Opossum Facts: 50 Teeth, 13-Day Pregnancy, No Ticks

• The only marsupial native to the United States and Canada. The Virginia opossum is the only marsupial native to the United States and Canada. Other marsupials — including the southern opossum and several mouse opossums — are native to Mexico; the rest of the world's marsupials live in Australasia and Central and South America.

• Fifty teeth, with a qualifier that keeps getting dropped. The dental formula is I 5/4, C 1/1, Pm 3/3, M 4/4 × 2 = 50 (Texas Tech NSRL, Mammals of Texas). Agency wording varies: Pennsylvania Game Commission says "the most of any North American mammal"; MassWildlife says "more than any other land mammal on this continent"; Illinois DNR drops the regional qualifier entirely and says "more teeth than any other land mammal." The defensible framing is the regional one, but it is not stated consistently.

• Pregnancy lasts 12 to 13 days. That is among the shortest gestations of any mammal. Females in warmer climates may have three litters per year (Smithsonian's National Zoo and Conservation Biology Institute).

• A newborn weighs about the same as a honeybee. A newborn opossum weighs about 0.0056 of an ounce (roughly 0.16 g) — about the mass of a honeybee (Illinois Department of Natural Resources, for the weight).

• More young are born than can possibly be fed. A litter may number 20 or more, but the female has only about 13 teats, and any newborn that fails to latch onto a working one dies (Texas Tech NSRL). The famous "13 nipples" figure is a typical maximum, not a fixed anatomical constant — the count varies and some teats are non-functional, so the real survivable number in a given litter is often fewer than 13.

• "Playing possum" is not acting. The National Park Service is blunt about it: the collapse is "an involuntary shock-like state that produces a foul smell from the rear of the animal." The smell comes from glands near the animal's rear, and the animal is not choosing any of it.

• An episode can last from one minute to six hours. Through it the opossum is motionless and unresponsive (Animal Diversity Web, University of Michigan Museum of Zoology).

• They run cool. Measured across ambient temperatures from 3 °C to 35 °C, the opossum held a core body temperature of 35.46 ± 0.66 °C — about 95.8 °F, several degrees below the typical placental mammal (McManus, Journal of Mammalogy, 1969).

• Not a rabies reservoir species. Of 6,154 rabid animals reported in the US in 2010, the seven groups that made up the bulk were raccoons (2,246), skunks (1,448), bats (1,430), foxes (429), cats (303), cattle (71) and dogs (69). Opossums appear only as a footnote of the wildlife tally — 3 animals, all in states where raccoon rabies is enzootic.

• The low-temperature explanation is a hypothesis, not a mechanism. MassWildlife writes that "it is extremely rare for opossums to contract rabies; it is thought that their low body temperature presents an inhospitable environment in which the virus cannot thrive." Note the hedge — it is thought that — which the viral versions of this fact quietly delete. Extremely rare is also not impossible, as the three 2010 cases show.

• Snake-venom resistance belongs to one branch of the family, not to opossums generally. It is concentrated in the tribe Didelphini, whose members are the opossums documented eating pit vipers, and it traces to rapid adaptive evolution in von Willebrand Factor, a blood-clotting protein that pit-viper venom targets (PLOS ONE, 2011).

• Two climbing tools most North American mammals lack. Opossums have a prehensile, gripping tail and an opposable, thumb-like toe on each hind foot (Alabama Cooperative Extension System, Auburn University).

• They do not hibernate, and northern animals pay for it. At the cold edge of the range it is common to see frostbite damage on the bare ears, nose and tail (MassWildlife).

• West Coast opossums are not native. The species was introduced to California in 1910 and has since spread across the state west of the Sierra (National Park Service, Presidio of San Francisco).

• Southwestern Ontario is the northernmost place any marsupial on Earth naturally lives. The IUCN records it as "the northernmost locality reached by a marsupial," and the range is still pushing northward, limited mainly by winter temperature and snow depth.

• The lineage is genuinely ancient. The evolutionary split between the ancestor of opossums and the ancestor of all other living marsupials happened at least 65 million years ago, around the extinction of the non-avian dinosaurs (Florida Museum of Natural History).

• The name means "white beast." Per San Diego Zoo Wildlife Alliance, the word comes from the Algonquin name "aposoum," meaning "white beast."

