The Virginia opossum (Didelphis virginiana), also known as the North American opossum, is a member of the opossum family found from southern Canada to northern Costa Rica (making it the northernmost marsupial in the world). Commonly referred to simply as the possum,[7] it is a solitary nocturnal animal about the size of a domestic cat, and a successful opportunist.
Opossums are familiar to many North Americans as they frequently inhabit settled areas near food sources like trash cans, pet food, compost piles, gardens or housemice. Their slow, nocturnal nature and their attraction to roadside carrion make opossums more likely to become roadkill.
The Virginia opossum is the original animal named "opossum", a word which comes from Algonquian wapathemwa, meaning "white animal". Colloquially, the Virginia opossum is frequently just called a "possum".[8] The term is applied more generally to any of the other marsupials of the families Didelphidae and Caenolestidae. The generic name (Didelphis) is derived from Ancient Greek: di, "two", and delphus, "womb".[9]
The possums of Australia, whose name derives from their similarity to the American species, are also marsupials, but of the order Diprotodontia.
The Virginia opossum is known in Mexico as tlacuache, tacuachi, and tlacuachi, from the Nahuatl word tlacuatzin.
The Virginia opossum's ancestors evolved in South America, but spread into North America as part of the Great American Interchange, which occurred mainly after the formation of the Isthmus of Panama about 3 million years ago. Didelphis was apparently one of the later migrants, entering North America about 0.8 million years ago.[10] It is now found throughout Central America and North America from Costa Rica to southern Ontario and is expanding its range northward, northwesterly and northeasterly at a significant pace.[8]
Its pre-European settlement range was generally as far north as Maryland; southern Ohio, Indiana and Illinois; Missouri and Kansas. The clearing of dense forests in these areas and further north by settlers allowed the opossum to move northward. Elimination of the opossum's main predators in these areas also contributed to their expansion. Since 1900, it has expanded its range to include 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.
Areas such as Rhode Island and Waterloo Region and Simcoe County in southern Ontario rarely had sightings of opossums in the 1960s, but now have them regularly; some speculate that this is likely due to global warming causing winters to be warmer.[11] Some people speculate the expansion into Ontario mostly occurred by opossums accidentally being transferred across the St. Lawrence, Niagara, Detroit and St. Clair rivers by motor vehicles or trains they may have climbed upon. As the opossum is not adapted to colder winters or heavy snow, its population may be significantly reduced if a colder winter with heavier snow occurs in a particular northern region.
The Virginia opossum was not originally native to the West Coast of the United States. It was intentionally introduced into the West[8] during the Great Depression, probably as a source of food,[12] and now occupies much of the Pacific coast. Its range has been expanding steadily northward into British Columbia.
Virginia opossums can vary considerably in size, with larger specimens found to the north of the opossum's range and smaller specimens in the tropics. They measure 33–55 cm (13–22 in) long from their snout to the base of the tail, with the tail adding another 25–54 cm (9.8–21.3 in). Males are slightly larger, with an average body length of 40.8 cm (16.1 in) with an average tail length of 29.4 cm (11.6 in), while females are 40.6 cm (16.0 in) long with a 28.1 cm (11.1 in) tail. Weight for males ranges from 2.1–2.8 kg (4.6–6.2 lb) and for females from 1.9–2.1 kg (4.2–4.6 lb).[13]
Their coats are a dull grayish brown, other than on their faces, which are white. Opossums have long, hairless, prehensile tails, which can be used to grab branches and carry small objects. They also have hairless ears and a long, flat nose. Opossums have 50 teeth, more than any other North American land mammal,[14] and opposable, clawless thumbs on their rear limbs. Opossums have 13 nipples, arranged in a circle of 12 with one in the middle.[15][16]
The dental formula of an opossum is 5.1.3.44.1.3.4.[17] No other mammal in North America has more than 6 upper incisors, but the Virginia opossum has 10.
