Marsupials are pouched animals that form a
distinctive group within the class Mammalia.
They possess the diagnostic features of typical
mammals, including high and stable body
temperature, furry pelt, simple lower jaw, and
mammary glands. However, there are other features
that distinguish them from what are considered
to be typical mammalian features.
The kangaroo is the most commonly known
marsupial, but a vast array of marsupials exist.
Most marsupials are crepuscular or nocturnal, so
most zoo visitors are unable to observe them.
Most marsupials are found in Australia and New
Zealand. Outside of Australia, it is rare to see marsupials
in zoos. Australian authorities impose
strict export sanctions to protect their numerous
endangered species. The only naturally occurring
marsupial found in the United States is the opossum,
Didelphis marsupialis. The opossums of North
and South America are the most diverse of three
families of extant marsupials outside of Australia.
There are three families of marsupials, Didelphidae,
Microbiotheriidae, and Caenolestidae,
that inhabit South and Central America. One species
of didelphid, the Virginia opossum, extends
across North America and beyond the Canadian
border. The American marsupials alive today are
mostly small, ranging from mouse to rabbit size.
These are generally either carnivorous or omnivorous,
living in forests and feeding on insects.
Marsupial Development
Marsupials are an example of adaptive radiation.
This adaptation to their varied habitats has led to
their enormous diversity of forms and niches. They
are also an example of convergent evolution, as indicated
by the similarities between marsupials and
placentals in the rest of the world. The marsupial
gliders resemble the flying squirrels and lemurs,
the Tasmanian tiger or thylacine was doglike, and
marsupial moles resemble eutherian moles. There
are many physiological similarities as well.Wombats
process grasses and sedges as horses do and
numbats feed on termites as anteaters do.
With few exceptions, marsupials are not conspicuous
in coloration or any external physical attributes.
The greatest majority of them are small,
ranging in size between that of a mouse and of a
small rabbit. They developed from small carnivores into herbivores the size of hippopotamuses.
The larger marsupials died out only several thousand
years ago.
For the most part, marsupials have remained
curiosities for the general public. Humans have
not traditionally exploited marsupials. They have
never been kept as pets, the meat of larger kangaroos
is mostly used for dog and cat
food, and the furs of only a few marsupials
have commercial value.
Marsupials include 18 families,
76 genera, and over 266 species, but
these divisions and categorizations
are currently being debated. Marsupials
are the only order in the subclass
Metatheria. There is no other
group within the higher mammals
that contains such a diversity of
higher species, genera, and families
as the marsupials.
There are marsupials that spring
about on their hind legs, as well as
climb, glide, burrow and even swim,
and they range in adult size from
147 pounds to only 0.1 ounce. They
are found in habitats as diverse as
freshwater, alpine areas, hot deserts,
and tropical rain forests. Their diet
ranges from purely insects to vertebrates,
fungi, underground plant
roots, bulbs, rhizomes and tubers,
plant exudates such as saps and
gums, seeds, pollen, terrestrial
grasses, herbs and shrubs, and tree
foliage. Because of this vast diversity
it is impossible to categorize marsupials
with a simple description. Instead,
the physiology of marsupials
must be used to categorize them.
Physiology
There are three types of reproductive
patterns in mammals. There are
monotremes, which are egg-laying
mammals, such as the duck-billed
platypus. There are placentals, whose
embryo develops inside the uterus, and the placenta
formed in the uterus provides nutrients
to the developing embryo. In placentals, the offspring
are born completely developed, as in
humans. Finally, there are marsupials, which
functionally fall in between monotremes and
placentals. Marsupials are often thought of as pouched
mammals. Their embryo develops inside the
uterus but, unlike placental mammals, the marsupial
is born very early in its development. It completes
its embryonic development outside the
mother's body, attached to teats of abdominal
mammary glands, which are often but not always
enclosed in a pouch called the marsupium. The
helpless embryonic form has forelimbs that are
strong enough to climb from the birth canal to the
mother's nipples, where it grabs on and nurses for
weeks or months depending on the species.
When the young are born, their eyes and ears
are closed, hind limbs and tails are stumps, and
they are completely hairless. Their olfactory senses
are greatly developed, as are their tactile senses,
allowing them to navigate their way to the marsupium.
The marsupium is formed in diverse ways,
ranging from the "primal pouch" (the annular
skin creasing around each teat), to common marsupial
walls surrounding all teats, and finally to a
closed marsupium, which can be opened to the
front or to the rear. Marsupials are usually woolly,
with shortened forelimbs and elongated hind
limbs. In kangaroos, these physical features allow
locomotion in a hopping movement only. However,
at an equivalent speed, allowing for the differences
in weight, a hopping kangaroo uses less
energy than a running horse or dog.
In several families, second and third toes of
the hind foot function as grooming claws and the
first toe is always clawless, except in the shrew
opossum. Vision is usually poorly developed and
olfactory, tactile, and auditory senses are well developed.
The gestation period is eight to forty-two days,
after which the young is carried in the marsupium
for between thirty days and seven months. Litter
sizes range from one to twelve per birth. The
young are weaned anywhere between six weeks
and one year. The relationship between mother
and offspring is long lasting in many species. Sexual
maturity is reached between ten months and
four years, depending on the species. The longer
range is associated with the male koala.
Behavior
Marsupials range from pure carnivores to pure
herbivores, with all the intermediate stages in between.
They are usually nocturnal and crepuscular.
Some species are solitary, while others live in
family groups.
