Metatherian mammals, also known as marsupials, comprise around 272 species. They are an ancient group, very diverse in body form, and they occupy an enormous range of ecological niches. Today, most marsupials are found in Central and South America (around 70 species) and Australasia (around 200 species). Radiations took place on both of these continents during the Cenozoic, at a time when there were few placental competitors. Present marsupial faunas are very diverse, with some startling parallels with placental mammals (e.g., marsupials with similar morphologies and life histories as moles, anteaters, shrews, primates, carnivores, and many others). Some marsupial life histories and morphologies are seemingly without placental mammal parallels, for example, kangaroos. Past marsupial faunas were even more incredible. In Australia, for example, were rhinoceros-sized marsupial herbivores, kangaroos nearly 10 feet tall, and carnivorous lion-like forms with shearing teeth and retractile claws. In South America, where parallel radiations of large placental herbivores may have denied these herbivorous niches to marsupials, marsupials filled many carnivore niches (including a sabretooth marsupial "cat") and many rodent-like forms. It seems clear on both continents that invasion by placental mammals is correlated with a decline in number and diversity of marsupials. However, it is unclear whether placental mammals caused the disappearance of marsupials through competition or the apparent pattern of replacement is the result of random historical events. (Moeller, 1990; Nowak, 1991; Vaughan, et al., 2000; Wilson and Reeder, 1993)
Today, most marsupials are found in Central and South America (around 70 species) and Australasia (around 200 species). One species is found in temperate North America (Didelphis virginianus). Metatheres diverged from the lineage leading to eutherian (placental) mammals by the Middle Cretaceous. Early diversification of metatheres is thought to have taken place in North America although, by the middle Miocene, the lineage became extinct on that continent, only reappearing around the time that North and South America regained contact in the Pliocene. The earliest marsupials are believed to have resembled North American opossums and other members of the family Didelphidae. A few fossil marsupials are known from Europe, Africa, and Asia, but this group was never well established on those continents. (Moeller, 1990; Nowak, 1991; Vaughan, et al., 2000)
From deserts and dry scrubland in Australia to tropical rain forests in South America, there are at least a few, and often many, species of Metatheria present. These animals occupy an enormous variety of terrestrial habitats throughout these two continents. The single species found in temperate North America (Didelphis virginia) naturally inhabits moist woodlands, but is common in towns and small cities. Metatheres have evolved to fill many niches in many habitats. Many species are fully terrestrial, many are arboreal, and at least one species, yapoks, is semi-aquatic. (Nowak, 1991; Vaughan, et al., 2000)
Marsupials differ from placental mammals in a number of important and obvious ways. The palate of marsupials is usually "fenestrated," that is, it contains large gaps or spaces in its bony surface. The angular process of the dentary is inflected (bent) medially in almost all marsupials. The braincase is small and narrow. It houses a relatively small and simple brain compared to that of similar-sized placental mammals. The jugal is large, extending posteriorally so that it contacts, and forms part of, the glenoid fossa. The lacrimal canal is slightly anterior to the orbit so that it opens on the surface of the face rather than inside the orbital space. The bullae are sometimes not ossified. When they are, they are formed largely by extensions from the alisphenoid.
Tooth form varies considerably among species of marsupials, but an easy and reliable character for recognizing members of the group is that the number of incisors in the upper jaw is different from the number in the lower (except in one family, the Vombatidae). The number is equal in most (but not all) placental mammals. Also, the maximum number of incisors (seen in several families) is 5/4, in contrast to 3/3 in placentals. The number of premolars and molars also differs between the groups (3/3 4/4 in marsupials, 4/4 3/3 in placental mammals), and the pattern of tooth replacement (milk teeth by adult teeth) differs, but these traits are difficult to use to recognize specimens. Postcranial skeletons of marsupials differ from those of placental mammals in that modern marsupials have epipubic bones in the body wall, projecting anteriorally from the pelvis. Epipubic bones are vestigial in recently extinct thylacines and were absent in at least one extinct group. The presence of epipubic bones is shared with monotremes. (Moeller, 1990; Nowak, 1991; Vaughan, et al., 2000)
Distinguishing among most of the orders and families of modern marsupials is not difficult. Two frequently-used characteristics are the conformation of the feet, and the number and position of the lower incisors. The second and third toes of syndactylous species are mostly enclosed in a sheath of skin and appear fused, except for the claws. Members of the orders Peramelemorphia and Diprotodontia are syndactylous. Others have separate toes, sometimes referred to as polydactylous. Members of the Diprotodontia and Paucituberculata have a pair of enlarged, forward-projecting (procumbent) lower incisors, a condition called diprotodonty. Other groups are polyprotodont, with numerous small and unspecialized lower incisors. Marsupial moles (Notoryctemorphia) are an unusual group, probably because of their extreme specialization to a fossorial mode of life. Their incisor morphology is not clearly diprotodont or polyprotodont and their feet are neither polydactylous nor syndactylous. (Moeller, 1990; Nowak, 1991; Vaughan, et al., 2000)
Mating systems of metatheres vary considerably. Many species are solitary throughout the year, only coming together to mate. This pattern of social behavior probably reflects promiscuous mating systems. Males of some species defend their access to several females. Koalas are an example of this polygynous mating behavior. Polygyny can also take the form of male dominance hierarchies in highly social species such as whiptail wallabies, which can live in groups of up to 50 individuals. Monogamy is also present within Metatheria. For example, Petauroides volans live in small family groups that consist of a mated pair and their offspring. (Nowak, 1991)
