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In-Depth Information
9.3 Biomarkers of Endocrine Disruption in Amphibians
9.3.1 Introduction
For more than 20 years, the phenomenon of amphibian population decline has provided
the impetus for research into the effects of agricultural chemicals in amphibians. Despite
the fact that not a single population decline can be unreservedly ascribed to the effects of
exposure to agricultural chemicals, suspicions remain strong—possibly a legacy of con-
cerns left in the wake of Rachel Carson's
Silent Spring
(1962), which still stands today as an
important milestone in our understanding of the dangers posed by pesticides in wildlife,
and a publication that spawned five decades of ecotoxicological research.
When it comes to amphibians, there is an irony that needs to be recognized—that agri-
culture waters (dams, weirs, irrigation channels, rice paddies, drainage channels, etc.)
provide breeding habitat for amphibians (Knutson et al. 2004). Whether indeed these artifi-
cially maintained and regulated waters are suitable for the full range of amphibian species
in a geographical region is debatable (Shulse et al. 2010), but frogs and other amphibians
do successfully colonize and persist in wetlands associated with agriculture (Herzon and
Helenius 2008; Choung et al. 2011), despite the often high concentrations of various agri-
cultural pesticides (Harris et al. 1998; Bishop et al. 1999; Bérubé et al. 2005; McDaniel et al.
2008; Bishop et al. 2010a; Spolyarich et al. 2011).
One of the difficulties with trying to discover the underlying causes of extinctions and
population declines, is that the primary witnesses are unavailable for questioning, and
the smoking gun can seldom be identified. Invariably, we need to look for clues among
remnant species and individuals. The persistence of amphibian populations in agricul-
tural landscapes provides a paradigm for the examination of pesticide effects in wildlife
in general. A recently perceived threat to wildlife is that of endocrine disruption, which is
epitomized by the highly publicized feminization of fish downstream of sewage treatment
plants (Jobling et al. 1996; Tarrant et al. 2008) and alligators exposed to pesticides in Lake
Apopka, Florida (Milnes and Guillette 2008).
Since the 1990s, amphibians living in agricultural landscapes have provided a labora-
tory for the assessment of the potential of biomarkers that reflect the risks associated
with pesticide exposure. One particular pesticide that has drawn a great deal of attention
is atrazine. Indeed, there are few more controversial issues in the discipline of ecotoxi-
cology than that surrounding atrazine and the consequences of exposure in amphibians,
and the story is signposted by numerous biomarkers that have been used to build the
case for pesticide induced endocrine disruption. The amphibian-atrazine story provides
a valuable case study for which several biomarkers of exposure and effect have played a
central role, particularly as biomarkers of the risk to recruitment, because almost all of
the research has specifically examined the effects of atrazine on the reproductive poten-
tial of amphibians. Much has already been written on this issue and numerous reviews
are available—some of them partisan (Solomon et al. 2008; Hayes et al. 2011), some of
them less so (Mann et al. 2009; Bishop et al. 2010b; Rohr and McCoy 2010), but the picture
is far from complete.
9.3.2 Ambiguous Gender and Gonadal Histology
In 2002, two studies came to light regarding apparent endocrine effects of atrazine in frogs.
The first was a study by Tavera-Mendoza (2001), some of which was later published as two
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