"A captive-animal study estimated opossums eat about 5,500 larval ticks per week, so having one in your yard protects you from Lyme disease." This is the single most-repeated opossum claim online, and it does not survive contact with wild animals. The 5,500-per-week figure came from a captive study; the authors of the wild-animal follow-up note that animals unaccustomed to lab conditions can behave atypically, and recommend groundtruthing lab behavior studies in the field. When Hennessy and Hild dissected 32 wild opossums from central Illinois and searched the stomach contents exhaustively under a microscope, they found no ticks and no tick parts. They then reviewed 23 published opossum diet studies — none of those found ticks either (Ticks and Tick-borne Diseases, 2021). Opossums are still worth having around; tick control just is not why.

"Opossums sleep hanging upside down from tree branches by their tails, like bats." The tail is genuinely prehensile and can curl around a branch, but the muscles in it are "not strong enough to support an adult's weight for more than a moment" (Indiana Public Media, 2023) — never mind all night. Young opossums do sometimes dangle briefly, which is probably where the cartoon image came from. What the tail is actually for is balance, gripping while climbing, and carrying objects.

"Opossums are giant rats — basically oversized rodents with pointy faces." They are not rodents at all, and they are not even close. Opossums are marsupials, in the infraclass that includes kangaroos and koalas; Mass Audubon puts it flatly, that the Virginia opossum "is more closely related to kangaroos than to any other mammal in North America." The resemblance to a rat is pure convergence on a small-scavenger body plan, right down to the scaly tail.

"'Possum' and 'opossum' are just two spellings of the same animal." They are different animals on different continents. The American animal is the opossum, family Didelphidae. True possums are a separate group of marsupials in Australia and New Zealand — furry-tailed, and not closely related. "Possum" survives in American English as informal shorthand, which is why the confusion persists, but a brushtail possum and a Virginia opossum are no more the same creature than a robin in London and a robin in Ohio.

"An opossum hissing and drooling in your yard is a rabid, dangerous animal." The drooling, hissing, teeth-baring display is a bluff — part of the same defensive repertoire that ends in the involuntary collapse people call "playing possum." State wildlife biologists rate opossums as posing very little risk to people, and rabies in the species is extremely rare: three animals out of 6,154 rabid animals reported across the US in 2010.

The opossum's cultural footprint is mostly linguistic. "Playing possum" entered English as a phrase for strategic faking, which is the opposite of what the animal does — the National Park Service describes the collapse as an involuntary shock-like state, complete with a foul smell released from the rear. The idiom credits the animal with a cunning it does not have.

The name itself is borrowed. Per San Diego Zoo Wildlife Alliance, "opossum" comes from the Algonquin word aposoum, meaning "white beast." The clipped form "possum" then took on a second life in American English as informal shorthand, which is why so many people assume the North American animal and the Australian possums are the same group of marsupials. They are not, and MassWildlife notes that using "possum" for the Virginia opossum is technically incorrect.

The modern folklore is statistical rather than narrative. Two claims do most of the circulating: that opossums are walking tick vacuums, and that they hang from branches by their tails overnight. Both are traceable — the first to a captive-animal study whose figure did not hold up when wild stomachs were examined, the second to the fact that young opossums can dangle for a moment before the tail muscles give out. Beyond the idiom, the etymology and these viral claims, the verified record behind this page carries no named opossum characters.

There is no named individual opossum in the verified research behind this page. What the species has instead is a short list of documents that changed what is known about it.

Kerr formally described Didelphis virginiana in 1792, and the binomial still carries his name. In 1969, John J. McManus published temperature measurements in the Journal of Mammalogy showing that the opossum holds a core temperature near 35.46 ± 0.66 °C across ambient conditions from 3 °C to 35 °C — the number now invoked, usually too confidently, to explain why opossums rarely contract rabies.

In 2009 the Florida Museum of Natural History reported that the split between the ancestor of opossums and the ancestor of all other living marsupials happened at least 65 million years ago. A 2011 paper in PLOS ONE traced pit-viper resistance to adaptive evolution in von Willebrand Factor, and showed the trait sits in one branch of the family, the tribe Didelphini, rather than in opossums as a whole.

The most consequential recent paper is the least flattering to the animal's reputation. In 2021, Hennessy and Hild examined the stomach contents of 32 wild opossums, found no ticks or tick parts, reviewed 23 earlier diet studies with the same result, and quietly ended the most popular opossum fact on the internet.