Temporal range: Early Miocene–Recent 
|Virginia opossum Didelphis virginiana|
Several; see text
The opossums, also known by their scientific name Didelphimorphia //), make up the largest order of marsupials in the Western Hemisphere, including 103 or more species in 19 genera. Of South American ancestry, they entered North America following the connection of the two continents. Their unspecialized biology, flexible diet, and reproductive habits make them successful colonizers and survivors in diverse locations and conditions.
The word "opossum" is borrowed from the Virginia Algonquian (Powhatan) language, and was first recorded between 1607 and 1611 by the Jamestown colonists John Smith (as "opassom") and William Strachey (as "aposoum"). The word ultimately derives from the Proto-Algonquian word *wa˙p- aʔθemw, meaning "white dog" or "white beast/animal".
They are also commonly called possums, particularly in the Southern United States and Midwest. However, the term "possum" was borrowed into use to describe distantly related Australian marsupials (specifically those of the suborder Phalangeriformes) when Australia became known to Europeans. 
Didelphimorphs are small to medium-sized marsupials, ranging in size from a large house cat to a small mouse. They tend to be semi-arboreal omnivores, although there are many exceptions. Most members of this taxon have long snouts, a narrow braincase, and a prominent sagittal crest. The dental formula is: 184.108.40.206. 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, opossums have prehensile tails. Like all marsupials, the fur consists of awn hair only, and the females have a pouch. The tail and parts of the feet bear scutes. The stomach is simple, with a small cecum. Notably, the male opossum has a forked penis bearing twin glandes.
Opossums have a remarkably robust immune system, and show partial or total immunity to the venom of rattlesnakes, cottonmouths, and other pit vipers. Opossums are about eight times less likely to carry rabies than wild dogs, and about one in eight hundred opossums is infected with this virus.
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. The 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. 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.
Reproduction and life cycle
As a marsupial, the opossum has a reproductive system including a divided uterus and marsupium, which is the pouch. The average menstrual cycle of the opossum is about 28 days. Opossums do possess a placenta, but it is short-lived, simple in structure, and, unlike that of placental mammals, is not fully functional. The young are therefore born at a very early stage, although the gestation period is similar to many other small marsupials, at only 12 to 14 days. Once born, the offspring must find their way into the marsupium to hold on to and nurse from a teat.
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, 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 two to four years. Senescence is rapid.
The species are moderately sexually dimorphic with males usually being slightly larger, much heavier, and having larger canines than females. 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). 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.
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.
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 between 40 minutes and 4 hours, a process that begins with slight twitching of the ears.
Adult opossums do not hang from trees by their tails, as sometimes depicted, though babies may dangle temporarily. Their semi-prehensile tails are not strong enough to support a mature adult's weight. Instead, the opossum uses 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. 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. If threatened, the baby will open its mouth and quietly hiss until the threat is gone.
Opossums are remarkably resistant to snake venom, ricin, and botulinum.
In hunting and foodways
In Dominica, Grenada, and Trinidad the common opossum or manicou is popular and can only be hunted during certain times of the year owing to overhunting. 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 Mexico, opossums are known as "tlacuache" or "tlacuatzin". Their tails are eaten as a folk remedy to improve fertility.
Opossum pelts have long been part of the fur trade.
