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.
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.
Biogeographic Regions: nearctic (Native ); neotropical (Native ); australian (Native )
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.
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.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike; male larger; sexes colored or patterned differently
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.
Habitat Regions: temperate ; tropical ; terrestrial ; freshwater
Terrestrial Biomes: desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Aquatic Biomes: lakes and ponds; rivers and streams
Other Habitat Features: urban ; suburban ; agricultural ; riparian
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.
Primary Diet: carnivore (Eats terrestrial vertebrates, Insectivore , Eats non-insect arthropods); herbivore (Folivore , Frugivore , Granivore , Lignivore, Nectarivore ); omnivore
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.
Ecosystem Impact: disperses seeds; pollinates; creates habitat; soil aeration
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.
Life History and Behavior
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.
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: pheromones
Perception Channels: visual ; tactile ; acoustic ; chemical
While some species of metatheres live only 1 to a few years, some species, such as coarse-haired wombats, have lived up to 26 years in captivity.
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.
Mating System: monogamous ; polygynous ; polygynandrous (promiscuous)
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.
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.
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).
Key Reproductive Features: iteroparous ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
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.
Parental Investment: altricial ; pre-hatching/birth (Provisioning: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female); post-independence association with parents
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.).
Relevance to Humans and Ecosystems
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.
Negative Impacts: crop pest
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).
Positive Impacts: pet trade ; food ; body parts are source of valuable material; ecotourism ; controls pest population
Marsupials are common pollinators outside of the United States, specifically in Australia. The honey possum (Tarsipes rostratus) pollinates plants in the genus Banksia (Family: Proteaceae) and the two are thought to have co-evolved. In fact, the abundance of honey possums fluctuates in concert with Banksia flower production. The honey possum is well adapted for collecting pollen and nectar and it feeds exclusively on these resources. Long filiform papillae, or projections, are found on the surface of the tongue and are used to collect pollen and nectar. The honey possum inserts its pointy snout and long brush-tipped tongue inside flowers and incidentally collects pollen on its fur, which is then transported to other flowers as the honey possum feeds. The honey possum also pollinates species in the myrtle (Family: Myrtaceae) and heath (Family: Epacridaceae) families.
Other marsupials known to pollinate plants in the family Proteaceae include the eastern pygmy-possum (Cercartetus nanus), western pygmy-possum (Cercartetus concinnus), sugar glider (Petaurus breviceps), feathertail glider (Acrobates pygmaeus), and several species of dasyurid marsupials of the genus Antechinus. The feathertail glider is known to pollinate plants in eucalyptus forests of eastern Australia in a similar manner to the honey possum. These nocturnal marsupials feed on pollen and nectar by sticking their heads and long tongues inside of flowers. Their tongues are brushed with fine hairs to aid in collecting pollen and nectar. When they stick their heads into the flower, their fur collects pollen. The next flower they feed upon is then pollinated by the pollen stuck to the animal's fur. Likewise, the eastern pygmy-possum is a significant pollinator of Banksia and other native flowering plants by transferring pollen stuck to its face.
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