Environmental Engineering Reference
In-Depth Information
Reproductive Effects in Birds
(Egg-Injection Studies)
density of muscarinic cholinergic receptors and negative
correlations between brain Mehg and density of nMda
receptors (scheuhammer
et al., 2008). a study of lower-
brain stem-tissue from 87 greenlandic polar bears found
surprisingly low brain hg concentrations in this top arctic
predator; nevertheless, a significant negative association
between brain total hg and nMda receptor concentra-
tion was observed (Basu et al., 2009). currently, studies are
underway to examine similar neurochemical end points
in a variety of other fish and wildlife species, including
sharks, dolphins, and seals. the demonstrated associations
between hg and neurochemical changes are of physiologic
importance because muscarinic cholinergic and nMda
receptors play important roles in normal memory, learn-
ing, and locomotion (ozawa
et al., 1998; Wess, 2004), all
of which are essential to normal neurobehavioral develop-
ment, survival, and reproduction. taken together, these
findings indicate that ecologically relevant levels of Mehg
may be exerting subtle neurologic damage in a diverse
group of fish-eating wildlife species, the long-term conse-
quences of which are not yet understood.
some of the correlations between brain hg concentra-
tions and neurochemical changes reported in wild ani-
mals have been verified using controlled feeding studies in
which juvenile male mink were fed diets containing 0, 0.1,
0.5, 1, and 2 µg g
-1
Mehg for 89 days (Basu et al., 2006a,
2007b, 2008). overt histologic lesions were found only in
the brains of mink exposed to the highest dietary Mehg
concentrations (1 and 2 µg g
-1
Mehg) (lyn ferns dVM,
nova scotia department of agriculture, Veterinary services
section, personal communication); and similar dietary lev-
els of Mehg caused brain lesions and neurobehavioral tox-
icity in captive mink in other studies (Wobeser et al., 1976;
Wren et al., 1987). at lower, more ecologically relevant
exposure levels (0.1-0.5 µg g
-1
dietary Mehg), histologic
changes were not seen, but significant changes in neuro-
chemistry were observed. at dietary Mehg concentrations
as low as 0.1 µg g
-1
—a level commonly found or exceeded
in many north american fish species (Kamman et al.,
2005)—hg-dependent increases in muscarinic cholinergic
receptor levels (total receptors and specific receptor sub-
types) and cholinesterase activity (Basu et al., 2006a, 2008),
and striking decreases in nMda receptor levels (Basu
et al.,
2007b) were observed in several brain regions. the cellu-
lar mechanisms underlying these neurochemical changes
are discussed in the references provided. relationships
between tissue hg accumulation and neurochemical changes
observed in captive mink not only validate the results of
field studies, but also help establish a continuum of Mehg
neurotoxicity whereby uptake of Mehg is followed first by
neurochemical changes, and then neurobehavioral tox-
icity, neurologic impairment, reproductive dysfunction,
and ultimately death at higher levels of exposure (Basu
et al., 2007a). the monitoring of neurochemical changes is
an emerging tool for assessing the early subclinical effects
of Mehg exposure in wildlife.
among wildlife, some bird species seem to be particularly
sensitive to Mehg, and the developing embryo is espe-
cially sensitive to hg exposure in birds (scheuhammer,
1987; thompson, 1996; Wiener et al., 2003). consequently,
much research has been devoted to determining how much
Mehg in the diet of breeding birds, or in their eggs, rep-
resents a threshold for reproductive harm. field studies
have implicated Mehg as a cause of reproductive impair-
ment in common loons (Burgess and Meyer, 2008; Evers
et al., 2008), common terns (
Sterna hirundo
) (fimreite,
1974), california clapper rails (
Rallus longirostris obsoletus
)
(schwarzbach et al., 2006), white ibises (
Eudocimus albus
)
(heath and fredrick, 2005), and snowy egrets (
Egretta
thula
) (hill et al., 2008). however, results from field stud-
ies are always complicated by the complex assemblage of
environmental factors that may accompany changing lev-
els of hg contamination. it is desirable to study the effects
of Mehg on bird reproduction within the context of the
natural environment in which these effects are occurring,
but the complexity of field conditions often makes it dif-
ficult to isolate the effects of hg from those of other stress-
ors. factors such as disease, food shortage, predation, and
weather may cause reduced reproduction or interact in
unknown ways with Mehg exposure. in one field study,
hill et al. (2008) reported that Mehg seemed to reduce
the reproductive success of snowy egrets, but only during
drought years. in another study, the relative contributions
of lake acidity and Mehg exposure to reduced reproductive
success in common loons were initially unclear (Meyer et
al., 1998). field studies, by their labor-intensive nature, also
tend to be very time consuming and expensive.
to overcome the complexities of field studies, and to
determine how much Mehg in the diet of breeding birds
or in their eggs is needed to impair reproduction, research-
ers sometimes turn to captive breeding studies. in these
controlled studies, a colony of breeding birds is established
and randomly selected groups are fed either a control
diet or diets containing various concentrations of Mehg.
the obvious advantage of these controlled breeding stud-
ies is that all variables, except the dietary concentration
of Mehg, are equivalent for all groups. therefore, there
should be no confounding effects (disease, food supply, pre-
dation, or weather) on the outcome. such controlled studies
have yielded very useful information about dietary and egg
levels of hg that are associated with reproductive impair-
ment in birds (tejning, 1967; fimreite, 1971; finley and
stendell, 1978; heinz 1979; albers et al., 2007).
unfortunately, controlled breeding studies suffer from
their own set of weaknesses. Perhaps the most obvious is
that, in controlling for all other factors that might affect
reproduction, interactions of hg with other environmental
stressors are generally not tested. for example, it is unlikely
that the interaction between drought and methylmercury
