Impairments of Endocrine Functions: Case Studies - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 245

et al. 2011). As a consequence, the reader cannot help but be confused by the complexi-
ties of the issue. Nevertheless, a recent meta-analysis of all studies indicate that atrazine
consistently affects amphibian development (Rohr and McCoy 2010), which likely occurs
as a consequence of altered steroid balance. The most pronounced and dramatic outcome
of these imbalances is complete sex reversal (Oka et al. 2008; Hayes et al. 2010), but more
commonly results in retarded or abnormal gonadal development (e.g., Storrs-Méndez and
Semlitsch 2010) and reduced reproductive success (Hayes et al. 2010).
All evidence for an endocrine effect in amphibians is reliant on the correct interpreta-
tion of biomarker data. Knowledge of various potential morphological, behavioral, and
biochemical biomarkers is slowly building and will eventually lead to a better understand-
ing of the complex nature of endocrine disruption in amphibians and other fauna. Along
the way, several promising biomarkers have already emerged into prominence and sub-
sequently receded to become but one part of the greater picture. The occurrence of TOs in
testicular tissue was one of the first biomarkers to come into our view, and is likely to be
a strong biomarker of developmental stress and possibly endocrine disruption after expo-
sure to agricultural pesticides and other anthropogenic chemicals. However, in the case
of atrazine exposure, the occurrence of TOs may well have been a “red herring” because
of the inconsistencies between studies, and the occurrence of TOs as a natural phase in
the sexual development of many species. Similarly, the hypothesis that the endocrine dis-
ruptive action of atrazine is mediated by the induction of aromatase in amphibians is not
strongly supported by the literature, and other enzymatic pathways may provide better
mechanistic explanations for the effects of atrazine.
Biomarkers can only be determined in animals that continue to live in an environment,
and as indicated at the beginning of this discussion, amphibian populations in agricul-
tural landscapes provide a convenient laboratory for examining these issues. However,
we must be careful about interpreting data about endocrine disruption in amphibians as a
cause for amphibian population decline. Low-land amphibians that colonize agricultural
and other anthropogenic landscapes are generally r-strategists with high fecundity and
rapid reproduction. The occurrence of developmental abnormalities in even a large pro-
portion of individuals is an evolutionary trade-off offset by rapid adaptation to otherwise
hostile environments. The reader of the handful of studies that have documented histo-
logical abnormalities among field-collected frogs, cannot help but observe that the authors
were able to collect a large number of frogs (e.g., McCoy et al. 2008; McDaniel et al. 2008;
Skelly et al. 2010), and it is unlikely that the species being examined in these studies are
actually those species that we need to be concerned about.
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