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Gender Bending in the Rockies

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Naturally masculine female marmots could shed light on the ways environment can impact biology

Usually, it’s easy for scientists to tell male and female yellow-bellied marmots apart simply by looking. First of all, the furry cousins of the groundhog are pretty good-sized, and they have prominent genitalia. Second, boy and girl marmots behave differently: males roughhouse more, groom one another more, and stray farther from home.

But Raquel Monclus, one of the biologists working on a 50-year marmot observation project at the Rocky Mountain Biological Laboratory, in Crested Butte, Colorado, says that the gender of some tiny newborn Marmota flaviventris can be hard to distinguish. Using a trusted method -- measuring the distance between the rodents’ anuses and genitalia -- on creatures of indeterminate gender, she and her colleagues occasionally found themselves surprised in the long run. Though a longer "AG distance" typically signals a male, some marmots that at first seemed obviously masculine turned out to be females.

And those females tended to have something in common -- a lot of brothers, according to a study by Monclus recently published in Biology Letters. It seems that a female marmot surrounded in the womb by males absorbs the testosterone given off by her brothers through the placenta. Not only does the hormone elongate her AG-distance, it affects her later behavior. She’ll engage in more play-fights and roam farther than normal females. She’ll also have trouble reproducing. (Boy marmots surrounded by sisters don’t have the reverse problem, because they begin making testosterone much earlier than females do estrogen.)

The discovery of the tomboy marmots shows that small hormonal changes in the womb can have effects that last for generations. And that has researchers worried that "endocrine disruptors" -- chemicals such as pesticides, the BPAs in plastics, and the PCBs in coolants and flame-retardants, which can mimic the behavior of hormones in animals and humans -- could harm marmot and other populations in unpredictable ways.

Monclus says it’s "not very clear" what makes some females give birth to mostly-male versus mostly-female litters, but it could have to do with their own hormonal experience in the womb. It likely also has to do with the mothers' overall health. Strong, robust females can afford to "invest" in male offspring -- which are bigger and require more energy to nurse and care for -- to pass on more of their genes. In a given year, a male can mate with many females, producing some 50 pups, whereas a female will only give birth to a litter of about five. Sons will also grow up and move away, instead of staying nearby to compete for food and nesting space with their mothers, as daughters do. On the flip side, male marmot pups are more sensitive to deprivations. Stressed females, Monclus says, tend to give birth mostly to girls, perhaps because female pups are more likely to survive with fewer resources.

All that cost-benefit analysis means that a marmot’s biology has to be pretty tuned in to its environment from year to year. If a harsh winter makes food scarce -- or if toxins appear -- it could change the gender, behavior, and reproductive capabilities of the next generation, and the next, and the next, in ways that scientists have only begun to anticipate.

It turns out that humans may share similar concerns. A new study in the journal Pediatrics shows that young girls exposed to BPA in the womb tend to have more behavioral problems than other children -- including aggression, hyperactivity, and distractibility, which are more typical of their male peers.

Do our surroundings -- as much as genes and upbringing -- make us who we are? If tomboy marmots are any indication, the age-old nature versus nurture debate is about to get a lot more complicated.

image of Kim Tingley
Kim Tingley is a regular contributor to OnEarth and the New York Times Magazine. She has an MFA in nonfiction writing from Columbia University and in 2012 received a Rona Jaffe Foundation Writer’s Award, given annually to six female writers who dem... READ MORE >