Marsupials and placental mammals differ strongly in their reproductive anatomy and pattern. In females, the reproductive tracts of marsupials are fully doubled. The right and left vaginae do not fuse to form a single body, as they do in all placental mammals, and birth takes place through a new median canal, the pseudovaginal canal. Right and left uteri also are unfused (varying degrees of fusion are found in placental mammals). Also, in the developing marsupial embryo, the arrangement of ducts that become the female reproductive tract is different in marsupials compared to placentals. In some (but by no means all) species of marsupials, females develop a pouch or marsupium in which the young are nursed. In males, the penis, like the female vagina, is bifid, or doubled. The scrotum lies in front of the penis instead of posterior to it, as in placental mammals. (Moeller, 1990; Parker, 1977; Vaughan, et al., 2000)
Perhaps the most conspicuous difference between marsupials and placental mammals is in the degree of development of the young at birth. Marsupial young are tiny at birth; litters always weigh less than 1% of the mother's body weight and individual young sometimes weigh only a few milligrams. They are born after a very short gestation period (8 to 43 days, depending on species; always less or equal to the length of an estrus cycle), and in what seems to our placental-biased point of view to be an extraordinarily underdeveloped state. A placenta is formed in only a few species and, even in those, the gestation period is extraordinarily short. At the time newborn marsupials emerge from their mother's reproductive tract and crawl to the pouch, they are tiny and have just begun forming functional organs. The forelimbs are fairly well developed, as they are required for the young pull to themselves along the mother's belly by grasping hairs with the forelimbs, but the hindlimbs are mere paddles. The heart, kidneys, and lungs are all barely functional. Even the brain is at an early ontogenetic stage. Most development takes place in the pouch and the lactation period is prolonged. (Moeller, 1990; Parker, 1977; Vaughan, et al., 2000)
It has been suggested that the marsupial pattern of reproduction is primitive for metatherian and eutherian mammals (Lillegraven, 1975). Lillegraven (1975) argues that marsupial young must be born quickly, before the mother's immune system can respond the presence of foreign tissue in the form of a developing embryo. Most development takes place in the pouch, safe from maternal immune attack. Eutherians "solved" the problem of immune rejection through the evolution of a complex set of interactions that take place in the trophoblast, a part of the developing egg of eutherians that is not found in marsupials. Whether this is likely to be true -- and whether retaining a primitive style of reproduction suggests any kind of competitive inferiority -- has been hotly debated (for example, Parker, 1977). (Lillegraven, 1975; Parker, 1977; Vaughan, et al., 2000)
Much of development in metatheres occurs after parturition while the young are nursing. In about 50 percent of marsupial species, young develop within the confines of a marsupium, or pouch. Weaning may take place after a year or more in some species (e.g., Macropus). Thus, female metatheres invest very little energy and resources into gestation, but lactation requires a substantial investment. The pouch itself (or protective folds of skin in many species) may be permanent, or may only develop at the onset of reproduction. In either case, resources must be devoted to producing and maintaining the structures that will protect the developing young.
Young generally do not associate directly with their mothers for much more than several weeks once they are fully independent of the pouch. This is generally true for both non-social and social species. In at least one species (Marcopus rufogriseus), extended associations between females and their independent young are known to reduce the success of future reproduction. (Clutton-Brock, 1991; Nowak, 1991; Vaughan, et al., 2000)
Metatheria has undergone two major adaptive radiations (in Australia and South America) and exhibit a vast array of behavior as a consequence of evolving to fill a variety of ecological niches on these two continents. Thus, generalization of their behavior is difficult. Marsupials may be arboreal, terriculous, or fossorial and at least one species is semi-aquatic (yapoks or water opossums). Their locomotion may include walking, gliding, hopping, running and swimming. Marsupials have not evolved powered flight. Activity patterns also run the full spectrum of possibilities: marsupials may be diurnal, nocturnal or crespuscular. Seasonal behavior may include hibernation in some species, while others remain active throughout the year. Some species are social, while others are solitary. (Nowak, 1991; Vaughan, et al., 2000)
As with mammals in general, vision, olfaction, hearing and touch are all important to varying degrees among species of Metatheria. Communication can take many forms as well. Some marsupials communicate with acoustic signals, particularly during mating or territorial encounters. Many species have conspicuous color patterns that may convey information about sex or species identity. Pheromones may also used in communication of reproductive receptivity. (Nowak, 1991)
Ominivory, insectivory, carnivory, and herbivory are all common food habits among Metatheria. Some groups that exhibit omnivory are American oppossums (Didelphidae), Australian possums (Phalangeridae) and bandicoots (Peramelidae). Insectivory and/or carnivory can be observed in several groups as well (e.g., Dasyuridae). Many marsupials, such as koalas (Phascolarctidae), wombats (Vombatidae), and kangaroos (Macropodidae), are strictly herbivorous. (Nowak, 1991)
Except for a very few species (e.g., the extinct thylacine wolf), marsupials are rarely top carnivores and thus are subject to predation by a host of mammalian, reptilian and avian predators wherever they occur.
