Suburban yards might be influencing more than just the biodiversity of what species live there; they might also be changing the sex ratio of those animals that still call them home. A new study looking into frogs in suburban ponds has found that estrogen in areas where there are shrubs, vegetable patches and lawns is driving up the number of female green frogs, and lowering the number of males.
“In suburban ponds, the proportion of females born was almost twice that of frog populations in forested ponds,” explained Max Lambert, a doctoral student at Yale University who led the study to be published in the Proceedings of the National Academy of Sciences, in a statement. “The fact that we saw such clear evidence was astonishing.”
The effect of estrogens on frog sex ratios has already been observed in agricultural regions and wastewater plants, but no one quite expected the results to be so clear cut in the suburban environment. Many commercial pesticides contain estrogen mimics, which act as an “endocrine disrupting compounds,” interfering with the hormone system of amphibians, specifically promoting the development of female characteristics. But interestingly, pesticides might not be the main source for those found in backyards.
The researchers found large quantities of what are called “phytoestrogens” in suburban ponds when compared with forest ponds. These estrogenic chemicals are produced by certain plants, such as clover and other legumes (beans and peanuts, amongst others). The researchers suspect that the cultivation of lawns – which often contain clover plants among the grass – and other non-native plants in yards might be contributing to an increase in phytoestrogens. Since these can mimic estrogen, they could therefore affect the sexual development of the frogs.
“Our work shows that, for a frog, the suburbs are very similar to farms and sewage treatment plants,” said Lambert. “Our study didn't look at the possible causes of this, partly because the potential relationship between lawns or ornamental plantings and endocrine disruption was unexpected.”
This study builds on research conducted last year that found intersex male green frogs – males with eggs in their testicles – in every suburban pond sampled. So common were they, the researchers estimate that one in five male frogs in backyards might be intersex, which is in sharp contrast to forest ponds where intersex males are almost unheard of.
These findings have implications not just for other amphibians that live in the ‘burbs, but also for animals that use the ponds, such as birds, fish and turtles, something which the authors of the study hope to look into. While the researchers didn't investigate the effect that this shift in sex ratio might have on future frog populations, if the effect really is as pronounced as they found, it might be a worth studying.
This article was edited on 09/09/2015. It originally stated that pesicides contained estrogen, and these acted as "endocrine disruptors." In fact, pesticides contain contain compounds that mimic estrogen, and they are called "endocrine disrupting compounds."