sure to environmental toxicants during early development in
the womb. EEDs can interfere with the normal hormonal cues
required in the developing embryo, and can profoundly affect
the development of the reproductive system. Not surprisingly,
EEDs are having a major impact on wildlife.
The effect of EEDs on aquatic species has been studied exten-
sively, but their effects are much less well understood in terres-
trial animals. Furthermore, there are almost no studies that
have examined the impact of these chemicals on Australia’s
marsupials.
Australian marsupials are exposed to a wide range of EEDs
from agricultural and industrial contaminants as well as natural
toxins in their diet. Research has shown that marsupials are
especially sensitive to alterations to their hormonal environ-
ment during development. These hormonal alterations can
have a profound effect on sex determination, causing sex reversal
in some cases and reduced fertility. Thus it appears that EED
exposure is of major concern to marsupial health.
Furthermore, many of Australia’s marsupial species are expe-
riencing population decline. While habitat loss and species
introduction is undoubtedly a major contributor to this decline,
exposure to foreign chemicals, such as EEDs, has the potential
to have a major impact on marsupial species in certain envi-
ronments.
Marsupials have many unique features of their development
and physiology that affect their ability to tolerate EED expo-
sures and consequently make them more susceptible to their
negative effects than eutherian/placental mammals. Marsu-
pials are born in a highly under-developed state. Most marsu-
pial growth and development occurs after birth, in the pouch,
while the young is dependent on milk. If the mother’s nutritional
intake becomes contaminated with EEDs, the young will be
compromised because they have no ability to remove or detoxify
these chemicals due to the immaturity of their organs.
Even before birth, marsupials have only a very short-lived
placenta, providing little potential for detoxifying EEDs
absorbed from the mother’s circulation. This is in contrast to
most eutherian mammals, including humans, that develop a
complex and long-lived placenta that is able to break down
many hormones and their metabolites.Eutherian mammals also undergo differentiation of their
organ systems before birth, while they remain coupled to the
mother’s detoxification systems, providing some additional
protection from EED exposures. In humans, many EEDs accu-
mulate in fatty tissues and can be passed through the milk
during lactation. Consequently, the extended period of lacta-
tion in marsupials while the young develop in the pouch could
cause an increased and prolonged exposure to EEDs.
Of greater concern is the fact that the marsupial urogenital
tract and gonads do not differentiate until around the time of
birth and are especially susceptible to endocrine disruption at
this time.
Some of the most destructive EEDs are those that mimic
the naturally occurring oestrogen hormone. Oestrogen is typi-
cally thought of as a female hormone, but is actually essential
for reproductive health and fertility in both males and females
and is tightly regulated during our developing and adult lives.
Oestrogen-mimicking chemicals are at an all time high in
Australian waterways. The majority of these contaminants
come in the form of oestrone and oestradiol, which remain as
active compounds in run-off from agriculture and wastewater-
treatment plants. Terrestrial animals that rely on these water
sources will be exposed to these active oestrogen compounds.
The pesticide endosulphan, which has potent oestrogen-
mimicking properties, was only banned in Australia in 2010.
Before its ban, 260 tonnes per year were sprayed on Australian
crops.
In addition, oestrogenic EEDs can be found from natural
origins. Subterranean clover (Trifolium subterraneum), which
is seeded as animal fodder in areas with poor quality soil, covers
literally millions of hectares across every state in Australia. The
clover produces high levels of a chemical called genistein and
lower quantities of daidzein, both of which are potent oestrogen-
like compounds that cause many reproductive defects in sheep
and mice when ingested in large quantities. Grazing marsupial
species could potentially consume these plant,s leading to
oestrogen exposure to their gestating young or young in the
pouch.
Oestrogen compounds can drastically affect sexual devel-
opment in marsupials. High levels of oestrogen can cause
complete male-to-female sex reversal, while lower levels cause
abnormalities to testis development and infertility. Even slight
alterations to the development of the early testis or ovary can
have profound effects on the masculinisation or feminisation
of the resulting young, further leading to reduced fertility and
lower fitness. Thus, oestrogenic EEDs are likely to pose a signifi-
cant risk to marsupial populations in the environment.
In addition to oestrogen-like EEDs, some environmental
pollutants have anti-androgenic properties, and again these
pose a potential health risk to our native fauna. Androgens are36 | JAN/FEB 2016
Many groups are now examining
the potential for endocrine
disruptors to alter our DNA’s
epigenetic code in ways that might
cause defects to persist for multiple
generations.