Herbicide Atrazine Morphing Male Frogs

The herbicide atrazine is wreaking havoc on the sex of adult male frogs, a study out of the University of California, Berkeley has found. About three-quarters of the frogs studied were chemically emasculated, while another 10 percent or more essentially became females—to the extent that they are able to reproduce with male frogs. The dual effects of Atrazine are potentially devastating to frog populations.

Atrazine appears to effect testosterone levels in frogs, according to the study, which was published in the journal Proceedings of the National Academy of Sciences. "These male frogs are missing testosterone and all the things that testosterone controls, including sperm. So their fertility is as low as 10 percent in some cases, and that is only if we isolate those animals and pair them with females," Tyrone B. Hayes, a professor of integrative biology at UC Berkeley, said in a press release. "In an environment where they are competing with unexposed animals, they have zero chance of reproducing."

These chemically castrated males are essentially unable to reproduce, which means the ability of the species to continue to thrive is greatly compromised. At the same time, when the male-turned-female frogs reproduce with unaffected male frogs, they can only produce male offspring because the "female" frogs are genetically male.

"In a population, the genetically male females can decrease or wipe out a population just because they skew sex ratios so badly," Hayes said.

While the experiment was performed in a lab with the African clawed frog, a species commonly used in laboratory study, Hayes says field studies have demonstrated similar effects in the wild, and it may play a key role in amphibian population drops around the world.

"These kinds of problems, like sex-reversing animals skewing sex ratios, are much more dangerous than any chemical that would kill off a population of frogs," Hayes explained. "In exposed populations, it looks like there are frogs breeding but, in fact, the population is being very slowly degraded by the introduction of these altered animals."

Atrazine is a strong endocrine disruptor—or in layman's terms, it affects hormone levels. It is one of the most widely used weed killers: about 80 million pounds of atrazine are used in the U.S. each year, with corn and sorghum crops using the most. While less common, atrazine is also used on residential lawns, particularly in Florida and the Southeast, according to the EPA.

Atrazine contamination of surface and drinking water is widespread in the Midwest and Southern United States according to an analysis of water data conducted by the National Resources Defense Council and published in a report in August 2009. An earlier extensive U.S. Geological Survey study found atrazine in about 75 percent of stream water and about 40 percent of groundwater samples from agricultural locations.

Various studies have linked atrazine to hormone disturbance in amphibians, fish, reptiles, birds, rodents, and human cell lines. The European Union banned the chemical in 2004 due to concerns with groundwater contamination, and the EPA is now reviewing atrazine usage regulations.