Gray short-tailed opossum

A small marsupial (family Didelphidae) native to South America (Brazil, Bolivia, Paraguay). Adults weigh 80–150 g with a body length of 12–18 cm and a short, prehensile tail (~7 cm). They have soft, gray-brown fur, a pointed snout, and small rounded ears. Uniquely among marsupials, females lack a pouch—pups cling directly to the mother's teats. They are solitary, nocturnal, and terrestrial/semi-arboreal. Omnivorous, eating insects, small vertebrates, fruit, and seeds. They have become the primary laboratory marsupial, used extensively in biomedical research since the 1970s. Their advantages include ease of laboratory breeding, small size, and being the first marsupial with a fully sequenced genome (2006, 2n=18). They are used in developmental biology (studying differences between marsupial and eutherian development), immunology, UV-induced melanoma (they are uniquely susceptible), genetics, and neuroscience. Females produce large litters (up to 13 pups) after a 14-day gestation—one of the shortest mammalian gestations. Neonates are extremely altricial. Lifespan: 4–6 years in captivity.