Marsupials

Types of animal science: Anatomy, classification, ecology, evolution, geography, reproduction

Fields of study: Anatomy, ecology, physiology, zoology

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 they are often inconspicuous even in zoos. Most marsupials are found in Australia and New Zealand. Australian authorities impose strict export sanctions to protect their numerous endangered species.

There are three families of marsupials, Didelphidae, Microbiotheriidae, and Caenolestidae, that inhabit South and Central America. The opossums of North and South America are the most diverse of three families of extant marsupials outside of Australia. 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. The only naturally occurring marsupial found in the United States is the Virginia opossum, Didelphis virginiana, which ranges extends across North America and beyond the Canadian border.

For the most part, marsupials have remained curiosities for the general public. Humans have not traditionally exploited marsupials on a large scale; they have not commonly been kept as pets, the meat of larger kangaroos is mostly used only for dog and cat food, and the furs of only a few marsupials have commercial value. However, hunting as well as habitat destruction and the introduction of competing species have posed threats to some marsupials, especially in Australasia.

Classification and Physiology

Marsupial divisions and categorizations are the subject of ongoing debate, but classifications commonly include around nineteen families, eighty-three genera, and over 270 species. Marsupials make up the infraclass Metatheria in the subclass Theria, with seven orders. There is no other group within the higher mammals that contains such a diversity of species, genera, and families as the marsupials.

Marsupials are an example of adaptive radiation. Their 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 in Australasia and placental mammals 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. They range in adult size from 147 pounds to only 0.1 ounce, though the majority of species 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.

Marsupials exhibit a diverse range of traits. They are found in habitats as diverse as freshwater, alpine areas, hot deserts, and tropical rain forests. While kangaroos and their relatives famously spring about on their hind legs, other species climb, glide, burrow and even swim. Their diets range 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.

Reproduction and Development

There are three types of reproductive patterns in mammals. There are monotremes, which are egg-laying mammals, such as the 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 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. 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.

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. 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.

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.

While reproductive traits are the best-known distinguishing feature, other physiological features are also characteristic. 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.

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. A female’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. Many of 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. Several 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 fossil record suggests that marsupials first evolved in North America before spreading to Australasia, where they particularly thrived. The earliest marsupials found in Australia are dated from approximately twenty-three million years ago. Most modern families and forms were clearly established by that time. The relative 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 large-scale modification of the marsupial’s habitat in Australia. The first major change, however, 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 humans arrived, and hunting and the use of fire by Aboriginal Australians may or may not have factored into their extinction.

European colonization, in contrast, is known to have had a direct impact on marsupial ecology. In addition to outright hunting and killing to prevent damage to crops, marsupials have been pressured by general habitat destruction and development. Perhaps even more destructive has been the introduction of foreign species that prey on or compete with marsupials. Approximately nine marsupial species have become extinct in Australia and fifteen to twenty have suffered gross reduction in range in the modern era. 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 have tended to fare reasonably well despite human population growth and encroachment. Indeed, some, such as the common opossum, have thrived. However, the destruction of habitats in South and Central America are considered a long-term risk to many species.

Bibliography

Dawson, Terence. Kangaroos: The Biology of the Largest Marsupials. Ithaca, N.Y.: Comstock, 1995. A very comprehensive book covering all aspects of the most familiar marsupial, the kangaroo. Easy-to-understand language covers the social organization, activities, habitat, and general characteristics of kangaroo life. Also covers kangaroos’ history of coexistence with humans.

Hume, Ian D. Marsupial Nutrition. New York: Cambridge University Press, 1999. Discusses all food resources utilized by marsupials, as well as their digestion and metabolism. Provides nutrition and digestion information applicable to all mammals.

Lee, Anthony K., and Andrew Cockburn. Evolutionary Ecology of Marsupials. New York: Cambridge University Press, 1985. A good discussion of the rich diversity of marsupials and the various coping techniques each has developed throughout evolution.

"Marsupial." Animals & Plants, San Diego Zoo, 2018, animals.sandiegozoo.org/animals/marsupial. Accessed 31 May. 2018.

Myers, P., et al. "Metatheria: Marsupial Mammals." Animal Diversity Web, University of Michigan Museum of Zoology, 2018, animaldiversity.org/accounts/Metatheria/classification/. Accessed 31 May. 2018.

Paddle, Robert. The Last Tasmanian Tiger: The History and Extinction of the Thylacine. New York: Cambridge University Press, 2000. An insightful and up-to-date examination of the history of one of Australia’s most enduring folkloric beasts—the thylacine, otherwise known as the Tasmanian tiger.

Saunders, Norman, and Lyn Hinds, eds. Marsupial Biology: Recent Research and New Perspectives. Sydney, Australia: New South Wales University Press, 1997. Twenty-two articles written by leading experts in the field. Articles are divided by subject matter: reproduction and development, genetics, ecology, pathology and homeostasis, and developmental neurobiology. A good reference text for graduate students.

Tynedale-Biscoe, Hugh, and Marilyn B. Renfree. Reproductive Physiology of Marsupials. New York: Cambridge University Press, 1987. An advanced discussion of reproductive characteristics and behaviors of marsupials.