NSF PR 02-26 - April 16, 2002
Popular Weed Killer Disrupts Frogs' Sexual Development
The nation's top-selling weed killer, atrazine, disrupts
the sexual development of frogs at concentrations
30 times lower than levels allowed by the Environmental
Protection Agency (EPA), raising concerns about heavy
use of the herbicide on corn, soybeans and other crops
in the Midwest and around the world.
In the April 16 issue of Proceedings of the National
Academy of Sciences, developmental endocrinologist
Tyrone Hayes and colleagues from the University of
California, Berkeley, report that atrazine at levels
often found in the environment demasculinizes tadpoles
and turns them into hermaphrodites creatures with
both male and female sexual characteristics. The herbicide
also lowers levels of the male hormone testosterone
in sexually mature male frogs by a factor of 10, to
levels lower than those in normal female frogs. The
research was funded by the National Science Foundation
"Researchers are investigating a number of possible
causes for declining populations of amphibians," said
William Zamer, program director in NSF's division
of integrative biology and neuroscience, which funded
the research. "Hayes and colleagues appear to have
identified another potential agent. Their findings
indicate that atrazine has significant endocrinological
effects on frogs at very low doses."
As Hayes discovered, many atrazine-contaminated ponds
in the Midwest contain native leopard frogs with the
same abnormalities. "Atrazine-exposed frogs don't
have normal reproductive systems," he said. "The males
have ovaries in their testes and much smaller vocal
organs," which are essential in calling potential
mates. It is unclear whether these abnormalities lead
to reduced fertility. Hayes now is trying to determine
how the abnormalities affect the frogs' ability to
"The use of atrazine in the environment is basically
an uncontrolled experiment - there seems to be no
atrazine-free environment," Hayes said. "Because it
is so widespread, aquatic environments are at risk."
Because the herbicide has been in use for 40 years
in some 80 countries, its effect on sexual development
in male frogs could be one of many factors in the
global decline of amphibians, he added.
The findings come at a time when the EPA is re-evaluating
allowable levels of atrazine in drinking water, which
stand today at 3 parts per billion (ppb), and has
drafted new criteria for the protection of aquatic
life, limiting four-day average exposures to 12 ppb.
Hayes found hermaphroditism in frogs at levels as
low as 0.1 ppb. Even with today's limits, levels of
40 ppb atrazine have been measured in rain and spring
water in parts of the Midwest, while atrazine in agricultural
runoff can be present at several parts per million.
The herbicide also contaminates drinking water supplies
in many communities in the Midwest, leading some environmental
groups to voice concern about its effect on children,
infants and the fetus. France, Germany, Italy, Sweden
and Norway are among countries that have banned the
use of atrazine.
To date, atrazine's effects on mammals and amphibians
have been tested only at large doses, not at doses
commonly found in the environment.
In their journal article, Hayes and his colleagues
write, "the effective doses in the current study ...
demonstrate the sensitivity of amphibians relative
to other taxa, validate the use of amphibians as sensitive
environmental monitors/sentinels, and raise real concern
for amphibians in the wild."
Hayes doubts that atrazine has such severe effects
on humans, because the herbicide does not accumulate
in tissue and humans don't spend their lives in water
like frogs do. Nevertheless, the effects of atrazine
on frogs could be a sign that the herbicide is subtly
affecting human sex hormones, too, interfering with
androgens, such as testosterone, that control male