Global ChangeIntersection of Nature and Culture

Herbicide exposure causes male frogs to turn into female frogs

Monday, March 1st, 2010

How do you turn a male frog into a female frog that breeds with other male frogs?   Expose them to herbicides that are routinely sprayed on agricultural fields worldwide.

Last year, Tyrone Hayes from UC Berkeley gave a talk at Bowdoin about his career’s work studying the impacts of endocrine-disrupting chemicals on amphibian development.

This week’s Early Edition of the Proceedings of the National Academy of Sciences features some of this research.¹

Excerpts:

Atrazine is one of the most widely used pesticides in the world. Approximately 80 million pounds are applied annually in the United States alone, and atrazine is the most common pesticide contaminant of ground and surface water. Atrazine can be transported more than 1,000 km from the point of application via rainfall and, as a result, contaminates otherwise pristine habitats, even in remote areas where it is not used.  In fact, more than a half million pounds of atrazine are precipitated in rainfall each year in the United States.

In addition to its persistence, mobility, and widespread contamination of water, atrazine is also a concern because several studies have shown that atrazine is a potent endocrine disruptor active in the ppb (parts per billion) range in fish, amphibians, reptiles, and human cell lines, and at higher doses (ppm) in reptiles, birds, and laboratory rodents. Atrazine seems to be most potent in amphibians, where it is active at levels as low as 0.1 ppb.  Although a few studies suggest that atrazine has no effect on amphibians under certain laboratory conditions, in other studies, atrazine reduces testicular volume; reduces germ cell and Sertoli cell numbers; induces hermaphroditism; reduces testosterone; and induces testicular oogenesis. Furthermore, atrazine contamination is associated with demasculinization and feminization of amphibians in agricultural areas where atrazine is used and directly correlated with atrazine contamination in the wild.

Using an experiment where his team exposed frogs to a 2.5 parts per billion atrizine solution, here’s what they found:

Atrazine-exposed males were both demasculinized (chemically castrated) and completely feminized as adults. Ten percent of the exposed genetic males developed into functional females that copulated with unexposed males and produced viable eggs. Atrazine exposed males suffered from depressed testosterone, decreased breeding gland size, demasculinized/feminized laryngeal development, suppressed mating behavior, reduced spermatogenesis, and decreased fertility. These data are consistent with effects of atrazine observed in other vertebrate classes. The present findings exemplify the role that atrazine and other endocrine-disrupting pesticides likely play in global amphibian declines.

The main implication of this chemically induced sex switching is that it has the potential to disrupt breeding and contribute to the amphibian declines observed worldwide:

Although many studies have focused on death from disease and its role in global amphibian declines and sudden enigmatic disappearances of populations, virtually no attention has been paid to the slow gradual loss of amphibian populations due to failed recruitment. The present study suggests several ways that exposure to endocrine disruptors such as atrazine may lead to population level effects in the wild and contribute to amphibian declines. Certainly, the inability to compete for females and the significant decline in fertility in exposed males, as reported in the present study, will have a direct impact on exposed populations.

¹Hayes, T. et al (2010) Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). Proceedings of the National Academy of Sciences. doi:10.1073/pnas.0909519107

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