Tyrone B. Hayes (born July 29, 1967) is an American biologist and professor of Integrative Biology at University of California, Berkeley known for his research findings concluding that the herbicide atrazine is an endocrine disruptor that demasculinizes and feminizes male frogs. He is also an advocate for critical review and regulation of pesticides and other chemicals that may cause adverse health effects. He has presented hundreds of papers, talks, and seminars on his conclusions that environmental chemical contaminants have played a role in global amphibian declines and in the health disparities that occur in minority and low income populations. His work has been contested by Syngenta, the Swiss manufacturer of atrazine and the Australian Pesticides and Veterinary Medicines Authority. It was used as the basis for the settlement of a multimillion-dollar class-action lawsuit against Syngenta.

Education and career

Born in Columbia, South Carolina, Hayes spent his childhood studying frogs and lizards and won a state science fair with research that showed anole lizards had to be awake to change color. After graduating from Harvard University, Hayes was a technician and freelance consultant from 1990–1992 for Tiburon, California based Biosystems, Inc. Hayes has held an academic appointment (professorship) at the University of California, Berkeley since completing his doctoral research there in 1992; He was hired as a graduate student instructor in 1992, became an assistant professor in 1994, associate professor in 2000, and professor in 2003 in the Department of Integrative Biology, Molecular Toxicology, Group in Endocrinology, Museum of Vertebrate Zoology, Energy and Resources Group, University of California, Berkeley.

Atrazine research

In 1997, the consulting firm EcoRisk, Inc. paid Hayes to join a panel of experts conducting studies for Novartis (later Syngenta) on the herbicide atrazine. When Hayes' research found unexpected toxicities for atrazine, he reported them to the panel, however the panel and company were resistant to his findings. He wanted to repeat his work to validate it but Novartis refused funding for further research; he resigned from the panel and obtained other funding to repeat the experiments.

In 2002 Hayes published findings that he says replicate what he found while he was working for EcoRisk, that developing male African clawed frogs and leopard frogs exhibited female characteristics after exposure to atrazine, first in the Proceedings of the National Academy of Sciences (PNAS) and then in Nature.

In 2007, Hayes was a co-author on a paper that detailed atrazine inducing mammary and prostate cancer in laboratory rodents and highlighted atrazine as a potential cause of reproductive cancers in humans. At a presentation to the U.S. National Institute of Environmental Health Sciences in 2007, Hayes presented results of his studies that showed chemical castration in frogs; individuals of both sexes had developed bisexual reproductive organs.

In 2010, Hayes published research in PNAS describing laboratory work showing how exposure to atrazine turned male tadpoles into females with impaired fertility.

The U.S. Environmental Protection Agency (EPA) and its independent Scientific Advisory Panel (SAP) examined all available studies on this topic and concluded that "atrazine does not adversely affect amphibian gonadal development based on a review of laboratory and field studies.". The EPA and its SAP made recommendations concerning proper study design needed for further investigation into this issue. As required by the EPA, Syngenta conducted two experiments under Good Laboratory Practices (GLP) and inspection by the EPA and German regulatory authorities. The paper concluded "These studies demonstrate that long-term exposure of larval X. laevis to atrazine at concentrations ranging from 0.01 to 100 microg/l does not affect growth, larval development, or sexual differentiation." A report written in Environmental Science and Technology (May 15, 2008) cites the independent work of researchers in Japan, who were unable to replicate Hayes' work. "The scientists found no hermaphrodite frogs; no increase in aromatase as measured by aromatase mRNA induction; and no increase in vitellogenin, another marker of feminization."

In 2010, the Australian Pesticides and Veterinary Medicines Authority (APVMA) responded to Hayes' 2010 published paper, by stating that his findings "do not provide sufficient evidence to justify a reconsideration of current regulations which are based on a very extensive dataset.".


Based on his research findings, Hayes has become an advocate for banning atrazine.

