the neck, with their sharp teeth. They can successfully take prey up to their own size. They are nocturnal, being most active in the first three hours after dusk. Although they may occasionally shelter in natural crevices in the rock, they normally spend the day in concealed nests constructed of leaves, bark, and other available materials. The nests of females are more complex and tightly woven than those of males. They are solitary, coming together only to mate, and with each individual occupying a home range of , flagged with scent marks. The approach of another member of the species is commonly met with hissing and screeching, which may escalate to defensive strikes launched while the animal is standing on its hind legs. Reproduction The opossums breed year round when the climate is suitable, being able to raise up to six litters of six to eleven young each during a good year. Females only come into oestrus when exposed to male pheromones, with ovulation being induced only by physical contact with the male. Gestation lasts fourteen days, after which the young attach to a teat, where they remain for the next two weeks. Like all marsupials, the young are born undeveloped; in this species they are just in length and weigh at birth. The young grow hair at around three weeks, open their eyes about a week later, and are weaned at eight weeks Gray short-tailed opossums are sexually mature at five to six months of age, and live for up to forty-nine months in captivity. Laboratory opossum The gray short-tailed opossum possesses several features that make it an ideal research model, particularly in studies of marsupials, as well as the immunological and developmental research on mammalian systems. It breeds relatively easily in laboratory settings, and neonates are exposed and can be readily accessed because, unlike other marsupial species, female opossums lack a pouch: neonates simply cling to the teats. Opossums are born at a stage that is approximately equivalent to 13- to 15-day-old fetal rats or 40-day-old human embryos. Like other marsupials, the inadequacies of the neonate's immune system function make it an ideal model for both transplant and cancer research, as well as general investigations into immune system development. Its genome was sequenced and a working draft published in May 2007: the decoding work, directed by MIT and Harvard, reveals the opossum to have between 18,000 and 20,000