Perhaps surprisingly for such a widespread and successful species, the Virginia opossum has one of the lowest encephalization quotients of any marsupial.[18]Its brain is one-fifth the size of a raccoon's.[19]
Virginia opossum tracks generally show five finger-like toes in both the fore and hind prints.[20] The hind tracks are unusual and distinctive due to the opossum's opposable thumb, which generally prints at an angle of 90° or greater to the other fingers (sometimes near 180°). Individual adult tracks generally measure 1.9 in long by 2.0 in wide (4.8 × 5.1 cm) for the fore prints and 2.5 in long by 2.3 in wide (6.4 × 5.7 cm) for the hind prints. Opossums have claws on all fingers fore and hind except on the two thumbs (in the photograph, claw marks show as small holes just beyond the tip of each finger); these generally show in the tracks. In a soft medium, such as the mud in this photograph, the foot pads clearly show (these are the deep, darker areas where the fingers and toes meet the rest of the hand or foot, which have been filled with plant debris by wind due to the advanced age of the tracks).
The tracks in the photograph were made while the opossum was walking with its typical pacing gait. The four aligned toes on the hind print show the approximate direction of travel.
In a pacing gait, the limbs on one side of the body are moved simultaneously, just prior to moving both limbs on the other side of the body. This is illustrated in the pacing diagram, which explains why the left-fore and right-hind tracks are generally found together (and vice versa). If the opossum was not walking (perhaps running), the prints would fall in a different pattern. Other animals that generally employ a pacing gait are raccoons, bears, skunks, badgers, woodchucks, porcupines, and beavers.
When pacing, the opossum's 'stride' generally measures from 7 to 10 in, or 18 to 25 cm (in the pacing diagram the stride is 8.5 in, where one grid square is equal to 1 in2). To determine the stride of a pacing gait, measure from the tip (just beyond the fingers or toes in the direction of travel, disregarding claw marks) of one set of fore/hind tracks to the tip of the next set. By taking careful stride and track-size measurements, one can usually determine what species of animal created a set of tracks, even when individual track details are vague or obscured.
If threatened, an opossum will either flee or take a stand. To appear threatening, an opossum will first bare its 50 teeth, snap its jaw, hiss, drool, and stand its fur on end to look bigger.[21] If this does not work, the Virginia opossum is noted for feigning death in response to extreme fear.[22] This is the genesis of the term "playing possum", which means pretending to be dead or injured with intent to deceive.[23]
In this inactive state it lies limp and motionless on its side, mouth and eyes open, tongue hanging out, and feet clenched.[21] Fear can also cause the opossum to release a green fluid from its anus with a putrid odor that repels predators.[22][24] Heart rate drops by half, and breathing rate is so slow and shallow it is hardly detectable.[21] Death feigning normally stops when the threat withdraws, and it can last for several hours.[21][24] Besides discouraging animals that eat live prey, playing possum also convinces some large animals that the opossum is no threat to their young.[22] "Playing possum" in response to threats from oncoming traffic often results in death.[25]
Opossums are omnivorous (sometimes said to be insectivorous) and eat a wide range of plant-based food, as well as animal-based food like small invertebrates, carrion, eggs, fish, amphibians, reptiles, birds, small mammals, and other small animals.[26]
Insects such as grasshoppers, crickets, and beetles make up the bulk of the animal foods eaten by opossums.[27] It has been stated that opossums eat up to 95% of the ticks they encounter[25][28] and may eat up to 5,000 ticks per season, helping to prevent the spread of tick-born illnesses, including Lyme disease and Rocky Mountain spotted fever.[28] This interpretation has been challenged.[29] A widely publicized 2009 study by the Cary Institute indicated that Virginia opossums in a laboratory setting could eat thousands of ticks per week grooming.[30][31] However, subsequent studies of the stomach contents of wild Virginia opossums have not found any ticks in their diet.[32]
Small animals include young rabbits, meadow voles, mice, rats, birds, snakes, lizards, frogs, fish, crayfish, gastropods, and earthworms.[33][34][35][36][37] The Virginia opossum has been found to be very resistant to snake venom.[38][28] Attracted to carrion on the side of the highway, opossums are at an increased risk of being hit by motor vehicles.[24]
Plant foods are mainly eaten in late summer, autumn, and early winter.[27] These include raspberries, blackberries, apples, acorns, beechnuts, seeds, grains, bulbs, and vegetables.[27][37] Persimmons are one of the opossum's favorite foods during the autumn.[39] Opossums in urban areas scavenge from bird feeders, vegetable gardens, compost piles, garbage cans, and food dishes intended for dogs and cats.[27][33]