Marsupial

Marsupials are the group of mammals commonly thought of as pouched mammals (like the wallaby and kangaroo at left).They give live birth, but they do not have long gestation times like placental mammals. Instead, they give birth very early and the young animal, essentiallya helpless embryo, climbs from the mother's birth canal to the nipples. There Red-necked Wallaby. Photo by Gerald and Buff Corsi, 2002 California Academy of Sciences. Red Kangaroo. Photo by Gerald and Buff Corsi, 2002 California Academy of Sciences. it grabs on with its mouth and continues to develop, often for weeks or monthsdepending on the species. The short gestation time is due to having a yolk-type placenta in the mother marsupial. Placental mammals nourish the developing embryo using the mother's blood supply, allowing longer gestation times.

marsupial

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The only naturally occurring marsupial in the United States is the opossum,Didelphis virginiana. In the past, however, marsupials were quite common. During the Mesozoicmarsupials were very common in North America; more common, in fact, than placental mammals. They persisted here until the mid- to late-Tertiary.

In South America and Australia, however, marsupials continued to be an important group of land mammals. Many South Americanforms are similar to the North American opossum. The marsupials of South America began to go extinct in the late Miocene and Early Pliocene when a land connection with North America formed, allowing placental mammals to cross into South America. In Australia, though, marsupials continue to be very diverse, and are the dominant native mammals. They include kangaroos, koalas (above left), tasmanian devils, wombats (above right), and other typical Australian mammals.Until recently, they also included the marsupial wolf, Thylacinus (below). Likethe quagga, the marsupial wolf is now extinct. The last individualwas seen in Tasmania in the 1950s.

Though marsupials today do not have as many species as do the placental mammals, they are quite structurally diverse. They range from small four-footedforms like the marsupial mole, Notoryctes, to the large two-legged kangaroos.

There are several cases of convergent evolution between marsupials and placental mammals, in which the two animals have evolved to fill the same ecological niche in different parts of the world. There are burrowing forms, grazing forms, gliding forms, and even long-snouted ant-eating forms which have evolved independently in the two groups.

Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pouch. Living marsupials include opossums, Tasmanian devils, kangaroos, koalas, wombats, wallabies, and bandicoots among others, while many extinct species, such as the thylacine, are also known.

Comprising over 300 extant species, several attempts have been made to accurately interpret the phylogenetic relationships among the different marsupial orders. Studies differ on whether Didelphimorphia or Paucituberculata is the sister group to all other marsupials.[4] Though the order Microbiotheria (which has only one species, the monito del monte) is found in South America, morphological similarities suggest it is closely related to Australian marsupials.[5] Molecular analyses in 2010 and 2011 identified Microbiotheria as the sister group to all Australian marsupials. However, the relations among the four Australidelphid orders are not as well understood. The cladogram below, depicting the relationships among the various marsupial orders, is based on a 2015 phylogenetic study.[4]

DNA evidence supports a South American origin for marsupials, with Australian marsupials arising from a single Gondwanan migration of marsupials from South America, across Antarctica, to Australia.[6][7] There are many small arboreal species in each group. The term "opossum" is used to refer to American species (though "possum" is a common abbreviation), while similar Australian species are properly called "possums".

Most female marsupials have a front pouch, which contains multiple teats for the sustenance of their young. Marsupials also have other common structural features. Ossified patellae are absent in most modern marsupials (though a small number of exceptions are reported)[8] and epipubic bones are present. Marsupials (and monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres.[9]

A pouch is present in most, but not all, species. Many marsupials have a permanent bag, whereas in others the pouch develops during gestation, as with the shrew opossum, where the young are hidden only by skin folds or in the fur of the mother. The arrangement of the pouch is variable to allow the offspring to receive maximum protection. Locomotive kangaroos have a pouch opening at the front, while many others that walk or climb on all fours have the opening in the back. Usually, only females have a pouch, but the male water opossum has a pouch that is used to accommodate his genitalia while swimming or running.

