The Virginia opossum is North America's only marsupial. A marsupial is an animal with a pouch, like a kangaroo or a koala. 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, although normally about the size of a house cat. The fur is generally gray but can vary from white to brown to black because their guard hairs can range in color and thickness. Opossums have long, hairless, prehensile tails, which is adapted for grasping items such as leaves to build a den or for stabilization to wrap around branches while they are in a tree. They also have hairless ears and a long, flat snout and pink nose. Opossums have 50 teeth, the most out of any North American mammal, and opposable, clawless thumbs on their rear limbs.
The Virginia opossum is found throughout Central America and North America east of the Rockies from Costa Rica to southern Ontario; it seems to be still expanding its range northward and has been found farther north than Toronto. In recent years their range has expanded west and north all the way into northern Minnesota. Originally native to the eastern United States, it was intentionally introduced into the West during the Great Depression, probably as a source of food, and now occupies much of the Pacific coast.
Opossums live in or near wooded areas, especially those near water. They are adapted to edge habitats and are often found in urban areas. Opossums do not build their own shelters. They take cover in abandoned dens or nests of other animals such as from skunks and foxes. They will also take cover in sheds or old buildings, cavities in rocks, brush piles, and hollow trees or fallen logs. However, they will carry nesting material to the shelter they find.
Opossums have no highly developed or specialized mechanisms for attack or aggressive offense. They are not fast runners but can climb trees and swim. However, defensive behavior is highly developed in the opossum. Intimidation (bluff) displays are commonly observed when an opossum is cornered and consist of the animal crouching down or sitting back on its haunches and opening the large mouth as much as possible to display its formidable fifty teeth. As this occurs the animal hisses, growls or screeches in that order and with increased intensity.
If caught and handled the animal tends to defecate and release a pea-green secretion from the two paracloacal glands located on either side of the cloaca near the base of the tail. If faced with an extreme threat such as a dog attack, the animal may feign death or “play opossum” for which this behavior is named. Playing opossum is a passive defensive tactic used by the opossum as well as other species in the animal kingdom, such as the hog-nosed snake. Feigning death functions to turn off all potential signals that might trigger the predator to attack or continue its attack. Opossums are extremely tough animals and can withstand considerable abuse. Opossum skeletons that have been examined often reveal well-healed bone fractures which equivalent sized eutherian mammals never would have survived. During feigned death the opossum often lies on its side curled up with its eyes and mouth open and teeth bared. The tongue may hang out of the mouth and the animal continues to drool considerably. The toes of the forepaws are closed and grasp anything in contact with them, even the other forepaw. The opossum appears to be in a catatonic state and this condition may occur for as little as a few minutes or last for several hours. Poking and shaking will not revive the animal from its catatonic state nor will it flinch from abuse while in this state. Younger animals are more apt to play opossum than older ones.
The skull of the Virginia opossum is considered primitive (because it retains many of the cranio-facial features of early fossil therian mammals) and is characterized by a small brain case. The size of the opossum’s brain case has been measured by filling the cranial cavity with dried beans and then counting the number of beans it took to fill the cavity. It was found that if the brain case of an opossum, a raccoon and a house cat were compared using this method, the opossum brain case held 5 dried beans; the cat brain case held 15 dried beans and that of a raccoon held 150 dried beans. Thus, the opossum has one of the smallest brain-to-body size ratios among mammals and it was generally assumed that the larger the brain size to total body ratio the more intelligent the animal is.
The opossum brain differs somewhat externally from a typical mammalian brain but in general the internal structure and pathways of its central nervous system show only minor dif- ferences from that of eutherian mammals. However, the external surface is smoother and has fewer folds and groves than the brains of eutherian mammals of similar size. The opossum brain features a pair of large, elongated olfactory lobes, which is not surpris- ing because of this animal’s keen, well-developed sense of smell. The opossum brain like that of all other marsupials is characterized by one major distinction: the complete absence of a corpus callosum. The corpus callosum is a large band of nerve fibers that functions to connect the cortical areas of the two cerebral hemispheres of the eutherian brain. It is the relative small brain size and the extremely shy and non-aggressive behavior of the opossum in comparison to other mammals that has contributed to the myth that the opossum is a very “stupid” animal. For example, when captured and held by hand, usually by the tail or the back of the neck, the opossum usually does not struggle or attack its captor. If held by the neck and the other hand used to support its back, the opossum simply gives up, relaxes and often clasps its forepaws together in a prayer-like pose. Such a method of handling another wild mammal of equivalent size such as a raccoon or fox would be difficult if not impossible and be met with aggression. It is this non-aggressive behavior, together with the knowledge of its relatively small brain size, that have been the major contributing factors in labeling the opossum as be- ing stupid or of less mental capacity than other more aggressive animals. In addition, it must be remembered that the opossum is a nocturnal species. Many of the published observations and stories concerned with the behavior of the opossum were made during the daylight hours at a time when the opossum is normally asleep. If one considers the time of observation it is quite understandable that the opossum gained the reputation of being a bit sluggish in some of the early accounts.