In all mammals, because of the milk produced
by the mother, male assistance in feeding the
young is less important than in birds, for example.
In many marsupials, the role of the male is further
reduced because the pouch takes over the functions
of carrying and protecting the young and
keeping it warm.Afemale's need for assistance in
rearing young does not appear to be an important factor promoting the formation of long-lasting
male-female pairs or larger social groups. The majority
of marsupial species mate promiscuously.
There are few examples of long-lasting bonds and
they do not live in groups. Some species form monogamous
pairs and harems. It is hypothesized
that the lack of frequent examples of this sort is
due to the lack of external pressures.
Evolution
Marsupial evolutionary development is not yet
clearly understood. Fossil records suggest that
they may have evolved simultaneously with the
placental animals about 100 million years ago,
in the Cretaceous period. The oldest geological
finds come from the recent Upper Cretaceous of
North America, about seventy-five million years
ago. Although there was some development of
marsupials in North America, they later declined
as placentals increased in diversity. In contrast,
South America has a considerable diversity of
marsupial fossil forms, indicating their persistence
for more than sixty million years. Seven families
of living and fossil marsupials are known from
South America. About two to five million years
ago, a land connection between the two Americas
was established again, and more
placental animals reached South
America, including carnivores
such as the jaguar. In the face of
such competition, the large carnivorous
marsupials disappeared, but
the small omnivores have persisted
successfully to the present day.
Some of them moved north to colonize
in North America.
The earliest marsupials found in
Australia are dated from twentythree
million years ago. Most modern
families and forms were clearly
established by that time. There isno
clear evidence to establish whether
marsupials originated in North
America, South America, or Australia.
The lack of fossil records of
marsupials in Asia or Africa makes
the most likely route of migration from South
America to Australia via Antarctica. At that time,
all three southern continents were united in the
land mass known as Gondwanaland. This mass of
land began breaking up 135 million years ago,
with South America and Antarctica still being
connected until about 30 million years ago. One
land mammal fossil has been found in Antarctica
which is a marsupial dated to be forty million
years old. The Australian plate then gradually
drifted northward for another thirty million years
before reaching its current latitude. This long isolation
allowed the extensive development of the
marsupials in Australia in the absence of competition
from other placentals.
As marsupials evolved in Australia, so did the
placentals in the rest of the world, filling the same
ecological niches. In many cases, they adopted
similar morphological solutions to ecological
problems. One example is the convergent evolution
of the carnivorous Tasmanian devil, a marsupial,
and placental wolves of other continents. The
marsupial mole is very similar in form to the placental
mole. The marsupial sugar glider and the
two flying squirrels of North America are also
very similar.
Habitat
The arrival of European settlers and the influx of
new species-sheep, cattle, rabbits, foxes, cats,
dogs, donkeys, and camels-have caused a largescale
modification of the marsupial's habitat in
Australia. The first major change was in the late
Pleistocene, with the extinction of whole families
of large terrestrial marsupials. Included in this extinction
was Diprotodon, the largest browsing
kangaroo. It is likely that the climatic fluctuations
increased aridity and reduced the available favorable
habitat. Many of the species were already under
stress when man arrived.
Approximately nine species have become extinct
in Australia and fifteen to twenty have suffered
gross reduction in range. The most affected
have been small kangaroos, bandicoots, and large
carnivores such as the thylacine and native cats.
Not all the environmental changes have been
unfavorable for marsupials. Many of the larger
herbivores have fared well with the advent of
ranching and available grazing land and watering
holes already set up for stock animals. As
these marsupials become competition for sheep
and cattle, Australian authorities have developed
programs to keep their population controlled by
allowing a certain number to be shot. Most species
of marsupials have little or no importance
as pests and their continued existence depends
largely on the maintenance of sufficient habitat to
support secure populations. The control of feral
foxes and cats is very important to keep predation
limited.
Marsupials outside Australia appear to have
suffered no ill effects due to the destruction of habitat
in North and South America.
Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Subclass: Theria
Infraclass: Metatheria
Order: Marsupialia
Suborders: Polyprotodonta and Diprotodonta
Families: Didelphidae (American opossums, eleven genera,
seventy-five species); Microbiotheriidae (Monito del montes);
Caenolestidae (shrew or rat opossums, three genera,
seven species); Dasyuridae (quolls, dunnarts, and marsupial
mice, eighteen genera, fifty-two species); Myrmecodoiidae
(numbats); Thylacinidae (thylacines); Notoryctidae (marsupial
mole); Peramelidae (bandicoots, seven genera, seventeen
species); Phalangeridae (cuscuses and brishtails, three genera,
fourteen species); Burramyidae (pygmy opossums, four genera,
seven species); Pseudocheiridae (ringtail opossums, two
genera, sixteen species); Petauridae (gliders, three genera,
seven species); Macropodidae (kangaroos and wallabies,
eleven genera, fifty species); Potoridae (rat kangaroos, five
genera, ten species); Phascolarctidae (koalas); Vombatidae
(wombats, two genera, three species); Tarsipedidae (honeypossums)
Geographical location: Australian region; North, Central, and
South America
Habitat: Varied depending on species, but includes major terrestrial
habitats and some arboreal habitats
Gestational period: Characteristically short, as the newborn
completes its development attached to a nipple inside the
marsupium
Life span: Varies by species
Special anatomy: The marsupium, or abdominal pouch, is characteristic,
although some forms, such as the murine opossum
of Central and South America, lack one
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