Berkeley - Native male leopard frogs throughout
the nation's Corn Belt are being feminized by an herbicide,
atrazine, used extensively to kill weeds on the country's
leading export crops, corn and soybeans, according to a survey
conducted by University of California, Berkeley, biologists
and reported this week in Nature.
The UC Berkeley scientists also showed that male leopard
frogs raised in laboratory tanks contaminated with atrazine
develop egg cells in their testes and essentially turn into
hermaphrodites. These sexual abnormalities were observed
at atrazine levels as low as 0.1 parts per billion (ppb),
30 times lower than the current allowable limit for atrazine
in drinking water set by the Environmental Protection Agency.
These findings, added to earlier evidence that atrazine
demasculinizes two other species of frog, suggest that the
herbicide could be a factor in the decline of frogs and
other amphibians in the United States and around the world,
the authors say. Atrazine has been used on crops since 1956
and currently is the most widely used herbicide in the nation.
"These studies clearly indicate that atrazine is detrimental
to amphibians," said study author Tyrone Hayes, associate
professor of integrative biology at UC Berkeley.
At the least, he said, atrazine is altering amphibian populations
in large areas of the United States. His field studies show
that frogs seem to adapt, since leopard frogs (Rana pipiens)
- the most common frogs in the Midwest - are often abundant
in some of the corn growing regions where atrazine is used
most. Atrazine may feminize male tadpoles and turn them
into female frogs, he said, or it may render some males
infertile. Alternatively, atrazine may favor tadpoles that
delay sexual differentiation until after they've turned
into frogs and leave the contaminated water.
"Atrazine is potentially destroying biodiversity," said
Hayes, now engaged in studies to determine the ultimate
fate of these feminized tadpoles. "In my opinion, this is
an unacceptable risk."
Hayes and his colleagues sampled leopard frog tadpoles
in eight separate ponds, ditches, rivers and streams in
the Midwest during the summer of 2001 and found feminized
male frogs at every site with measurable levels of atrazine.
The current laboratory detection limit is 0.1 parts per
The sites were scattered through the Corn Belt and beyond,
including in Utah, Wyoming, Nebraska and near the Iowa-Illinois
border. Several ponds chosen as controls because they are
in nonagricultural areas also had measurable levels of atrazine
and feminized frogs, while only one, in Utah, had neither
detectable atrazine nor affected frogs.
The site with the highest concentration of feminized frogs
was along the North Platte River in Wyoming. There, 92 percent
of male frogs showed sex reversal. This area of Wyoming
reports little use of atrazine, but the river is fed by
streams that carry runoff from Colorado farms, which do
use significant amounts of the herbicide.
Earlier this year, Hayes and his colleagues reported that
a common laboratory frog, the African clawed frog (Xenopus
laevis), became demasculinized when raised in lab tanks
with concentrations of atrazine of 0.1 ppb or higher. The
current allowable limit set by the Environmental Protection
Agency for atrazine in drinking water is 3 ppb, and the
proposed chronic exposure limit for aquatic life is 12 ppb.
The new experiment, reported in the Oct. 31 issue of Nature,
is a repeat of these experiments using the leopard frog
. The new report also contains a summary of Hayes' survey
of Midwestern frog ponds.
A field study by another group showed similar gonadal abnormalities
in atrazine-exposed northern cricket frogs (Acris crepitans).
Just as in Hayes' earlier experiment, the male tadpoles
developed female characteristics in their sex organs, or
gonads. Up to 29 percent of males developed female egg cells
(oocytes) in their testes, becoming hermaphrodites. In one
experiment, more than a third of the male frogs exposed
to 0.1 ppb atrazine showed under-developed testes.
Details of the experiment are reported in the November
issue of the journal Environmental Health Perspectives (EHP).
"The testes in these male frogs are obviously not functioning
normally, because if they were, egg cells would not be able
to grow in them," Hayes said. "Some testes are so invaded
by ovary cells it looks like they are converted, and technically,
they could be considered ovaries."
Hayes suspects that atrazine boosts the activity of an
enzyme, aromatase, that converts male sex hormones, or androgens,
to female hormones, or estrogens. The lowered androgens
and increased estrogens allow egg cells to grow within the
testes, which is normally impossible. Atrazine's effects
on aromatase have been demonstrated in fish, reptiles and
mammals, but not yet in amphibians.
Atrazine is so widespread that it can be found far from
agricultural areas and even in rainwater and snow. At one
site in Nebraska, Hayes found that rain and tap water contained
enough atrazine to disrupt normal male development in amphibians.
"The current data raise concern about the effects of atrazine
on amphibians in general and the potential role of atrazine
and other endocrine-disrupting pesticides in amphibian declines,"
the authors wrote in their EHP article.
The leopard frog studies are supported by grants from the
National Science Foundation, the W. Alton Jones Foundation,
the World Wildlife Fund, The Homeland Foundation, the Rose
Foundation and the Howard Hughes Medical Institute. Hayes'
coauthors are former or current undergraduate students -
Kelly Haston, Mable Tsui and Cathryn Haeffele - postdoctoral
fellow Anhthu Hoang and research associate Aaron Vonk.
Hayes is a member of UC Berkeley's Health Sciences Initiative,
a broad-based effort to bring the physical and biological
sciences together to tackle health problems of the 21st