Common Herbicide Suspected in Frog Sex Changes

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The Berkeley study, led by Tyrone Hayes, says atrazine changes genetically male frogs into functional females or mixed-sex individuals. Hayes continuously exposed a group of 40 African clawed frogs, starting when they hatched, to 2.5 ppb of atrazine. He found 10% of the male frogs became females, even producing viable eggs. Eighteen other frogs showed fewer testicular tubules with sperm bundles. In addition, his atrazine-exposed frogs were less successful in mating. To be sure to block out the effects of natural variations - frogs sometimes change sex spontaneously -- Hayes used only male frogs. He also bred the functionally female, (but genetically male) frogs, and found they produced only male offspring.

The Canadian group used leopard frogs, common in Canada, and a type that has declined in agricultural regions. The tadpoles were exposed to both 0.1 ppb and 1.8 ppb of atrazine. They found the frog populations produced more females. The male-female ratio in the control was 1:0.6, while at the lower exposure it was 1:0.8, and at the higher it went to 1:1.4. Successful metamorphosis was also reduced-- fully 76 per cent of the controls reached metamorphosis while the atrazine-exposed frogs only did so 40 to 50 per cent of the time. Their research appeared in Environmental Health Perspectives.

Some scientists are unconvinced. Werner Kloas, an endocrinologist at Berlin's Humboldt University, studied the same African frog species in 2008, using more than 3,000 animals in two different labs. He found no effect of atrazine at four different concentrations ranging from 0.1 ppb to 100 ppb. Kloas says Hayes' experiment should be repeated with several atrazine levels. There is also the question of whether the effect depends on what stage of development the frog is in when it is exposed. Regarding the Canadian study, Kloas says his research showed little or no effect on frogs' thyroid systems, which play an important role in metamorphosis.

Alan Hosmer, senior technical advisor at Syngenta, was also critical of the University of Ottawa research. He says the Canadian study used water contaminated with the herbicide as its control. The type of atrazine was also different from that in the environment, and investigators should also have equalized the sex ratios from the beginning.

Kloas also noted South African scientists haven't found any drastic differences in sex ratios among clawed frogs in the wild, where they have been exposed for decades.

One of the authors of the Canadian research, University of Ottawa biologist Vance Trudeau, counters by noting most water supplies in North America have some atrazine in them, and it is often near or below detection limits. The important thing is that after addition of atrazine to the water the sex ratios changed. Trudeau also sees the metamorphosis results as more important. Frogs that fail to metamorphose on time can be vulnerable when ponds dry up over the course of the summer, or to predators.

Jason Rohr, of the department of integrative biology at the University of South Florida, published a meta-analysis of atrazine research in January. He says the balance of evidence is that there is some effect, though the mechanism and magnitude might be under dispute. Meanwhile the debate among scientists is tinged with acrimony; Hayes is decried as an "activist," Hayes says Syngenta-funded studies such as Kloas' can't be trusted.

American farmers use more than 30 million kilograms of atrazine per year. Don Coursey, a professor of pubic policy at the University of Chicago, estimates a ban on atrazine would mean the loss of between 21,000 and 48,000 jobs in the agricultural sector. Switching to another product would cost farmers between $26 and $56 per acre. And even a ban doesn't mean the end of weed killer; the substitute used in the European Union, terbuthylazine, is chemically quite similar.

Representative Keith Ellison (D-Minn.) introduced a bill to ban the chemical, citing human health concerns. That bill went to the House Agriculture Committee in June. Similar bills were introduced in previous sessions but were never passed.