Gender Bending in the ‘Burbs

A new study finds female frogs outnumber males 2:1 in suburban ponds. This is not normal.

Industrial chemicals and freshwater critters don’t mix well. Scientists have already shown that waterways polluted with hormone-disrupting substances—whether from pesticide runoff near farms or pharmaceutical-laden effluent from our sewage-treatment plants—can cause malformed limbs in frogs and intersex fish, respectively.

But new research shows that the risks posed by these chemicals extend beyond places around agricultural lands and wastewater facilities, where their sources are easier to identify. According to the study, which was published this week in the Proceedings of the National Academy of Sciences, endocrine disruptors of unknown origin are turning male green frogs into females in suburbia.

Photo: Geoff Giller/Yale UniversityA female green frog

Endocrine disruptors are found in everything from pesticides and food to detergents and toys, and they're particularly harmful to embryos and the young (human and otherwise). In people, the hormone mimickers have been linked to deformations, cognitive problems, and cancerous tumors, and—when it comes to frogs and fish—the feminizing of males, or the masculinizing of females.

Max Lambert and colleagues at Yale examined ponds in forested areas outside of New Haven, Connecticut, where they found that, on average, 63 percent of frogs were fellas. But when the team looked at suburban ponds, surrounded by landscaping and manicured laws, they discovered that about half of the frogs were females. The bias toward males in the more natural areas had disappeared, so the researchers checked hormone levels within the frogs’ habitats to see if it could be something in the water causing the gender discrepancy.

“We found a diversity of estrogens and phytoestrogens in suburban ponds that were entirely absent in forested ponds,” says Lambert. These substances could be coming from leaky septic tanks or sewer lines, pesticide or fertilizer runoff from yards, or even from popular landscaping plants, like clover, which contain chemicals called isoflavones that can mimic female hormones. Lambert says there’s no smoking gun, yet, when it comes to suburban estrogen pollution.

Previous studies in the lab have found that sex determination in amphibians is quite sensitive to estrogen exposure, but scientists weren’t sure how that translated to life in the wild. The latest results indicate that environmental surroundings may play a larger role in the gender balance of frogs than previously thought. And, the researchers say, the chemicals could also be messing with the reproductive development of other amphibians, as well as birds, reptiles, and mammals—as if sex lives weren’t complicated enough already.


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