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The water opossum and the now extinct Tasmanian tiger are the only two marsupials where the male also has a pouch (in order to protect their genitalia while swimming). [ 7 ] Some marsupials (e.g. phascogales ) lack the true, permanent pouches seen in other species.
Being a marsupial and at the same time an aquatic animal, the water opossum has evolved a way to protect its young while swimming. A strong ring of muscle makes the pouch (which opens to the rear) watertight, so the young remain dry, even when the mother is totally immersed in water. [6]
A pouch is present in most species. Many marsupials have a permanent bag, while in others such as the shrew opossum the pouch develops during gestation, where the young are hidden only by skin folds or in the maternal fur. The arrangement of the pouch is variable to allow the offspring to receive maximum protection.
As marsupials, female opossums have a reproductive system that includes a bifurcated vagina and a divided uterus; many have a pouch. [29] The average estrous cycle of the Virginia opossum is about 28 days. [30] Opossums do possess a placenta, [31] but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully ...
They also have hairless ears and a long, flat nose. Opossums have 50 teeth, more than any other North American land mammal, [15] and opposable, clawless thumbs on their rear limbs. Opossums have 13 nipples, arranged in a circle of 12 with one in the middle. [16] [17] The dental formula of an opossum is 5.1.3.4 4.1.3.4. [18]
Females have a gestation period of 16–18 days, after which they give birth to single young. [7] [8] A newborn brushtail possum is only 1.5 cm (0.6 in) long and weighs only 2 g (0.07 oz). As usual for marsupials, the newborn may climb, unaided, through the female's fur and into the pouch and attach to a teat.
Unlike most other marsupials, the gray short-tailed opossum does not have a true pouch. The scientific name Monodelphis is derived from Greek and means "single womb" (referring to the lack of a pouch) and the Latin word domestica which means "domestic" (chosen because of the species' habit of entering human dwellings). [3]
The tiny young, measuring only up to 12 millimeters, attach themselves to the mother's mammae where they may remain for around 30 days. [6] Unlike many marsupials, female mouse opossums do not possess a pouch to protect the young as they develop. The young are born so undeveloped that they don't open their eyes until 39–40 days after birth.