Classification based on Voss and Jansa (2009)
- Family Didelphidae
- Subfamily Glironiinae
- Subfamily Caluromyinae
- Subfamily Hyladelphinae
- Subfamily Didelphinae
- Tribe Didelphini
- Unnamed subgroup
- Unnamed subgroup
- Genus Didelphis
- Genus Philander
- Anderson's four-eyed opossum (Philander andersoni)
- Deltaic four-eyed opossum (Philander deltae)
- Southeastern four-eyed opossum (Philander frenatus)
- McIlhenny's four-eyed opossum (Philander mcilhennyi)
- Mondolfi's four-eyed opossum (Philander mondolfii)
- Olrog's four-eyed opossum (Philander olrogi)
- Gray four-eyed opossum (Philander opossum)
- †Genus Thylophorops
- Tribe Marmosini
- Genus Marmosa
- Subgenus Marmosa
- Heavy-browed mouse opossum (Marmosa andersoni)
- Isthmian mouse opossum (Marmosa isthmica)
- Rufous mouse opossum (Marmosa lepida)
- Mexican mouse opossum (Marmosa mexicana)
- Linnaeus's mouse opossum (Marmosa murina)
- Quechuan mouse opossum (Marmosa quichua)
- Robinson's mouse opossum (Marmosa robinsoni)
- Red mouse opossum (Marmosa rubra)
- Marmosa simonsi
- Tyleria mouse opossum (Marmosa tyleriana)
- Marmosa waterhousei
- Guajira mouse opossum (Marmosa xerophila)
- Marmosa zeledoni
- Subgenus Micoureus
- Alston's mouse opossum (Marmosa alstoni)
- White-bellied woolly mouse opossum (Marmosa constantiae)
- Woolly mouse opossum (Marmosa demerarae)
- †Marmosa laventica
- Tate's woolly mouse opossum (Marmosa paraguayanus)
- Little woolly mouse opossum (Marmosa phaeus)
- Bare-tailed woolly mouse opossum (Marmosa regina)
- Subgenus Marmosa
- Genus Monodelphis (translation of Spanish article)
- Sepia short-tailed opossum (Monodelphis adusta)
- Northern three-striped opossum (Monodelphis americana)
- (Monodelphis arlindoi)
- Northern red-sided opossum (Monodelphis brevicaudata)
- Yellow-sided opossum (Monodelphis dimidiata)
- Gray short-tailed opossum (Monodelphis domestica)
- Emilia's short-tailed opossum (Monodelphis emiliae)
- Amazonian red-sided opossum (Monodelphis glirina)
- Handley's short-tailed opossum (Monodelphis handleyi)
- Ihering's three-striped opossum (Monodelphis iheringi)
- Pygmy short-tailed opossum (Monodelphis kunsi)
- Marajó short-tailed opossum (Monodelphis maraxina)
- Osgood's short-tailed opossum (Monodelphis osgoodi)
- Hooded red-sided opossum (Monodelphis palliolata)
- Reig's opossum (Monodelphis reigi)
- Ronald's opossum (Monodelphis ronaldi)
- Chestnut-striped opossum (Monodelphis rubida)
- (Monodelphis sanctaerosae)
- Long-nosed short-tailed opossum (Monodelphis scalops)
- Southern red-sided opossum (Monodelphis sorex)
- Southern three-striped opossum (Monodelphis theresa)
- (Monodelphis touan)
- Red three-striped opossum (Monodelphis umbristriata)
- One-striped opossum (Monodelphis unistriata)
- Genus Tlacuatzin (translation of Spanish article)
- Grayish mouse opossum (Tlacuatzin canescens)
- Genus Marmosa
- Tribe Thylamyini
- Genus Chacodelphys
- Chacoan pygmy opossum (Chacodelphys formosa)
- Genus Cryptonanus
- Genus Gracilinanus
- Genus Lestodelphys
- Patagonian opossum (Lestodelphys halli)
- Genus Marmosops
- Bishop's slender opossum (Marmosops bishopi)
- Narrow-headed slender opossum (Marmosops cracens)
- Creighton's slender opossum Marmosops creightoni
- Dorothy's slender opossum (Marmosops dorothea)
- Dusky slender opossum (Marmosops fuscatus)
- Handley's slender opossum (Marmosops handleyi)
- Tschudi's slender opossum (Marmosops impavidus)
- Gray slender opossum (Marmosops incanus)
- Panama slender opossum (Marmosops invictus)
- Junin slender opossum (Marmosops juninensis)
- Neblina slender opossum (Marmosops neblina)
- White-bellied slender opossum (Marmosops noctivagus)
- Delicate slender opossum (Marmosops parvidens)
- Brazilian slender opossum (Marmosops paulensis)
- Pinheiro's slender opossum (Marmosops pinheiroi)
- Genus Thylamys (translation of Spanish article)
- †Thylamys colombianus
- Cinderella fat-tailed mouse opossum (Thylamys cinderella)
- (Thylamys citellus)
- Elegant fat-tailed mouse opossum (Thylamys elegans)
- (Thylamys fenestrae)
- Karimi's fat-tailed mouse opossum (Thylamys karimii)
- Paraguayan fat-tailed mouse opossum (Thylamys macrurus)
- †Thylamys minutus
- White-bellied fat-tailed mouse opossum (Thylamys pallidior)
- (Thylamys pulchellus)
- †Thylamys pinei
- Common fat-tailed mouse opossum (Thylamys pusillus)
- Argentine fat-tailed mouse opossum (Thylamys sponsorius)
- Tate's fat-tailed mouse opossum (Thylamys tatei)
- Dwarf fat-tailed mouse opossum (Thylamys velutinus)
- Buff-bellied fat-tailed mouse opossum (Thylamys venustus)
- †Thylamys zettii
- †Genus Zygolestes
- †Zygolestes tatei
- Genus Chacodelphys
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