Marsupials have adapted to many habitats, reflected in the wide variety in their build. The largest living marsupial, the red kangaroo, grows up to 1.8 metres (5 ft 11 in) in height and 90 kilograms (200 lb) in weight, but extinct genera, such as Diprotodon, were significantly larger and heavier. The smallest members of this group are the marsupial mice, which often reach only 5 centimetres (2.0 in) in body length.

Some species resemble placental mammals and are examples of convergent evolution. This convergence is evident in both brain evolution[11] and behaviour.[12] The extinct thylacine strongly resembled the placental wolf, hence one of its nicknames "Tasmanian wolf". The ability to glide evolved in both marsupials (as with sugar gliders) and some placental mammals (as with flying squirrels), which developed independently. Other groups such as the kangaroo, however, do not have clear placental counterparts, though they share similarities in lifestyle and ecological niches with ruminants.

Marsupials' reproductive systems differ markedly from those of placental mammals.[14][15] During embryonic development, a choriovitelline placenta forms in all marsupials. In bandicoots, an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas.

The evolution of reproduction in marsupials, and speculation about the ancestral state of mammalian reproduction, have engaged discussion since the end of the 19th century. Both sexes possess a cloaca,[15] which is connected to a urogenital sac used to store waste before expulsion. The bladder of marsupials functions as a site to concentrate urine and empties into the common urogenital sinus in both females and males.[15]

Most male marsupials, except for macropods[16] and marsupial moles,[17] have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females' two vaginas.[9][15][18][19][10][20][21] The penis is used only during copulation, and is separate from the urinary tract.[10][15] It curves forward when erect,[22] and when not erect, it is retracted into the body in an S-shaped curve.[10] Neither marsupials nor monotremes possess a baculum.[9] The shape of the glans penis varies among marsupial species.[10][23][24][25]

The shape of the urethral grooves of the males' genitalia is used to distinguish between Monodelphis brevicaudata, Monodelphis domestica, and Monodelphis americana. The grooves form 2 separate channels that form the ventral and dorsal folds of the erectile tissue.[27] Several species of dasyurid marsupials can also be distinguished by their penis morphology.[28]The only accessory sex glands marsupials possess are the prostate and bulbourethral glands.[29] Male marsupials have 1-3 pairs of bulbourethral glands.[30] There are no ampullae of vas deferens, seminal vesicles or coagulating glands.[31][18] The prostate is proportionally larger in marsupials than in placental mammals.[10] During the breeding season, the male tammar wallaby's prostate and bulbourethral gland enlarge. However, there does not appear to be any seasonal difference in the weight of the testes.[32]

Female marsupials have two lateral vaginas, which lead to separate uteri, but both open externally through the same orifice. A third canal, the median vagina, is used for birth. This canal can be transitory or permanent.[9] Some marsupial species are able to store sperm in the oviduct after mating.[33]

Marsupials give birth at a very early stage of development; after birth, newborn marsupials crawl up the bodies of their mothers and attach themselves to a teat, which is located on the underside of the mother, either inside a pouch called the marsupium, or open to the environment. Mothers often lick their fur to leave a trail of scent for the newborn to follow to increase chances of making it into the marsupium. There they remain for a number of weeks, attached to the teat. The offspring are eventually able to leave the marsupium for short periods, returning to it for warmth, protection, and nourishment.

Prenatal development differs between marsupials and placental mammals. Key aspects of the first stages of placental mammal embryo development, such as the inner cell mass and the process of compaction, are not found in marsupials.[34] The cleavage stages of marsupial development are very variable between groups and aspects of marsupial early development are not yet fully understood.

An early birth removes a developing marsupial from its mother's body much sooner than in placental mammals; thus marsupials have not developed a complex placenta to protect the embryo from its mother's immune system. Though early birth puts the tiny newborn marsupial at a greater environmental risk, it significantly reduces the dangers associated with long pregnancies, as there is no need to carry a large fetus to full term in bad seasons. Marsupials are extremely altricial animals, needing to be intensely cared for immediately following birth (cf. precocial). Newborn marsupials lack histologically mature immune tissues [41][42][43] and are highly reliant on their mother's immune system for immunological protection.[44] 041b061a72