With their great diversity of food habits, behavior and habitat use, metatheres can substantially impact their communities and ecosystems in a variety of ways. For example, metatheres may help pollinate plants, distribute seeds, or control pest populations. Most species are prey for other species and thus are an important component of many food webs. Species that dig burrows (e.g. wombats and marsupial moles) create habitat for other organisms and/or help aerate soil. Parasites of marsupials are certainly as diverse as their hosts.
Humans benefit from metatheres in a variety of ways. Many are eaten as food or their body parts are used as some sort of resource (e.g., leather is made from kangaroo hides and koalas and brushtail possums were once taken for their fur). Some species that eat mice or insects can help control agricultural pests. Metatheres are valuable for the ecotourism industry--drawing many tourists to Australia. Some species are even kept as pets (e.g. Petaurus outside of Australia). (Nowak, 1991)
Metatheres generally do not have large detrimental effects on human economics. Some herbivorous species may be minor crop pests. Wombats are extirpated in some areas because their burrows cause injury to livestock, or because European rabbits, which have become significant pests in Australia, use wombat burrows as dens. (Nowak, 1991)
Many marsupials are threatened or endangered. The International Union for the Conservation of Nature and Natural Resources (IUCN) currently lists over 200 species (i.e., more than 2/3 of all marsupials) as being of some level of concern. Habitat destruction, overexploitation, and competition with exotic species and livestock have greatly reduced many populations. A number of species have gone extinct within the past two centuries as a direct result of human activity (e.g., Thylacinus, Macropus spp.). (IUCN, 2004; Nowak, 1991)
Tanya Dewey (editor), Animal Diversity Web.
Matthew Wund (author), University of Michigan-Ann Arbor, Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
uses sound to communicate
living in landscapes dominated by human agriculture.
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
active at dawn and dusk
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
an animal that mainly eats leaves.
A substance that provides both nutrients and energy to a living thing.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
Referring to a burrowing life-style or behavior, specialized for digging or burrowing.
mainly lives in water that is not salty.
an animal that mainly eats fruit
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
An animal that eats mainly insects or spiders.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
Having one mate at a time.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
an animal that mainly eats nectar from flowers
active during the night
an animal that mainly eats all kinds of things, including plants and animals
the business of buying and selling animals for people to keep in their homes as pets.
chemicals released into air or water that are detected by and responded to by other animals of the same species
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
having more than one female as a mate at one time
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
specialized for leaping or bounding locomotion; jumps or hops.
scrub forests develop in areas that experience dry seasons.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
digs and breaks up soil so air and water can get in
living in residential areas on the outskirts of large cities or towns.
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
living in cities and large towns, landscapes dominated by human structures and activity.
uses sight to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
1990. Marsupials. Pp. 210-211 in B Grzimek, ed. Grzimek's Encyclopedia of Mammals, Vol. 1, 1st Edition. New York: Mcgraw-Hill.
Aplin, K., M. Archer. 1987. Recent advances in marsupial systematics with a new syncretic classification. Pp. xv-lxxii in M Archer, ed. Possums and Opossums: Studies in Evolution, Vol. 1. Chipping Norton: Surrey Beatty and Sons PTY Limited.
Clutton-Brock, T. 1991. The Evolution of Parental Care. Princeton: Princeton University Press.
IUCN, 2004. "2004 IUCN Red List of Threatened Species" (On-line). Accessed October 19, 2005 at http://www.iucnredlist.org.
Lillegraven, J. 1975. Biological considerations of the marsupial-placental dichotomy. Evolution, 29: 707.
Moeller, H. 1990. Marsupials: Introduction. Pp. 212-219 in B Grzimek, ed. Grzimek's Encyclopedia of Mammals, Vol. 1, 1st Edition. New York: Mcgraw-Hill.
Nowak, R. 1991. Order Marsupialia. Pp. 10-113 in Walker's Mammals of the World, Vol. 1, 5th Edition. Baltimore: Johns Hopkins University Press.
Parker, P. 1977. An ecological comparison of marsupial and placental patterns of reproduction. B Stonehouse, D Gilmore, eds. The Biology of Marsupials. London: T. Macmillan.
Thenius, E. 1990. Marsupials: Phylogeny. Pp. 219-231 in B Grzimek, ed. Grzimek's Encyclopedia of Mammals, Vol. 1, 1st Edition. New York: m.
Vaughan, T., J. Ryan, N. Czaplewski. 2000. Mammalogy, 4th Edition. Toronto: Brooks Cole.
Wilson, D., D. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Washington, D. C.: Smithsonian Institution Press.