According to Hayes, the link between atrazine and altered "aromatase and estrogen production has been demonstrated... in fish, frogs, alligators, birds, turtles, rats and human cells", and, "I believe that the preponderance of the evidence shows atrazine to be a risk to wildlife and humans. I would not want to be exposed to it, nor do I think it should be released into the environment." He travels and lectures extensively, both to academic, professional and lay audiences. He famously wrote a song called The Atrazine Rap to cleverly summarize and convey his message which he will often share in summation at the end of a talk: “So let me remind you / don't put this behind you / Atrazine ain't a good thing / it causes male frogs to grow eggs / contributes to extra legs / and exposed males don't want to sing / If that ain't enough / when you combine the stuff / with a few other pesticides / it causes greater than additive effects / unpredictable defects / exposed larvae don't grow / and they develop slow / and they contract diseases that otherwise could be beaten / you see this exposure effects their composure and determines who gonna eat and who gonna be eaten […] published that yet / and when I do / how will it affect you? / you may never know / because if the EPA / has their way / it may take 40 years or more / so if you're sitting there thinking / that the water you're drinking / is fine well that ain't the case / because you see this endocrine disruption that leads to biological disruption is relevant to all species / including the human race / so what? you might say / who cares anyway? / if somehow you still don't see the connection to you / I'm here to remind you that your son or daughter will develop in water / just like my tadpoles do / and so as we approach the hour I want to remind you that you've got the power / and that the whole world is waiting on your stance” He also has raised issues of environmental racism, "warning that the consequences of atrazine use [are] disproportionately felt by people of color. 'If you’re black or Hispanic, you’re more likely to live or work in areas where you’re exposed,' he has said."

Research published by Hayes and other scientists was used as evidence in a class action lawsuit against Syngenta by 15 water providers in Illinois that was settled for 105 million dollars in May 2012, which reimbursed more than 1,000 water systems for the costs of filtering atrazine from drinking water, although the company continues to deny any wrongdoing.

Conflict with atrazine manufacturer Syngenta

A long running conflict between Hayes and agricultural chemical manufacturer Syngenta was described as "one of the weirdest feuds in the history of science,” by Dashka Slater it in her 2012 profile of Hayes in Mother Jones magazine.

In 2014, New Yorker writer Rachel Aviv reported that Syngenta might have been orchestrating an attack not only on Hayes' scientific credibility, but on other scientists as well whose studies have shown atrazine to have adverse effects on the environment and/or human and animal health.

Aviv reported that Syngenta has criticized Hayes' science and conduct in press releases, letters to the editor, and through a formal ethics complaint filed at University of California-Berkeley. Internal Syngenta documents from 2005 released by a class-action lawsuit in 2014 show ways that Syngenta conspired to discredit Hayes, including attempting to get journals to retract his work, and investigating his funding and private life.

In one of the 2005 e-mails obtained by class-action lawsuit plaintiffs, the company’s communications consultants had written about plans to track Hayes' speaking engagements and prepare audiences with Syngenta's counterpoints to Hayes's message on atrazine. Syngenta subsequently stated that many of the documents unsealed in the lawsuits refer to "ideas that were never implemented."

In 2010 Syngenta forwarded an ethics complaint to the University of California Berkeley, complaining that Hayes had been sending sexually explicit and harassing e-mails to Syngenta scientists. Legal counsel from the university responded that Hayes had acknowledged sending letters having "unprofessional and offensive" content, and that he had agreed not to use similar language in future communications.

Popular culture

Hayes' work was featured in the 2008 documentary film Flow: For Love of Water. He appeared in the 2012 documentary film Last Call at the Oasis.

Hayes is the subject of The Frog Hunter, a biographical book for children, first published in 2009.

Hayes was a biologist on the Public Broadcasting Service, National Geographic program Strange Days, where he expressed his concerns for human health, particularly that of minority and low-paid workers exposure to agricultural chemicals. He is a National Geographic Society Explorer.