Gray four-eyed opossum

It has a sharply defined white spot above each eye, hence the common name.[4][5] Its prehensile tail is bicolored, with a pale distal part and a longer proximal darker gray part, and is naked at the end. Its dorsal fur is gray, while its ventral fur, throat, and cheeks are cream-colored. Adults have ears that are black except at the base.[3] Wild specimens weigh 200–674 grams (7.1–23.8 oz), while captive specimens can weigh up to 1.5 kilograms (3.3 lb).[5] Body length ranges from 22.0 to 33.1 centimetres (8.7 to 13.0 in) with a tail in a similar size range, 19.5 to 35.5 centimetres (7.7 to 14.0 in).[5]

The species is nocturnal, solitary and partly arboreal.[2] It is usually found in moist areas, often near streams, although it wanders in many different vegetation types.[2] It is a good swimmer.[2]

Gray four-eyed opossums do not have a well defined territory, and home range stability depends on the availability of adequate resources.[4] They are omnivorous, feeding on small animals and vegetation, such as leaves, seeds, and fruits.[5]

The gray four-eyed opossum does not "play dead" like the North American Virginia opossum. Instead it is aggressive and fights with potential predators.[6] Some displays of aggression include opening the mouth wide and hissing loudly.[6] It is known to be "the fiercest fighter of the opossums".[6] The gray four-eyed opossum is a nocturnal animal but can be active during the day. Although it is terrestrial, it is very good at climbing and swimming.[6] It has agile and swift movements, and seems more alert than other didelphids.[6]

Gray four-eyed opossums build nests out of dry leaves in hollow trees, tree forks, fallen logs and in ground burrows.[6] They roll up into a ball while sleeping and although their eyes are actually closed, the white patches of fur above their eyes gives them the appearance of an awake animal.[6]

Little is known about gray four-eyed opossums mating habits but they are in the family Didelphidae and so most likely to be polygynous.[5] This means males compete with other males. There are no courtship displays or pair bonds formed in didelphids.[5]

Reproduction is typically seasonal, with more young being born during the rainy season when there is an abundance of fruit.[5] During the dry season, fewer babies are born due to the lack of available fruit.[5] Litter sizes averaging between four and five young, with each female producing between two and four litters per year. Many young die while nursing in the mother's pouch.[5] This death rate is especially high during the dry season.[5] A major factor that determines survival of young is the mother's age; there are many deaths when the mother is less than 11 months.[6]

The average gestation period for the gray four-eyed opossum is 13 to 14 days, and each newborn weighs about 9 grams (0.32 oz).[5] They nurse in their mother's pouch until they are 68 to 75 days old.[5] Once weaned, they stay in their mother's nest for a further 8 to 15 days before their mother becomes aggressive and expels them.[5]

The gray four-eyed opossum has an omnivorous diet containing fruits, nectar, insects, small mammals (such as mice), birds, reptiles, amphibians, crustaceans, snails, and earthworms.[6] Its diet varies depending on the season.[6]

With such a varied diet, the gray four-eyed opossum will both encounter and eat venomous snakes. While the bites of these snakes may be harmful to most animals, the gray four-eyed opossum is able to overcome the toxic effects due to its immunity to the toxins.[7] The immunity was initially thought to come from an immune response leading to the production of antibodies, but in fact it comes from toxin-neutralizing proteins found in opossum serum.[7][8] These proteins are produced by the opossum prior to any encounter with a venomous snake, thus this immunity is not learned but inherited.[7]