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Didelphidae; Temporal range: Early Miocene – Recent PreꞒ Ꞓ O S D C P T J K Pg N
	Virginia opossum, Didelphis virginiana, the only U.S. and Canadian species (mother with nine young)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Marsupialia
Superorder:	Ameridelphia
Order:	Didelphimorphia; Gill, 1872
Family:	Didelphidae; J. E. Gray, 1821
Type genus
	Didelphis; Linnaeus, 1758
Genera
	Several; see text
Diversity
	126 species

Opossums (/əˈpɒsəmz/) are members of the marsupial order Didelphimorphia (/daɪˌdɛlfɪˈmɔːrfiə/) endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 126 species in 18 genera. Opossums originated in South America and entered North America in the Great American Interchange following the connection of North and South America in the late Cenozoic.

The Virginia opossum is the only species found in the United States and Canada. It is often simply referred to as an opossum, and in North America it is commonly referred to as a possum^[3] (/ˈpɒsəm/; sometimes rendered as 'possum in written form to indicate the dropped "o"). The Australasian arboreal marsupials of suborder Phalangeriformes are also called possums because of their resemblance to opossums, but they belong to a different order. The opossum is typically a nonaggressive animal and almost never carries the virus that causes rabies.^[4]

Etymology

The word opossum is derived from the Powhatan language and was first recorded between 1607 and 1611 by John Smith (as opassom) and William Strachey (as aposoum).^[5] Possum was first recorded in 1613. Both men encountered the language at the English settlement of Jamestown, Virginia, which Smith helped to found and where Strachey later served as its first secretary.^[6] Strachey's notes describe the opossum as a "beast in bigness of a pig and in taste alike," while Smith recorded it "hath an head like a swine ... tail like a rat ... of the bigness of a cat."^[6] The Powhatan word ultimately derives from a Proto-Algonquian word (*wa·p-aʔθemwa) meaning "white dog or dog-like beast."^[6]

Following the arrival of Europeans in Australia, the term possum was borrowed to describe distantly related Australian marsupials of the suborder Phalangeriformes,^[7] which are more closely related to other Australian marsupials such as kangaroos.

They similarly have didelphimorphia, two (di) wombs (delphus).^[8]

Evolution

Opossums are often considered to be "living fossils",^[9] and as a result they are often used to approximate the ancestral therian condition in comparative studies.^[9]^[10] But this is a mistake, because the oldest opossum fossils are from a more recent epoch, the early Miocene (roughly 20 million years ago).^[11] The last common ancestor of all living opossums dates approximately to the Oligocene-Miocene boundary (23 million years ago) and is at most no older than Oligocene in age.^[12]^[13] Many extinct metatherians, such as Alphadon, Peradectes, Herpetotherium, and Pucadelphys, were once considered to be early opossums, but it has since been recognized that this was solely on the basis of plesiomorphies; they are now considered to belong to older branches of Metatheria that are only distantly related to modern opossums.

Opossums probably originated in the Amazonia region of northern South America, where they began their initial diversification.^[14] They were minor components of South American mammal faunas until the late Miocene, when they began to diversify rapidly.^[12] Before that time, the ecological niches presently occupied by opossums were occupied by other groups of metatherians such as paucituberculatans^[15] and sparassodonts.^[13]^[16]^[17] Large opossums like Didelphis show a pattern of gradually increasing in size over geologic time as sparassodont diversity declined.^[16]^[17] Several groups of opossums, including Thylophorops, Thylatheridium, Hyperdidelphys, and sparassocynids developed carnivorous adaptations during the late Miocene-Pliocene, before the arrival of carnivorans in South America. Most of these groups, with the exception of Lutreolina, are now extinct.^[18] It has been suggested that the size and shape of the ancestral Didelphid’s jaw would most closely match that of the modern Marmosa genus.^[19]

Characteristics

Didelphimorphs are small to medium-sized marsupials that grow to the size of a house cat. They tend to be semi-arboreal omnivores, although there are many exceptions. Most members of this order have long snouts, a narrow braincase, and a prominent sagittal crest. The dental formula is: 5.1.3.44.1.3.4 × 2 = 50 teeth. By mammalian standards, this is an unusually full jaw. The incisors are very small, the canines large, and the molars are tricuspid.

