carbamates are the most widely used insecticides and birds frequently suffer nontar-
get exposure. Both types of insecticide act by inhibiting acetylcholinesterase (AChe)
resulting in an accumulation of acetylcholine in the synapses (Mineau 1991). AChe
inhibition is usually measured in the brain, since this is the principle site of action
and brain AChe inhibition can be related directly to behavioral effects (Hart 1993).
Measuring brain AChe inhibition is obviously destructive. Nondestructive assess-
ment, using inhibition of blood AChe (or butyrylcholinesterase) may be more
acceptable, and can be used on free-living birds (e.g. Parsons et al. 2000). However,
the relationship to inhibition of brain AChe is complex. A further complication is
that exposure to other pesticides can act synergistically to enhance the toxic effects
of some insecticides ( Johnston 1995; Johnston et al. 1996).
Another commonly used biomarker is the induction of the heme containing
enzymes known as cytochromes P 450 (so-called because their spectral peak is at
450 nm). These form a large family of monooxygenases with a wide range of func-
tions including biosynthesis of endogenous compounds such as steroid hormones,
and metabolism of a range of endogenous and exogenous compounds. The latter
function is of relevance to ecotoxicology—they detoxify many anthropogenic
compounds. A wide range of chemicals induces P 450 activity in birds, including
polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB),
organochlorine pesticides and ergosterol biosynthesis inhibiting fungicides
(e.g. Ronis et al. 1998; Schlezinger et al. 2000). This makes P 450 a useful biomarker
to detect pollutant exposure. Conversely, it does not reveal the specific causative
pollutant. The most commonly used tissue for estimating P 450 activity is the liver,
which means that sampling is destructive.
An area of current interest in ecotoxicology is endocrine disruption—exogenous
chemicals that interfere with the normal functioning of the endocrine system.
There is clear evidence that fish exposed to phytooestrogens in pulp mill effluent
and to human-derived oestrogens in sewage outflows suffer endocrine disrup-
tion. There is also evidence that synthetic chemicals can also have endocrine
disrupting effects, but it is uncertain whether this can be caused by environ-
mentally realistic levels of exposure. Whether there are any examples of endocrine
disruption in free-living birds (and other terrestrial vertebrates) remains contro-
versial (Dawson 2000).
9.4 Endocrinology
Most organisms live in environments that fluctuate on a predictable schedule
(seasonal cycles). Individuals must therefore adjust to maximize their survival
over a wide range of environmental conditions. The annual cycle comprises
222 |Techniques in physiology and genetics