In spite of their apparent primitiveness and small brain size, opossums have a remarkable capacity to find food and remember where it was found. When tested for their ability to remember, opossums scored better than rats, rabbits, dogs, and cats but did not score as well as humans. Opossums can remember the taste of noxious or toxic substances even a year after a single encounter.
Visual discrimination tests have shown that the opossum can learn to discriminate black versus white, different colors, patterns, and geometric forms. Additional studies designed to measure the opossum’s ability to solve maze problems indicate that mature opossums were superior to most species (rats, cats) in maze learning tasks.
Nests or Dens
The opossum is primarily a nocturnal mammal (active at night) and forages for food shortly after dark, a behavior that continues until dawn. However, opossums may become active during daylight hours during cold weather (winter usually) when food is more difficult to obtain and its metabolic need is greater. The opossum, as is the case for most other mammals active during cold weather, needs a greater caloric intake in order to survive. Like most nocturnal animals, the opossum must build a rough nest or find a den in which to rest during the daylight hours. Depending on the location, the opossum den can take a variety of forms: an abandoned burrow or underground tunnel, cavities in hollow trees, abandoned squirrel nests, crevices in rocks, or crawl spaces under houses, in attics or in some other dark, hidden spaces of buildings. Opossums do not dig their own burrows, but occupy abandoned burrows of other animals. They will seek some quite dark space that is dry. The dens are most often temporary but if a food source is nearby may be occupied for several weeks. A single opossum may have several dens that it will use periodically. Females with young tend to be the exception to this behavior and use the same den site for several weeks at a time. The dens usually are lined by grass or thin twigs mixed with dry leaves. In warmer months its construction is similar to that of a bird’s nest. In the cooler months of fall and winter its construction resembles a hollow sphere and is similar in appearance to a mouse or squirrel nest. There have been numerous observations of opossums gathering denning materials. The materials are gathered using the mouth and forepaws, then passed to the rear under the abdomen to the hind feet, which the opossum uses to organize the gathered materials into a bundle. The bundle is then placed in a loop of the tail and the prehensile tail used to drag the material to the opossum den.
My apologies if the image below is rather graphic to you, but I think it shows just how versatile opossums are in what they will eat in order to survive.
The opossum is an opportunistic feeder and will eat whatever is available in its environment at a given time and its diet will change with the seasons. When foraging at night the opossum keeps its nose to the ground, relying primarily on smell and touch. Vision does not appear to play a large role in locating food during their night prowling although they can see distant objects. The opossum is omnivorous, consuming both animal and plant material. They will eat just about anything and because of this are really great for the environment as a clean up crew. Its diet includes a variety of insects, earthworms, slugs, snails, crayfish, snakes and lizards, frogs, small rodents, small birds, eggs, grasses, vegetables, fruits, berries, grains, and human garbage. Many people enjoy having them around their home because they will get rid of cockroaches and rats, two pests that many people despise. The prehensile tail and opposable thumb on the hind feet help the opossum climb trees in search of bird eggs and young. They will also eat carrion, dead animal material. Although since so much carrion is roadkill, opossums are often killed by cars while looking for food on roadways. Opossums also eat snakes, including venomous snakes. Which brings me to my next subject.
Resistance to Snake Bite During casual field studies on mammals indigenous to the Everglades region of Florida, a natural bite by a 160 cm eastern diamondback rattlesnake on an adult opossum was observed. The opossum displayed no apparent distress. After this initial documented observation, several field experiments were conducted manually causing snakes to inflict actual bites on captured opossums. Bites were inflicted using timber rattlesnake (Crotalus h. horridus), eastern diamondback rattlesnake (Crotalus adamanteus), cottonmouth moccasin (Agkis- trodon p. piscivorus), Russell’s viper (Vipera russelli), and the common Asiatic cobra (Naja n. kaouthia). None of the opossums developed observable local reactions or systemic effects other than trauma attributable to fang penetration. Because the amount of venom injected by an actual snakebite is highly variable, another series of experiments was conducted in which a known volume of snake venom was injected directly into the opossum blood stream. These studies demonstrated that the Virginia opossum has a remarkable physiological tolerance to both snakebite and massive intravascular infusion of venom. Thus, the opossum appeared resistant to venomous snakebites, particularly from those snakes that share the same range and habitat with the opossum. If a copperhead, water moccasin, or rattle snake bites the opossum, the reaction is only a small local swelling similar to that of a bee sting. In some regions of the United States poisonous snakes represent an important part of the opossum’s diet.
