Opossum (Didelphis virginiana Kerr)
From: Saunders, D. A. 1988. Adirondack Mammals. State University of New York, College of Environmental Science and Forestry. 216pp.
Description: A long, scaly, round tail, conical head, and the female’s fur-lined pouch (marsupium) distinguish the Adirondack’s only marsupial. The long, coarse fur is a grizzled grayish white on the body, palest on the underparts. The legs are black to blackish brown; the face white, sometimes tinged with yellow. The small, beady eyes and the thin, prominent ears are black, the latter naked and rimmed with white. The prehensile, tapered tail, 22-33 cm (9-13 in) in length, is black at the base with the remainder grayish white. The first or innermost toe (hallux) of each hind foot is clawless and opposable (thumblike). Average-sized adults are 71.2 cm (28 in) in total length, and weigh approximately 3.6 kg (8 lb). Males are larger than females, and may weigh as much as 5.4-6.9 kg (12-15 lb).
Range and Habitat: The range is southeastern Canada, the eastern U.S., and parts of Mexico and Central America. The opossum is an introduced species in the western U.S. Its presence in Canada and much of the Northeast, including the periphery of the Adirondacks, is from a northern expansion of the range during this century, especially since the 1950’s. The advent of farming and the availability of agricultural crops for food are the most likely reasons for this range expansion, although deliberate and accidental releases may have contributed to it. The opossum is found in the southern Adirondacks and around the Park’s boundaries. It is generally absent in most of the Park, but occurs sporadically and perhaps as a vagrant after mild winters in the interior to elevations of at least 473 m (1550 ft) (R Masters, pers. Comm.). Where it does occur in the interior, it is associated with Village dumps.
Farmlands, wooded lowlands near water, and residential areas are places the opossum prefers. Hollow trees and logs, the abandoned dens of others mammals; piles of rubbish, rocks, hay, and wood; storm sewers; and spaces under or within barns, sheds, houses, and derelict buildings provide temporary dens. The opossum may construct nests by collecting dry leaves or other plant debris with the mouth and front feet, then wrapping the tail around the bedding material to carry it into a den.
Food and Feeding Behavior: The opossum is an opportunistic omnivore, scavenging the flesh of dead animals (carrion), hunting small animals, and gleaning fruits and seeds. Insects such as grasshoppers, crickets, and beetles make up the bulk of animal foods, but these also include snails, slugs, earthworms, crayfish, small mammals, birds and birds’ egg, amphibians and reptiles. Plant foods, taken chiefly in late summer, autumn, and early winter include raspberries, blackberries, apples, acorns, beechnuts, and where available, vegetables, and grain. The opossum forages by searching rank vegetation, decomposing logs and stumps, and by climbing trees and shrubs. In urban areas, opossums may visit bird feeding stations to eat sunflower seeds, raid garbage cans or dumpsters for refuse, and consume food intended for dogs or cats.
Activity and Movement: The opossum usually moves by walking on the soles of the feet, the tail swinging from side to side as it progresses in a slow, awkward amble, but it can run at speeds up to 7.4 km (4.6 mph). The opossum climbs trees to seek food or avoid predators, and uses the tail as a balance during arboreal activity, and sometimes hangs head downward by wrapping the tail around a large branch to free the feet for drawing food to the mouth. Although the opossum is mainly terrestrial or arboreal, it may enter water and swim well both on the surface and under water.
Most activity takes place at night, especially between 11:00 pm - 2:00 am, throughout the year. The opossum is more active in spring and summer than in winter when it may spend several days or more in a den if the temperature is below -7 to -4 degrees C (20-15 degrees F). An opossum is not well adapted to cold; the ears and tail are prone to frostbite. Cold intolerance and the scarcity of winter foods accounts for the infrequent occurrence of this species in most of the Adirondacks.
Reproduction: The breeding season is from late January through late March, and mid-May to early July. Most females produce two litters, sometimes a third. The gestation period, 12.5-13 days, is the shortest of any Adirondack mammal, and the young are born at an early stage of development. At birth each newborn opossum is about the size of a honeybee, 14 mm (0.6 in) in length, weighs 0.16 g (0.0056 oz), and is pink to translucent in color. The front feet posses clawed toes; the hind feet are small stubs. While giving birth, the female adopts a sitting posture and licks the fur of the abdomen and pouch. When the young emerge from the vulva, they use their forelimbs to crawl the 50 mm (2 in) through the moistened fur and into the pouch where they attach to one of the 13 nipples. The journey into the pouch may require no more 16-17 seconds. Not all nipples may be functional and young that fail to attach to one perish. The average little size is 6-9, with extremes of 1-13.
Once a newborn opossum begins to nurse, the nipple enlarges within the mouth, making detachment unlikely. The young remain attached for 60-65 days, or until about the age their eyes begin to open. At 70 days, they begin to leave the pouch, accompanying the female during her trips away from the den, at first by clinging to her fur, and later by walking. Although the young begin to eat solid food at this age, intermittent nursing continues until 95-105 days, when the young become independent and disperse. They do not breed until the following spring. Opossums may live to 7 years, but most survive less than two years.
Predators: Large raptors, such as the great horned owl; bobcats, foxes, coyotes, and other mammals occasionally prey upon the opossum. Many wild mammals succumb to collisions with motorized vehicles, but the opossum is especially susceptible to this hazard because it searches roads for food and is slow moving. Road-killed opossums are often the only visible evidence of this species’ presence in many parts of the Adirondacks.
- Social system - Opossums are solitary during much of their short life span. Exceptions include adults that associate briefly while mating, female-young family groups, and groups of up to 8 individuals that may share a winter den. The opossum is one of the few mammals that does not occupy a fixed or permanent home range (living area) as an adult. Some adults are sedentary, but many wander - often following water courses and establishing temporary dens in new areas. Home range size and shape vary with the availability of food, water, and dens, and with the mobility of an individual. Home ranges vary from 1-40 ha (2.5-100 ac). In the most favorable habitats of the Midwest, opossums reach densities of 2-3 per ha (0.8-1.2 per ac).
- Communication - Chemical, vocal, visual, and tactile signals regulate the social encounters of the opossum. Of these four channels of communication, the first three are the most highly developed. In aggressive contexts, the opossum hisses, growls, screeches, and makes “clicking sounds” the latter also accompanying posturing (termed a “dance”) given by the male during courtship. Adults threaten each other and predators by baring the teeth, arching the back with the fur erected, and secreting a foul-smelling greenish fluid from the paired anal glands. Males have skin glands which impart a yellowish color to the fur on the chest and may play a role in chemical communication. This mammal is well known for “playing possum” or feigning death (catatonia) during which it falls over on its side, remains immobile for periods of a few minutes to 6 hours. This behavior tends to discourage or shorten the attack of a predator.
McManus, J. J. 1974. Didelphis virginiana. Mammalian Species, 40:1-6.
Will, G., and M. Brown. 1981. Adirondack furbearers. The New York State Conservationist, 35(4):32-37.