deborah Blum

Further studies showed that these changes in the body shape were indicators of other changes. For instance, a region of the brain called the hypothalamus, which is influential in mating behavior, is typically larger in male mice. It turned out that the 2M females also had larger hypothalamuses, had delayed puberty, had more irregular cycles, and were more independent-natured and aggressive. Notably, once the mice all reached mating age, if males were given a choice between a 0M and 2M female, they chose the more "female" mouse.

More like her twin brother

DURING THE 1980S, VANDENBERGH--NOW A PROFESSOR EMERITUS of zoology at North Carolina State University--decided to see what this meant in terms of how the mice interacted with others, including all important reproductive behaviors. With his colleagues, he let loose mice populations into the grassy cloverleafs of highways around Raleigh. Within each group, the scientists released about 20 0M and 20 2M females, marking them all for observation.

Over time, the zoologists discovered that the 2Ms were the most adventurous females; their home ranges were some 40 percent larger than the OMs, who tended to be homebodies. But being more assertive and adventurous didn't seem to lend a reproductive advantage. The home-loving 0Ms tended to raise larger litters, although Vandenbergh points out that in a less confined, harsher environment, the exploring tendencies of the 2M mice would offer a greater advantage. In other words, the variety itself offers an advantage in overall species survival.

"It demonstrates the importance of changes during fetal development," Vandenbergh says. "We all try to explain variability, why brains are different, and we're all trying to find explanations for some of that variation. This is at least a reminder that a lot can happen in the fetal environment--that it's a very sensitive time."

Animal researchers can directly experiment with such ideas. Emory's Wallen has demonstrated that by blocking testosterone at different times during gestation, he can alter both the physiology and behavior of monkeys, inducing little males, for instance, to vocalize like females. But as he and McFadden both note, comparable studies cannot be done with human research subjects.

"Animal research offers some really nice opportunities," McFadden says. Human researchers are generally "forced to capitalize on the manipulations of nature," mostly by studying people with genetic defects or variations that cause either an excess of androgens or an inability to absorb them.

Listening for differences

MCFADDEN BEGAN HIS OWN WORK ON FEMALE CO-TWINS IN the 1990s while running a series of tests on hearing, comparing different groups to try to sort out genetic influences. In particular, he was looking at a phenomenon called "otoacoustic emissions," in which vibrations within the ear, as it responds to sounds, create faint popping noises that can be registered on scientific instruments. In general, the more sensitive the hearing is, the greater the frequency of such sounds. Overall, women's ears pop more, men's ears less often. But when McFadden tallied up his results across a range of population groups, he discovered that one group of women looked markedly different. Females with a twin brother tended toward the male range of the tests.