Scientists have now identified the protective factor in opossum blood serum using high-pressure liquid chromatography and named this small protein (a proteinase inhibitor)lethal toxin-neutralizing factor (LTNF). The reason opossums are naturally resistant to the proteolytic effects of several venoms is that the proteinase inhibitors in their blood bind to and neutralize the venoms. The proteinase inhibitors are not antibodies but proteins that occur naturally in opossum serum. A series of experiments were conducted in which a predetermined lethal dose of toxins derived from animals, plants, and bacteria injected into mice were given LTNF. It was observed that LTNF neutralized the lethal effects of all snake venoms tested: western diamondback rattlesnake (Crotalus atrox), Thailand cobra (Naja kaouthia), Asian viper (Da- boia russelli), Australian taipan (Oxyuranus scutellatus); is effective against scorpion (Andro- cetonus australis) and honeybee (Apis mellifera) venoms; plant-derived ricin (one of the most toxic plant-derived toxins from castor seeds) as well as botulinum toxin from the bacterium, Clostridium botulinum. Recently, the active ingredient of LTNF (a ten amino acid peptide) has been sequenced and synthesized and tested on mice. The results proved that the synthetic form was effective in inhibiting the lethality of all the toxins tested. Lethality was inhibited even when the synthetic LTNF was administered two hours after toxin or venom injection. Most importantly, synthetic LTNF can now be made in abundance without depending upon the natural source, opossum serum. Thus, LTNF can be made in abundance and may prove to be a universal therapy against toxins from animals, plants, and bacteria to help save human lives.
Resistance to Rabies Opossums are rarely found to be rabid and appear to be resistant to many viral diseases such as distemper, parvovirus, and feline hepatitis normally found in domestic animals (cats and dogs) as well as some wild mammals. The resistance to many of the viral diseases is thought to be due, at least in part, to the slightly lower body temperature of the opossum as compared to other mammals.
Females have one to two litters per year, in some parts of the country they can have up to three litters. The average litter size is seven to eight young, although the females can support up to 13 young. The female gives birth to underdeveloped, embryo-like young approximately two weeks after mating. At this stage, the young are about the size of a bumblebee and they must crawl from the opening of the birth canal to the pouch using their well-developed forelimbs and clawed digits. where they continue to develop. During the birth process, the female usually sits on her haunches, curls over somewhat and approximates the opening of the birth canal to an area beneath the pouch thereby shortening the distance the newborn opossums must travel to locate and enter the pouch. The tiny forepaws of these newborn animals are capable of grasping, and with their clawed digits grasp the mother’s fur and wriggle their way to the pouch. The newborn opossum exhibits a swimming motion (overhand stroke) where the head and neck are flexed to one side followed by the forward movement of the opposite foreleg. The head and neck are then flexed to the opposite side while the deciduous claws of the forepaw clasp the abdominal fur of the mother. As the forepaw is pulled back toward the newborn opossum it is pushed forward. In a short while (the journey from the birth canal to the pouch usually takes between two and four minutes), the newborn opossum enters the relatively large space of the pouch, tangled with large, coarse curly hairs among which are usually thirteen tiny teats.
Young opossums leave the pouch when they are two to three months old but remain near the female so that they can return to her pouch to nurse. During this stage of their life, the young will then hang onto the mother's back as she forages and will begin to eat solid foods. By the time they are four months old, they will be weaned and on their own.
Baby opossums at different ages in mother's pouch.
