et al. demonstrate that at exposure levels far beneath
those found in the lakes, rivers, streams, drinking water and even
rainwater, atrazine causes frogs to mature with multiple, mixed
gonads and to become demasculinized. These effects occurred at exposure
levels 10,000 - 30,000 times beneath levels previously identified
as non-toxic to frogs.
impact on frogs appears to be caused by this herbicide's ability
to promote the conversion of testosterone to estrogen via activity
of the enzyme aromatase. This mechanism is found not just in frogs,
but other vertebrates as well, including mammals. Their study raises
important, and as yet unanswered, questions about the possible role
of atrazine in world-wide frog population declines, and about the
potential for atrazine to affect human health via the same enzymatic
is one of the most widely and heavily used agricultural chemicals
in the world. Each year, American farmers alone apply 60 million
pounds to U.S. farmland to prevent growth of weeds competing with
corn and other crops. The US EPA considers short-term exposure to
200 ppb acceptable for people and allows 3 ppb atrazine in drinking
water. In rainwater in regions where atrazine is applied, atrazine
contamination has been measured up to 40 ppb. It has even been found
at 1 ppb in rainwater in regions where atrazine is not
used. Hence it is virtually certain that frogs (and people)
living in the real world are regularly exposed to atrazine at levels
many times greater than what Hayes et al. report is sufficient
to harm frogs.
did they do? Hayes et al. used two different experimental
designs to study the effect of atrazine on the laboratory rat of
the amphibian world, the African Clawed Toad Xenopus laevus.
In both designs, Hayes took great care to make sure that the scientists
measuring the exposed animals were unaware of which treatment they
had received. Only after all the measurements were taken did a coding
system reveal what treatment each animal experienced.
sample sizes used in the experiments were quite large (90 animals
per treatment); these experiments were repeated four times; and
experiments with atrazine at these low levels were replicated 51
times in Hayes' lab with similar results.
1: expose developing tadpoles to different levels of atrazine and
examine individuals for morphological effects after metamorphisis.
The tadpoles were exposed to concentrations of atrazine from 0.01
ppb to 25 ppb.
2: expose adults directly to 25 ppb atrazine and measure testosterone
and estrogen levels. Adults were used because they found it not
possible to obtain enough serum from tadpoles to do the hormonal
did they find?
Experiment 1: When exposed as tadpoles to as little as 0.1 part
per billion atrazine, individuals mature with hermaphroditic deformities
in the reproductive tract. Normal adult frogs have two gonads, either
two testes if male or two ovaries if female. Between 16% and 20%
of animals treated with atrazine at 0.1 ppb and above had either
two many gonads and/or mixtures of ovaries and testes. This is not
normal. Hayes et al. comment that they had never seen these
effects in 6 years of study of Xenopus involving over 10,000
figures below, from Hayes et al. shows the deformed reproductive
tract of one hermaphroditic frog. This particular individual has
three testes and three ovaries. The microscopic photograph on the
left shows the entire complex of the animals gonads and kidneys.
The microphotograph on the right shows cross-sections through gonadal
material at places specified by the arrows on the left. T= testis;
tadpoles were exposed to atrazine at 1 ppb and above, Hayes et
al. noted a demasculinization of the adults secondary sexual
characteristics, specifically a reduction in the size of the males'
larynges (vocal chords). Normally male larynges are larger than
female. The atrazine-treated males had larynges that were intermediate
in size between normal male and normal female larynges. There was
no effect on the female larynges size.
2: Adult males exposed to 25 ppb atrazine showed a 10-fold
decrease in testostero