Didelphimorphs have a plantigrade stance (feet flat on the ground) and the hind feet have an opposable digit with no claw. Like some New World monkeys, some opossums have prehensile tails. Like most marsupials, many females have a pouch. The tail and parts of the feet bear scutes. The stomach is simple, with a small cecum.^[9] Like most marsupials, the male opossum has a forked penis bearing twin glandes.^[20]^[21]^[9]

Although all living opossums are essentially opportunistic omnivores, different species vary in the amount of meat and vegetation they include in their diet. Members of the Caluromyinae are essentially frugivorous; whereas the lutrine opossum and Patagonian opossum primarily feed on other animals.^[22] The water opossum or yapok (Chironectes minimus) is particularly unusual, as it is the only living semi-aquatic marsupial, using its webbed hindlimbs to dive in search of freshwater mollusks and crayfish.^[23] The extinct Thylophorops, the largest known opossum at 4–7 kg (8.8–15.4 lb), was a macropredator.^[24]^[25]^[26] Most opossums are scansorial, well-adapted to life in the trees or on the ground, but members of the Caluromyinae and Glironiinae are primarily arboreal, whereas species of Metachirus, Monodelphis, and to a lesser degree Didelphis show adaptations for life on the ground.^[27] Metachirus nudicaudatus, found in the upper Amazon basin, consumes fruit seeds, small vertebrate creatures like birds and reptiles and invertebrates like crayfish and snails, but seems to be mainly insectivorous.^[28]

Reproduction and life cycle

As marsupials, female opossums have a reproductive system that includes a bifurcated vagina and a divided uterus; many have a marsupium, the pouch.^[29] The average estrous cycle of the Virginia opossum is about 28 days.^[30] Opossums do possess a placenta,^[31] but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional.^[32] The young are therefore born at a very early stage, although the gestation period is similar to that of many other small marsupials, at only 12 to 14 days.^[33] They give birth to litters of up to 20 young.^[34] Once born, the offspring must find their way into the marsupium, if present, to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys.^[35] Female opossums often give birth to very large numbers of young, most of which fail to attach to a teat, although as many as thirteen young can attach,^[36] and therefore survive, depending on species. The young are weaned between 70 and 125 days, when they detach from the teat and leave the pouch. The opossum lifespan is unusually short for a mammal of its size, usually only one to two years in the wild and as long as four or more years in captivity. Senescence is rapid.^[37] Opossums are moderately sexually dimorphic with males usually being larger, heavier, and having larger canines than females.^[36] The largest difference between the opossum and non-marsupial mammals is the bifurcated penis of the male and bifurcated vagina of the female (the source of the term didelphimorph, from the Greek didelphys, meaning "double-wombed").^[38] Opossum spermatozoa exhibit sperm-pairing, forming conjugate pairs in the epididymis. This may ensure that flagella movement can be accurately coordinated for maximal motility. Conjugate pairs dissociate into separate spermatozoa before fertilization.^[39]

Behavior

Virginia opossum feigning death, or "playing possum"

Opossums are usually solitary and nomadic, staying in one area as long as food and water are easily available. Some families will group together in ready-made burrows or even under houses. Though they will temporarily occupy abandoned burrows, they do not dig or put much effort into building their own. As nocturnal animals, they favor dark, secure areas. These areas may be below ground or above.^[40]^[41]

When threatened or harmed, they will "play possum", mimicking the appearance and smell of a sick or dead animal. This physiological response is involuntary (like fainting), rather than a conscious act. In the case of baby opossums, however, the brain does not always react this way at the appropriate moment, and therefore they often fail to "play dead" when threatened. When an opossum is "playing possum", the animal's lips are drawn back, the teeth are bared, saliva foams around the mouth, the eyes close or half-close, and a foul-smelling fluid is secreted from the anal glands. The stiff, curled form can be prodded, turned over, and even carried away without reaction. The animal will typically regain consciousness after a period of a few minutes to four hours, a process that begins with a slight twitching of the ears.^[42]

Some species of opossums have prehensile tails, although dangling by the tail is more common among juveniles. An opossum may also use its tail as a brace and a fifth limb when climbing. The tail is occasionally used as a grip to carry bunches of leaves or bedding materials to the nest.^[43] A mother will sometimes carry her young upon her back, where they will cling tightly even when she is climbing or running.