Although the opossum has a high mortality rate at all stages of its life cycle, mortality is particularly high during the first year. The death rate of young while still being carried in the pouch is high and ranges between ten and twenty percent. Of those that survive until after weaning, to go out on their own, fewer that ten percent live beyond the first year. A few tagged wild opossums have been shown to have a life span of about two or three years. Thus, the turnover rate of the opossum population is rapid and the indigenous population of a given region is heavily weighted toward the young of the year. The most important causes of death leading to their relatively short life spans are: human-caused deaths due to automobiles as well as hunting and trapping, diseases and parasites, exposure and starvation. It is estimated that between four million and eight million opossums are killed each year in the United States by automobiles. Although predators such as large owls, foxes, coyotes, and domestic animals may reduce the numbers of young animals somewhat, predation does not appear to be a significant cause of mortality in the adult population. The opossum is host to a multitude of external and internal parasites and these generally are debilitating and increase the susceptibility of the opossum to malnutrition and disease. Parasites and human activities apparently are the leading cause of death for most adult opossums.
Opossums, for their size, are one of the shortest-lived animals in the world. Those individuals that do live into their second year show many of the classic signs of advanced aging such as weight loss, lessened motor coordination, and formation of cataracts. Why this occurs so early in this species is unknown. Captive opossums have been recorded as living about twice as long as wild opossums.
Threat to Human Health and Property
Although opossums often live in close proximity to human habitation, they seldom if ever are the cause of serious problems for farmers, ranchers, or suburban homeowners. Opossums are often blamed for raiding trash containers, but more often than not, they are secondary in such raids and are foraging in what remains following the previous activity of domestic animals or raccoons. When pets are routinely fed outside and any excess food remains, if discovered by an opossum, it will adapt that location as a feeding area. This activity can be discouraged by simply removing the food source and when this is done the opossum will move on to a new area in which to feed as is its habit.
The only potential threat opossums may pose to pets or domestic livestock is with regard to horses. Opossums should be considered a threat to the health of horses and other equines particularly if opossums den or feed where horses are being housed and/or have access to the horses’ food/water source that can be contaminated. A protozoan known as Sarcocystis neurona is thought to be a major contributing factor of a neurologic disease in horses known as equine protozoal myeloencephalitis or EPM. Opossums are the definitive hosts, and horses and other mammals are the aberrant hosts for this protozoan. If sporocysts (the larval form or infectious agent in the next host) from the intestinal tract (via feces) of infected opossums are ingested by horses in contaminated food or water, they are at high risk for contracting this disease. Studies that examined the blood serum for antibodies against Sarcocystis neurona of horses from the Rocky Mountain states demonstrated that these horses have a lower seroprevalence for this protozoan organism than horses from the eastern regions of the United States. This data corresponds to areas of opossum population density.
Damage Prevention and Control Measures
Opossums are most likely to cause problems by getting into garbage or raiding bird feeders. Though they do not usually cause damage, they can startle people when they take shelter in woodpiles, sheds, under decks and porches, or in other unexpected places.
Do not leave pet food outside overnight.
Remove spilled bird seed.
Remove brush or trash piles.
Store firewood away from buildings or fences and keep the wood at least a foot off of the ground.
Keep garbage stored correctly in secured cans with tightly fastened lids.
Close garbage dumpster lids each night to prevent opossums from climbing or falling into the dumpster.
Regularly check your home for signs that it needs repair and make the repairs quickly.
Repair loose fascia boards.
Check the integrity of chimney caps and attic vents, and repair any holes.
Deny access to sheds or to spaces underneath decks or porches. For porches or decks built within two feet of the ground, dig a trench at least 10 inches deep around the deck's perimeter. Attach ½" × ½" mesh hardware cloth or 1" × 1" welded wire from the top of the outside joists to the bottom of the trench. Leave six to eight inches of wire at the bottom and bend it out at a 90° angle. Fill the trench with soil or rocks. Add lattice or other cover for aesthetics.
When sleeping, opossums hang upside down by their tails in trees. Opossums are terrestrial animals and spend the vast majority of their time on the ground and not in trees. They climb up into trees primarily to escape predators or are in search of food. They do not sleep in the branches of trees like birds. Instead, they make dens in a variety of places on the ground and curl up to sleep within the protection of the den, as do many other mammals. The tail is prehensile and used as an aid to help with climbing and with balance during walking and running. It is also used to carry bundles of denning material. An adult opossum rarely if ever hangs solely by its tail and, if it did, could do so for only a limited amount of time due to its own body weight.
Opossums breed through the nose. The ridiculous notion that the male opossum copulates with the female through her nose is thought to have occurred during colonial times as a result of assumptions made by early European settlers. The idea for such a notion arose from the observation that penis of the male is bifid (forked) and the behavior that the female opossum grooms her pouch immediately prior to the birth of a litter. It was assumed that when the female placed her snout into the pouch (while grooming) she sneezed the young into the pouch where they then developed.
