The Virginia opossum is the only species found in the United States and Canada. It is often simply referred to as an opossum; in North America, it is commonly referred to as a possum<ref name="NationalGeographic">Template:Cite magazine</ref> (Template:IPAc-en; sometimes rendered as Template:'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.<ref>Template:Cite web</ref>
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).<ref>Template:Cite book</ref> 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.<ref name="Siebert">Template:Cite encyclopedia</ref> 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."<ref name="Siebert" /> The Powhatan word ultimately derives from a Proto-Algonquian word (*wa·p-aʔθemwa) meaning "white dog or dog-like beast."<ref name="Siebert" />
Opossums are often considered to be "living fossils",<ref name = "Krause"/> and as a result they are often used to approximate the ancestral therian condition in comparative studies.<ref name = "Krause"/><ref>Template:Cite journal</ref> But this is a mistake, because the oldest opossum fossils are from a more recent epoch, the early Miocene (roughly 20 million years ago).<ref>Template:Cite journal</ref> 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.<ref name=Jansa2014>Template:Cite journal</ref><ref name="BeckTaglioretti" /> 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.<ref>Template:Cite journal</ref> They were minor components of South American mammal faunas until the late Miocene, when they began to diversify rapidly.<ref name=Jansa2014/> Before that time, the ecological niches presently occupied by opossums were occupied by other groups of metatherians such as paucituberculatans<ref name = Engelman2017>Template:Cite journal</ref> and sparassodonts.<ref name="BeckTaglioretti" /><ref name=EngelmanCroft2014>Template:Cite journal</ref><ref name=Engelman2018>Template:Cite journal</ref>
Large opossums like Didelphis show a pattern of gradually increasing in size over geologic time as sparassodont diversity declined.<ref name=EngelmanCroft2014/><ref name=Engelman2018/> 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.<ref name="hyperdidelphys" /> 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.<ref>Template:Cite journal</ref>
Didelphimorphs are small to medium-sized marsupials that grow to the size of a house cat. They tend to be semi-arborealomnivores, 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: Template:DentalFormula teeth. By mammalian standards, this is an unusually full jaw. The incisors are very small, the canines large, and the molars are tricuspid.
As marsupials, female opossums have a reproductive system that includes a bifurcated vagina and a divided uterus; many have a pouch.<ref>Campbell, N. & Reece, J. (2005) Biology. Pearson Education Inc.</ref> The average estrous cycle of the Virginia opossum is about 28 days.<ref name="Opossum Society of the United States">Reproduction – Life Cycle. opossumsocietyus.org.</ref> Opossums do possess a placenta,<ref name=Enders2005>Template:Cite journal</ref> but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional.<ref name=Krause1985>Template:Cite journal</ref> 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.<ref name=EoM>Template:Cite book</ref> They give birth to litters of up to 20 young.<ref>Template:Cite web</ref> 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.<ref>Template:Cite journal</ref> Female opossums often give birth to very large numbers of young, most of which fail to attach to a teat, although as many as 13 young can attach,<ref name=smithsonian /> 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.<ref>Opossum Facts. opossum.org.</ref>
Opossums are moderately sexually dimorphic with males usually being larger, heavier, and having larger canines than females.<ref name=smithsonian>Virginia Opossum. Didelphis virginiana. Smithsonian National Museum of Natural History.</ref> 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").<ref>Template:Cite web</ref> 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.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>
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.<ref>Found an Orphaned or injured Opossum?. Opossumsocietyus.org. Retrieved on 2012-05-03.</ref>
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.<ref name="GonzálezClaramunt2000">Template:Cite journal</ref> 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.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref> 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.<ref>Template:Cite journal</ref> This adaptation seems to be unique to the Didelphini, as their closest relative, the brown four-eyed opossum, is not immune to snake venom.<ref>Template:Cite journal</ref> 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,<ref name = Voss2013/> 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.<ref name = Engelman2017/> 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.<ref name = Voss2013>Template:Cite journal</ref>
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.<ref>Template:Cite encyclopedia</ref> The Virginia opossum can often be found in wooded areas, though its habitat may vary widely.<ref>Template:Cite web</ref> Opossums are generally found in areas like forests, shrubland, mangrove swamps, rainforests and eucalyptus forests.<ref>Template:Cite web</ref> Opossums have been found moving northward.<ref name="Gardner2003"/><ref>Template:Cite news</ref>
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.<ref>Template:Cite web</ref> 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.<ref>Template:Cite book</ref> In the Yucatán peninsula they are known in the Yucatec Mayan language as "och"<ref>Template:Cite book</ref> and they are not considered part of the regular diet by Mayan people, but still considered edible in times of famine.
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