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.
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.
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.
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.
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.
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.
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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