Pesticides for macho frogs

The increasing demand for organic food sends a pretty clear message that we are starting to realize that pesticides and herbicides are bad. We are concerned that pesticide exposure may be linked to cancer, decreased fertility rates, etc. Unfortunately, the majority of living organisms can’t choose their level of exposure to these chemicals, and a recent study describes a most unusual consequence of pesticide exposure in frogs.

The researchers looked at the effects of exposure to atrazine, the most widely used pesticide in the world, and the most common pesticide contaminant in water. The study shows that when male frogs are exposed to atrazine they become demasculinized, meaning they lose male characteristics. This happens both at the physical, or “looks” level (the male frogs look like female frogs), and at the physiological, or “function” level (the male atrazine-exposed frogs have reduced testosterone levels and sperm count compared with control male frogs).

All this isn’t exactly new. For years, we have been aware that pesticide exposure interferes with normal hormonal function in a variety of organisms (salmon, frogs, rodents). What sets this study apart is that the researchers observed that in 10% of the atrazine-exposed male frogs, a complete reversal to a female frog happened. Everything about these initially male frogs now identified them as female, with the only exception of their genetic code (the equivalent of their Y chromosome). They looked 100% female, had female reproductive organs, and were able to mate with other males and produce eggs. If you’re in the market for an amphibian sex-change, look no further.

I know what you’re thinking. Surely the researchers exposed the frogs to crazy high levels of atrazine? No. They used a concentration of 2.5 parts per billion, which is consistent with what you can find in the environment. But frogs can be hermaphrodites, right? So it’s not a big stretch that they would switch over from hermaphrodite to female and just lose their “half male”? No. While these frogs can be hermaphrodites, the researchers only looked at genetic males, meaning males that could not be or become hermaphrodites. But the researchers looked at early stages of development, right? When the tadpole can go either way? Again, no. These were adult frogs.

Can we extrapolate these results to humans? Unlikely. Frogs absorb chemicals like atrazine through their skin, something mammals don’t do. Therefore, you would need exposure at a much higher concentration to achieve the same amount of atrazine in our system. Does this mean the study is not relevant to human health? Again, no (I’m so negative today!). The fact that atrazine is so potent at interfering with the frog’s hormonal system is definitely cause for concern. Interestingly, atrazine is banned in the European Union. The United States, on the other hand, slathered 76 million pounds of the stuff on cropland in 2003. (Organic) food for thought…

Reference: Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). (2010) Hayes TB, Khoury V, Narayan A, Nazir M, Park A, Brown T, Adame L, Chan E, Bucholz D, Stueve T, Gallipeau S. Proc Natl Acad Sci 107(10):4612-17.