The gray short-tailed opossum is a small South American member of the family Didelphidae. Unlike most other marsupials, the grey 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” (as a result of the species’ habit of entering human dwellings, not because it has been tamed and is different from its wild relatives). It was the first marsupial to have its genome sequenced. The grey short-tailed opossum is used as a research model in science, and is also frequently found in the exotic pet trade. It is also known as the Brazilian opossum, rainforest opossum and in a research setting the laboratory opossum.

Gray short-tailed opossums are omnivores, eating rodents, frogs, reptiles, and invertebrates, as well as some fruit. They hunt primarily by scent, poking their snout into vegetation in search of prey or dead animals to scavenge. Once they find living prey, they pounce onto it, holding it down with their forefeet while delivering a killing strike, often to the base of 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 1,200 to 1,800 m2, with scent marks along boundaries. 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.

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. Even only living to a maximum of 4 years, that means that each female is theoretically capable of producing over 200 offspring in that time. This means that this species is capable of recovering rapidly should their population drop for some specific reason. 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 1 centimetre (0.39 in) in length and weigh 0.1 grams (0.0035 oz) at birth. The young grow hair at around three weeks, open their eyes about a week later, and are weaned at eight weeks[11]

Gray short-tailed opossums are sexually mature at five to six months of age, and live for up to forty-nine months in captivity.

There do not appear to be many videos of this species in the wild (though there are plenty of it in captivity, furthermore in some places they have become an exotic pet.

They are listed as least concern, so their future survival is not currently remotely in doubt.