Opossums will attack pets and livestock. The opossum is a very shy, non-aggressive animal that usually avoids contact with other species unless they are smaller and represent potential prey. The opossum will not attack typical pets such as dogs or cats and certainly not larger livestock. Conflict may arise, however, if pets are fed outside and excess food is left in the feeding area. If the opossum discovers food it will enter such an area to feed on what is available. On occasion, the pets being fed (usually dogs) will surprise the feeding opossum and a conflict will arise. Even then the opossum will try to escape to avoid confrontation and is usually discovered cornered trying to defend itself.
Opossums are the dumbest of all mammals. Despite its small brain size relative to body mass, the opossum when tested scientifically performs as well as rats and other species on various tests that measure intelligence. Its shy, non-aggressive demeanor and ease of handling when captured have been interpreted by many as this species having a diminished mental capacity. Nothing could be further from the truth as recent testing with regard to the opossum’s intelligence has proven.
Opossums are dirty little animals and like rats, spread disease. The opossum like most other animals in the wild are well groomed. Indeed, their very survival depends on being clean and well groomed. If not, their protective fur coat will become matted and dirty and without such a protective layer the animal will eventually die of exposure. The opossum appears quite resistant to some viral diseases such as rabies, distemper, forms of feline hepatitis, and other diseases that plague domestic pets. However, the opossum is heavily parasitized by a wide variety of organisms, which are thought to contribute directly to its short life span.
Opossums are closely related to monkeys because they have similarly shaped hind feet. The opossum is classified as a marsupial; monkeys are classified as primates. The similarities of the hindfeet arose independently in these two widely different species. The opposable digit on the hindfoot of each species evidently provided a specific advantage for each animal in its environment.
When Rescue Is Needed
There are generally three situations that cause a baby opossum to be separated from his mother: either the mother is killed (by man, dogs or a natural predator), the baby falls off the mother, or the mother is prevented from returning to her nest.
Opossums hit by a car can have live babies in the pouch, babies still clinging to her back, or babies wandering lost and confused up to 50 feet or more away from their dead mother. If there are babies anywhere near, they need to be rescued, or they will die a slow death from hypothermia (loss of body heat), starvation, become prey for any nearby predators, or hit by another car. Contact a wildlife rehabber in your area immediately if you have found babies on or near their dead mother.
Opossum youngsters are remarkably good at holding on to their mother even as she’s climbing, but if they fall off they will make a sneezing, wheezing sound to alert their mother of their position. If you find a baby opossum and do not see its mother anywhere around you need to seek immediate assistance and get the baby warmed so it does not die of hypothermia.
If their babies are old enough to regulate their own temperature, possums will occasionally leave their young in the den to go out and forage for food. If that den happens to be in your garage or under your tool shed, you may inadvertently doom the babies by closing off their mother's access to them. Or you may frighten her as she's returning to the den, causing her to run off and abandon her babies. If you'd like the mother to take her babies somewhere else you need to make the area uninviting. Keep the area well lit 24 hours a day, remove any food sources (pet food, overripe fruit on the ground), leave a radio turned on low. Remember, you don't want to frighten the mother, you just want to encourage her to find a new home. Once the mother realizes that her choice for a den has deteriorated, she will move her babies to a new den. It may take her 3 - 4 days to get everyone resettled, so give her time. When you're certain all the babies are out of the den, close up the access routes so that the next opossum wandering through does not take up residence. If you have found a den of baby opossums but have not seen any sign of their mother, you need to seek the assistance of a wildlife rehabber asap.
Injured adult opossums require special handling. They are very strong, very fast, and have very powerful jaws with a lot of sharp teeth. In spite of all this, an experienced wildlife rehabilitator will gladly spend the time and energy necessary to help an injured opossum be released back into the wild. Opossums are very resilient animals, so if you find an adult lying immobile, whether it's in your yard or on the side of the road, its injuries are probably severe enough that he needs to be rescued. Adult opossums can be hit by automobiles and survive if given the proper medical treatment and care. Opossums trapped in garbage cans or trash dumpsters also sometimes need some rehabilitation time, since they can be dehydrated from not having water for a few days, or can have open wounds that may require a few stitches. If you have found an adult opossum that needs rescuing, seek the assistance of a wildlife rehabber asap.
If you have an opossum that needs rescued, please visit this page here for how-to's and contact a wildlife rehabber in your area immediately.
Be sure to check out the e-book, “The Opossum: Its Amazing Story” by William J. and Winifred A. Krause. Much of the information from this page has come from there.