Threatened opossums (especially males) will growl deeply, raising their pitch as the threat becomes more urgent. Males make a clicking "smack" noise out of the side of their mouths as they wander in search of a mate, and females will sometimes repeat the sound in return. When separated or distressed, baby opossums will make a sneezing noise to signal their mother. The mother in return makes a clicking sound and waits for the baby to find her. If threatened, the baby will open its mouth and quietly hiss until the threat is gone.^[44]

Diet

Opossums eat insects, rodents, birds, eggs, frogs, plants, fruits and grain. Some species may eat the skeletal remains of rodents and roadkill animals to fulfill their calcium requirements.^[45] In captivity, opossums will eat practically anything including dog and cat food, livestock fodder and discarded human food scraps and waste.

Many large opossums (Didelphini) are immune to the venom of rattlesnakes and pit vipers (Crotalinae) and regularly prey upon these snakes.^[46] This adaptation seems to be unique to the Didelphini, as their closest relative, the brown four-eyed opossum, is not immune to snake venom.^[47] Similar adaptations are seen in other small predatory mammals such as mongooses and hedgehogs. Didelphin opossums and crotaline vipers have been suggested to be in an evolutionary arms race. Some authors have suggested that this adaptation originally arose as a defense mechanism, allowing a rare reversal of an evolutionary arms race where the former prey has become the predator,^[48] whereas others have suggested it arose as a predatory adaptation given that it also occurs in other predatory mammals and does not occur in opossums that do not regularly eat other vertebrates.^[15] The fer-de-lance, one of the most venomous snakes in the New World, may have developed its highly potent venom as a means to prey on or a defense mechanism against large opossums.^[48]

Habitat

Opossums are found in North, Central, and South America. The Virginia opossum lives in regions as far north as Canada and as far south as Central America, while other types of opossums only inhabit countries south of the United States.^[50] The Virginia opossum can often be found in wooded areas, though its habitat may vary widely.^[51] Opossums are generally found in areas like forests, shrubland, mangrove swamps, rainforests and eucalyptus forests.^[52] Opossums have been found moving northward.^[49]^[53]

Hunting and foodways

Until the early 20th century, the Virginia opossum was widely hunted and consumed in the United States.^[54]^[55]^[56]^[57] Opossum farms have been operated in the United States in the past.^[58]^[59]^[60] Sweet potatoes were eaten together with the opossum in the American South.^[61]^[62] In 1909, a "Possum and 'Taters" banquet was held in Atlanta to honor President-elect William Howard Taft.^[63]^[64] South Carolina cuisine includes opossum,^[65] and President Jimmy Carter hunted opossums^[66]^[67] in addition to other small game.^[68]^[69]

In Dominica, Grenada, Trinidad, Saint Lucia and Saint Vincent and the Grenadines, the common opossum or manicou is popular and can only be hunted during certain times of the year owing to overhunting.^[70] The meat is traditionally prepared by smoking, then stewing. It is light and fine-grained, but the musk glands must be removed as part of preparation. The meat can be used in place of rabbit and chicken in recipes. Historically, hunters in the Caribbean would place a barrel with fresh or rotten fruit to attract opossums that would feed on the fruit or insects.

In northern/central Mexico, opossums are known as tlacuache or tlacuatzin. Their tails are eaten as a folk remedy to improve fertility.^[71] In the Yucatán peninsula they are known in the Yucatec Mayan language as "och"^[72] and they are not considered part of the regular diet by Mayan people, but still considered edible in times of famine.

Opossum oil (possum grease) is high in essential fatty acids and has been used as a chest rub and a carrier for arthritis remedies given as salves.^[73]^[74]^[75]

Opossum pelts have long been part of the fur trade.