Dr Tyrone Hayes
Professor of Integrative Biology at UC Berkeley. The scientist who discovered that atrazine chemically castrates and feminises male frogs at concentrations legal in US drinking water, and spent two decades fighting the manufacturer who tried to silence him.
Dr Tyrone Hayes is Professor of Integrative Biology at the University of California, Berkeley, where he has worked since completing his PhD in 1993. He grew up in South Carolina with a childhood fascination with frogs that eventually became the focus of a research career spanning three decades. His laboratory at Berkeley studies the effects of environmental chemicals on amphibian development, with a particular focus on the herbicide atrazine. His research has been published in the Proceedings of the National Academy of Sciences, Nature, Environmental Health Perspectives and dozens of other peer reviewed journals. He is one of the most compelling public speakers in the field of environmental health and has given hundreds of lectures worldwide. His story is also a case study in what happens when corporate interests and scientific truth collide directly.
Watch the Talk
In this TEDxBerkeley talk from 2018, Hayes covers his childhood fascination with frogs, how he came to study atrazine, what he found, the Syngenta suppression campaign, and why those who suffer most from chemical exposure are consistently the most economically vulnerable. One of his most complete and accessible public accounts of the full story.
This Democracy Now interview from 2014 focuses specifically on the Syngenta suppression campaign, recorded shortly after the New Yorker investigation using court documents was published. Hayes is direct and specific about what happened and what the internal documents revealed.
Content note: This interview includes Hayes's account of racial threats, homophobic threats and threats of sexual violence made against him and his family by individuals connected to Syngenta. Recounted in his own words as part of the documented suppression campaign.
The Research
In 1997 Hayes was approached by Novartis Agribusiness, a corporate predecessor of the pesticide manufacturer Syngenta, to study the effects of the herbicide atrazine on amphibian development. He assumed the company would not have hired him if there was anything to find. He was wrong.
Atrazine is the second most widely used herbicide in the United States and is widely applied to corn, sorghum and sugarcane crops. It is the most commonly detected pesticide contaminant in US ground water, surface water and drinking water. It was banned in the European Union in 2004 due to persistent groundwater contamination.
Hayes et al., Proceedings of the National Academy of Sciences, 2002. African clawed frogs exposed to atrazine at concentrations as low as 0.1 parts per billion, which is below the US Environmental Protection Agency's legal limit for atrazine in drinking water, showed significant disruption to sexual development. Male frogs developed abnormally small testes, had reduced testosterone, and showed elevated oestrogen levels. Some developed both male and female reproductive organs.
Hayes et al., Proceedings of the National Academy of Sciences, 2010. The definitive study. Long-term exposure of male African clawed frogs to atrazine at concentrations within the legal limit for US drinking water caused complete feminisation in 10% of genetic males, who developed into functional females capable of mating with other males and producing eggs. A further proportion showed severe demasculinisation without complete sex reversal. The mechanism identified was atrazine's activation of aromatase, an enzyme that converts testosterone into oestrogen.
DR TYRONE HAYES
Distinguished Scientist Lecture, Trinity University 2017
If you watch the full lecture separately, note that the talk begins at 3:04 after an introductory music and welcome section.
About 10% of genetically male frogs exposed to atrazine completely grew up to be females and lay eggs.
The aromatase enzyme activated by atrazine in frogs is the same enzyme present in human tissue. Elevated aromatase activity in humans is associated with reduced male fertility, disrupted testosterone production and increased risk of hormone-sensitive cancers including breast and prostate cancer. An independent EPA scientific advisory panel found suggestive evidence linking atrazine to certain cancers in humans. Hayes has consistently argued that the evidence is sufficient to justify precautionary action.
DR TYRONE HAYES
Distinguished Scientist Lecture, Trinity University 2017
His colleague Dr Shanna Swan found that men with atrazine in their urine at just 0.1 parts per billion had a low sperm count and could not get their partner pregnant.
