One of the most common weed killers in the world, atrazine, can chemically castrate male frogs, turning them into females that lay eggs say US researchers.
Professor Tyrone Hayes of the University of California, Berkeley and colleagues report their findings in this week's Proceedings of the National Academy of Sciences.
"Atrazine-exposed males were both demasculinised (chemically castrated) and completely feminised as adults," says Hayes.
Atrazine is widely used by farmers around the world as a herbicide, particularly in production of corn, sorghum and sugar cane.
Earlier studies have found that the chemical feminised zebra fish and leopard frogs and caused a significant decline in sperm production in male salmon and caiman lizards.
"Atrazine exposure is highly correlated with low sperm count, poor semen quality and impaired fertility in humans," say Hayes and colleagues.
While previous studies have shown atrazine adversely affects amphibian larval development, this latest study of African clawed frogs shows the process can go even further, says Hayes.
"Before, we knew we got fewer males than we should have, and we got hermaphrodites. Now, we have clearly shown that many of these animals are sex-reversed males," says Hayes.
Males produce eggs
Hayes and colleagues compared 40 male control frogs with 40 male frogs reared from hatchlings until full sexual maturity, in atrazine concentrations similar to those experienced year-round in areas where the chemical is found.
Of the male frogs exposed to atrazine 90% had low testosterone levels, decreased breeding gland size, feminised laryngeal development, suppressed mating behaviour, reduced sperm production and decreased fertility.
The remaining atrazine-exposed male frogs developed into functional females.
"10% of the exposed genetic males developed into functional females that copulated with unexposed males and produced viable eggs," the researchers wrote.
The larvae that developed from those eggs were all male.
"Atrazine has caused a hormonal imbalance that has made them develop into the wrong sex, in terms of their genetic constitution," says Hayes.
Hayes and colleagues exposed frogs to water contaminated with 2.5 parts per billion of atrazine.
In Australia, environmental groups have been concerned about the adequacy of testing of Australian waterways for chemicals such as atrazine.
For example, in 2008, Tasmanian Greens MP Tim Morris said Freedom of Information requests had revealed atrazine levels in the state's waterways as high as 7.42 parts per billion.
The National Health and Medical Research Council recently released revised guidelines that restrict the level of atrazine in drinking water to 20 parts per billion.
Whether the effects found in frogs translate to humans is far from clear.
Frogs have thin skin that can absorb chemicals easily and they literally bathe in the polluted water.
Australian regulatory response
The Australian Pesticides and Veterinary Medicines Authority (APVMA) says the link between atrazine and gonad development in amphibians is the subject of ongoing debate.
It says it is aware of these and other studies suggesting potential adverse environmental or human health impacts from exposure to atrazine.
"Should these evaluations reveal a causal link between atrazine and adverse impacts, the APVMA would consider appropriate regulatory responses," says Simon Cubit of the APVMA.
Atrazine is no longer approved for use in Europe and the US Environmental Protection Agency (US EPA) announced last year it would launch a new scientific evaluation of atrazine's effect on humans.
Cubit says the US EPA has indicated that its focus will be data generated since 2003, some of which suggests a potential association with birth defects, low birth weight and premature births.
"The APVMA is very interested in the work being undertaken by the US EPA and will liaise with the US regulator," says the spokesperson.