 (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 testosterone compared to controls. This effect
is highly significant statistically. Treated male testosterone
levels were indistinguishable from control females.
Hayes et al.
does it mean?
This research eliminates any doubt about atrazine as an endocrine
disruptor at extraordinarily low levels of exposure. It puts endocrine
disruption high onto the list of plausible factors contributing
to frog population declines. And because of the apparent mechanism
of action, via enhancement of aromatase conversion of testosterone
to estrogen, it raises important concerns about atrazine disrupting
the hormonal control of development in other organisms, including
Hayes et al. summarize in their paper, atrazine contamination
is extremely widespread , including, via atmospheric transport,
in rainwater in regions where atrazine is not used for agricultural
crops. The level of atrazine sufficient to cause reproductive abnormalities
in Xenopushermaphroditismis 1/30th of the level allowed
by the US EPA in drinking water:
recommended application level of atrazine ranges from 2,500,000
29,300,000 ppb, the allowable contaminant level for atrazine
in drinking water is 3 ppb, and short-term exposures of 200
ppb are not considered a health risk. Atrazine can be as high
as 21 ppb in ground water, 42 ppb in surface waters, 102 ppb
in river basins in agricultural areas, up to 224 ppb in Midwestern
streams, and up to 2,300 ppb in tailwater pits in Midwestern
agricultural areas. Atrazine can be found in excess of 1 ppb
in precipitation in localities where it is not used and up to
40 ppb in rainfall in Midwestern agricultural areas."
in many places in the world are undoubtedly exposed to atrazine
at these levels. It remains to be determined whether other frog
species are as sensitive to atrazine as is Xenopus, and whether
the effects are similar. Hayes' work should reinvigorate efforts
to understand the contribution of chemical exposure to frog declines.
has strongly implicated infection by a chytrid fungus as an important
factor driving frog extinctions