WASHINGTON - Male
frogs exposed to even very low doses of a common weed
killer can develop multiple sex organs - sometimes
both male and female - researchers in California have
"I was very much surprised,"
at the impact of atrazine on developing frogs, said
Tyrone B. Hayes of the University of California at
Atrazine is the most commonly
used weed killer in North America, he said, and can
be found in rainwater, snow runoff and ground water.
"There is virtually no
atrazine-free environment," Hayes said.
The Environmental Protection
Agency permits up to 3 parts per billion of atrazine
in drinking water.
But Hayes' team found
it affected frogs at doses as small as 0.1 part per
billion. As the amount of atrazine increased, as many
as 20 percent of frogs exposed during their early
development produced multiple sex organs or had both
male and female organs. Many had small, feminized
Hayes' research team concluded
that the effect on the frogs results from atrazine
causing cells to produce the enzyme aromatase, which
is present in vertebrates and converts the male hormone
testosterone to the female hormone estrogen.
The effects on frogs in
Hayes' study occurred at exposure levels more than
600 times lower than the dose that has been seen to
induce aromatase production in human cells.
Their research is reported
in Tuesday's issue of Proceedings of the National
Academy of Science.
Asked if atrazine might
also be a threat to people at low levels, Hayes said
he did not know, adding that, unlike frogs, "we're
not in the water all the time."
"I'm not saying it's safe
for humans. I'm not saying its unsafe for humans.
All I'm saying is it that it makes hermaphrodites
of frogs," he said.
Stanley I. Dodson of the
University of Wisconsin at Madison called the work
"the most important paper in environmental toxicology
"It shows the effect of
the most commonly used herbicide on amphibians in
environmentally relevant concentrations," he said.
Asked if people should
be worried, he also said: "We don't know."
"It's like a canary in
the mine shaft sort of thing," Dodson said, referring
to the former practice of miners of bringing canaries
with them as warnings of dangerous gases. The birds
are very sensitive to gases and would die before the
concentration of the gas was enough to harm the miners.
Dodson said that in his
research he had found that low exposures atrazine
changes the ratio of males to females among water
In addition to its effects
on developing frogs, the Berkeley researchers found
that male frogs exposed to atrazine after reaching
maturity had a decrease in testosterone to levels
equivalent to that found in females.