Chemicals in the environment can affect whether some animals develop as males or females.
Atrazine, a pesticide commonly found in ground water, can turn boy frogs into girls. It does this by decreasing male hormones and increasing the female hormone estrogen.
The disruptive effects of Atrazine on sex hormones is not limited to frogs. Other amphibians, fish, reptiles, and mammals can be affected.
A review article, led by Tyrone B. Hayes at Berkely, compiles all the research done showing the effects of Atrazine on sexual development in many animals. The article can be found here. It was published online March 22, 2011 in the Journal of Steroid Biochemistry and Molecular Biology.
By examining all of the research done on the effects of Atrazine on sexual development, Hayes and colleagues make a very strong argument that Atrazine directly causes changes in male gonads (the testes). These changes feminize the animal–they turn a male into a female. The most striking case is in frogs, where a previous study found that about 10% of males had a complete sex change after exposure to the pesticide. These “females” had male sex chromosomes, but were able to lay fertile eggs. The other 90% had lowered fertility and sometimes had eggs developing in their testes (ouch!).
Atrazine changes the sex of frogs by inducing expression of an enzyme called aromatase in males, where it’s normally turned off. Usually, aromatase is turned on only in female frogs and leads to the production of estrogen.
In the caiman, a reptile, exposure to Atrazine before hatching lowered testosterone levels in males and caused defects in the testes. In rats, Atrazine causes defects in sperm production and may have other health effects as well.
Using cells derived from humans, scientists have shown that Atrazine can induce aromatase, the enzyme that leads to estrogen production. This suggests there may be a risk to humans as well as other animals.
Atrazine is a herbicide usually used on corn fields, and is commonly found in drinking water. Previous research suggested that Atrazine may be linked to birth defects, as discussed in this 2009 New York Times article. Understandably, the company that makes Atrazine, Syngenta, disagrees.
The lead scientist on the review article, as well as much of the research on the reproductive consequences of Atrazine, is Tyrone B. Hayes. Dr. Hayes is a professor at the University of California, Berkeley.