Virginia opossum

The Virginia opossum (Didelphis virginiana), often called the North American opossum, is the most northerly living marsupial, ranging from southern Canada down to northern Costa Rica. Nocturnal and solitary, it is roughly the size of a house cat and thrives as an adaptable opportunist. It commonly lives near people, drawn to easy meals from trash cans, pet food bowls, compost heaps, gardens, or house mice. Its slow movements and habit of feeding on roadkill make it especially vulnerable to being hit by cars.

The name "opossum" originally referred to this species, coming from the Algonquian word *wapathemwa*, meaning "white animal." In everyday speech, it is often shortened to "possum." The scientific name *Didelphis* comes from Greek words for "two" and "womb." Australian possums, which are also marsupials but belong to a different order, were named for their resemblance to this American animal.

The Virginia opossum’s ancestors evolved in South America and moved north during the Great American Interchange, mainly after the Isthmus of Panama formed about 3 million years ago. *Didelphis* arrived later, entering North America roughly 0.8 million years ago. Today it lives from Costa Rica through Central America and North America up to southern Ontario, and its range is still expanding northward, northwestward, and northeastward. Before European settlement, it was generally found only as far north as Maryland; southern Ohio, Indiana, and Illinois; Missouri; and Kansas. When settlers cleared dense forests in those areas and farther north, the opossum moved in. The removal of its main predators also helped. Since 1900, it has spread into most of New England (including Maine), New York, extreme southwestern Quebec, most of southern and eastern Ontario, most of Michigan and Wisconsin, most of Minnesota, southeastern South Dakota, and most of Nebraska. Places like Rhode Island and parts of southern Ontario that rarely saw opossums in the 1960s now have them regularly, possibly because warmer winters from global warming have helped. Some think opossums reached Ontario by accidentally hitching rides on motor vehicles or trains across the St. Lawrence, Niagara, Detroit, and St. Clair rivers. Because they are not well adapted to cold winters or deep snow, a harsh winter can significantly reduce their numbers in northern areas.

The Virginia opossum is not originally native to the West Coast. It was deliberately brought there during the Great Depression, likely as a food source, and now lives along much of the Pacific coast, steadily moving north into British Columbia. Small, isolated groups also exist in eastern New Mexico and parts of southern and central Arizona, especially near Tucson, though it is unclear whether these are native or introduced.

Size varies, with larger individuals in the north and smaller ones in the tropics. From snout to tail base, they measure 33–55 cm (13–22 in), with the tail adding another 25–54 cm (9.8–21.3 in). Males are slightly larger, averaging 40.8 cm (16.1 in) body length and 29.4 cm (11.6 in) tail, while females average 40.6 cm (16.0 in) body and 28.1 cm (11.1 in) tail. Males weigh 2.1–2.8 kg (4.6–6.2 lb) and females 1.9–2.1 kg (4.2–4.6 lb). Their fur is dull grayish brown, with white faces. They have long, hairless, prehensile tails useful for gripping branches and carrying small items, plus hairless ears and a long, flat nose. With 50 teeth, they have more than any other North American land mammal, and they have opposable, clawless thumbs on their hind feet. Females have 13 nipples arranged in a circle of 12 with one in the middle. Their dental formula is 5.1.3.4 / 4.1.3.4, meaning they have 10 upper incisors—no other North American mammal has more than six. Despite being widespread and successful, the Virginia opossum has one of the lowest brain-to-body size ratios of any marsupial; its brain is only one-fifth the size of a raccoon’s.

Opossum tracks show five finger-like toes on both front and hind feet. The hind prints are distinctive because the opposable thumb usually angles 90 degrees or more from the other toes, sometimes nearly 180 degrees. Adult front prints average 1.9 inches long by 2.0 inches wide (4.8 × 5.1 cm), and hind prints average 2.5 inches long by 2.3 inches wide (6.4 × 5.7 cm). Claws are present on all toes except the two thumbs, and they often show in tracks as small holes beyond the fingertips. In soft ground like mud, the foot pads appear as deep, darker areas where the digits meet the rest of the foot. The tracks in a typical photograph were made while the animal walked with its usual pacing gait, with the four aligned toes on the hind print clearly visible.