Hayes has argued that atrazine is a significant contributing factor to the global decline of amphibian populations, which have fallen dramatically over the past half century. Amphibians are considered an indicator species, particularly sensitive to environmental chemical contamination. Their decline, Hayes argues, is a warning sign for the broader effects of pesticide contamination on ecosystems and on the species that depend on them, including humans.
The Corporate Suppression Campaign
When Hayes presented his initial findings to the manufacturer-convened panel and refused to alter his conclusions, Syngenta declined to publish the results. Hayes left the panel, replicated his research independently, and published in peer reviewed journals. What followed was a decade-long campaign by Syngenta to discredit both his science and his personal reputation.
Internal Syngenta documents obtained through litigation and subsequently made public revealed a detailed corporate strategy that included funding scientists to conduct studies aimed at contradicting Hayes's findings, monitoring his public appearances and communications, and working to undermine his credibility with regulatory agencies and academic institutions.
DR TYRONE HAYES
Democracy Now interview, 2014
On what happened when he refused to suppress his findings. The company moved to asking him to manipulate data, and ultimately told him he could not publish.
My hypothesis was that nothing was going to happen. I assumed they would not have hired me if there was something to find. But there was something to find. And once I found it, everything changed.
Dr Tyrone Hayes, Mother Jones interview
The Hayes and Syngenta story was investigated in depth by Rachel Aviv in a 2014 New Yorker article titled The Toad Who Could Foil Syngenta, which documented the full extent of the manufacturer's efforts to suppress and discredit his research. The article remains one of the most detailed accounts available of how corporate interests operate to delay chemical regulation.
DR TYRONE HAYES
Democracy Now interview, 2014
On the scientists paid to contradict his findings. Their opinions were written by the manufacturer. They were personalities for sale.
Why This Matters Beyond Frogs
The significance of Hayes's atrazine research extends well beyond any single chemical or any single species. His work established several principles that now underpin the broader understanding of endocrine disruption:
Atrazine causes significant reproductive disruption at 0.1 parts per billion, a concentration below the legal limit and far below the concentrations traditionally studied in toxicology. This directly challenges the assumption that only the dose makes the poison and supports the broader endocrine disruption research showing that very low concentrations of hormonally active chemicals can cause serious harm.
The concentrations at which Hayes documented harm in frogs are within the legal limits for atrazine in US drinking water. This is not a chemical causing harm at extreme exposure levels. It is causing harm at the levels present in the water supply that millions of people drink every day.
The Syngenta documents made public through litigation provide a detailed blueprint of how a manufacturer suppresses inconvenient scientific findings. The same playbook, funding counter-studies, targeting individual scientists, lobbying regulatory agencies, has been documented in tobacco, asbestos, PFAS and other industries. Hayes's case makes it visible and specific.
DR TYRONE HAYES
Distinguished Scientist Lecture, Trinity University 2017
Your grandchildren could be affected by chemicals that we are using today. That made him realise he had a much bigger responsibility than just a little boy who likes frogs.
Content note: This clip contains scientific images of the effects of atrazine on reproductive development, including images of birth defects in affected infants. Shown in a clinical research context.
Atrazine is banned in Europe and freely used in the United States. The science behind that ban and the story of what happened to the scientist who produced it are both worth understanding. Not because frogs are a sentimental concern but because what atrazine does to frog hormones at legal concentrations is a precise demonstration of what endocrine disrupting chemicals do, how they work, and why the regulatory response so often lags decades behind the evidence.
The frogs were the warning. We were not listening.
Sources: Hayes TB et al., PNAS (2002) | Hayes TB et al., PNAS (2010) | Rachel Aviv, The New Yorker, February 2014 | Union of Concerned Scientists, Syngenta and Tyrone Hayes (2013) | US EPA Atrazine Science Reevaluation | TEDxBerkeley, March 2018. Video courtesy of TED. Democracy Now interview, February 2014. Video courtesy of Democracy Now.