ORDER DIDELPHIMORPHIA –

AMERICAN OPOSSUMS

The order Didelphimorphia includes only New World marsupials, which are all species of opossums.  Marsupials, or pouched mammals, are fundamentally different from the placental mammals.  Placentals nourish their undeveloped young through the placental membrane in the uterus.  Marsupials differ from the placental mammals by the presence of a fur-lined pouch (marsupium) on the abdomen of most of the marsupial female. Young are born very small and incompletely developed after a very short gestation period. They complete their development over a period of weeks after attaching to a nipple in the marsupial pouch. Marsupials also have a skeletal system that is closer to that of the reptiles from which mammals evolved than placentals.   For example, the opossum has 50 teeth; more than any other mammal, where as most placental mammals have only 44.  

Members of this order have normally long, scaly, very scantily haired, and prehensile tails.  All members have four feet with five separate digits, with an opposable great toe.  They are mostly nocturnal or crepuscular, and solitary. They have a less efficient reproductive process and a more primitive brain than the placental animals.  In like-sized mammals, the size of the opossum brain is a third to half the size of other mammals (Man comes in at 7.5x larger.) In the ranking of the complexity of mammalian social systems, on a scale that starts at 2 for least complex and ranges to 20 for most complex, the opossum earned a 2.  Even the young noticeably don't play or interact and are, in general, extremely lethargic. However, results from some learning and discrimination tests rank opossums above dogs and more or less on a par with pigs in intelligence.

Both marsupials and placentals originated during the Cretaceous Period when dinosaurs ruled. Earliest known fossils date back 130 million years ago. 

While North and South America were separated by water (for 60 million years), the placentals exterminated the marsupials in North America, while marsupials, in the absence of carnivorous placentals, dominated South America and Australia. When the Isthmus of Panama landbridge was re-established 3 million years ago, the placental carnivores from North America quickly traveled south and established dominance, wiping out many of the herbivorous and all of the carnivorous marsupials in South America (including the marsupial version of the saber-toothed tiger).  85 marsupial (non-carnivorous) species in this opossum order are still found in Central and South America.

Worldwide, marsupials make up seven different orders and 22 families.  They include kangaroos, opossums, wallabies, wombats, koalas, Tasmanian devils, marsupial moles and marsupial wolves.   

Within the order Didelphimorphia, there are four families, 15 genera and 66 species found from southeastern Canada throughout the eastern US and Mexico, into South America to about 47 degrees S in Argentina (according to Nowak).  Almost every habitat in South and Central America has some sort of opossum.  There are both terrestrial and arboreal species and even an aquatic form; the yapok or water opossum, with a watertight pouch seal.  They are all generalized feeders.  Only about eleven reach as far north as Central America, and only one is distributed in North America. 

The one South American marsupial species that has successfully established a range in North America is the Virginia opossum. The success of the opossum is undoubtedly tied to it’s omnivorous diet, adaptable habitat, and high fecundity. (Only two other South American species have established a range in North America; the placental porcupine and nine-banded armadillo.)

Family Didelphidae – Pouched Opossums

As a result of recent changes (early 90's), the Didelphid family now only contains 8 species of opossum in 4 genera, found from southeastern Canada, through the eastern US and Mexico, to central Argentina.  There is much variation in pelage.  The snout is long and pointed, and the tail is usually prehensile (used for grasping).  Although not all families in the Didelphimorphia order have a marsupium, females of the didelphids usually have a distinctive pouch.  More specifically, our one genus, didelphis, does have well-developed  marsupium, with a soft fur lining and containing the mammae.  This family is known for its extremely short gestation period of only one or two weeks, with the undeveloped, or embryonic newborn, making its way from the base of the mother's tail to the pouch, where it attaches itself to a nipple.

Opossums, like the monotremes (echidna and duck-billed platypus), and some shrew, have only a single opening, or cloaca, closed by a sphincter muscle into which all of the fecal, urinal and genital products are discharged.

This ancient family (perhaps the most primitive surviving marsupial family) is known from North American fossils as long ago as 100 million years ago (the middle Cretaceous period).

The one member of this family; the Virginia opossum, has  opposable great toes, a naked, prehensile tail, and well-developed marsupium.  The presence of white-tipped guard hairs make this genus unique in the Didelpid family.

VIRGINIA OPOSSUM (Didelphis virginiana) (double womb; Virginia)

ARTICLES

Appalachian Region Distribution: Throughout.
Continental Range: Eastern US, from the Great Lakes region south to Costa Rica.  Also introduced into California and parts of Arizona, western Colorado and Idaho.  Four subspecies, with one inhabiting this study area (D. virginiana virginiana).
Abundance: Common, decreasing at higher elevations.
Size and Molt: Head and body 15 to 20 inches; 6-9 lbs. Males are larger and weigh about a third more than females (individual males have been found weighing up to 14 pounds).  It is often said that opossums grow throughout life.
Mammae: Six pair arranged in a horseshoe configuration with one mamma in the middle.  Records exist of females with as few as 9 or as many as 17 teats.  Additionally, not all mammae are functional. 
Habitat: Varied, but including farmlands, low, damp woodlands, along streams and backyards, both terrestrial and arboreal.
Population Density: Extremely variable, from  1/acre to 1/100 acres, depending on land use.  For forested areas likely to be found in the Appalachians, Chapman and Feldhamer reports an average of 1/2.5 acres found in oak-hickory forests and old-field habitats in Illinois.   One MD study (1974) found an average density of two opossum per 102 acres.  A Richmond, VA study found one opossum per 50 acres.  Forsyth reports a range from one in two acres to one in ten. Nowak reports 0.65/acre.
Home Range: Solitary and generally nomadic, staying in an area for six months to a year before moving.  Has a home range ranging between 15 - 40 acres, overlapping other individuals, but also wanders widely, especially in the fall. They are not territorial, but individuals will defend the area they occupy at a given time.  Male range is larger than the female.
Active Period: Primarily nocturnal. Females in general are more sedentary in habit.  Activity declines in winter (especially among females). During inclement winter weather, opossum may remain in their nests for several days.  They are occasionally seen in the daytime during warm spells.  Nightly foraging distance is about 1 to 1.5 miles.
Diet: Omnivorous.  Eats anything; plant or animal (more plants and worms in spring, insects and amphibians in summer, fruits and nuts in fall, and shrews and voles in winter). Carrion, with maggots, is a year-round treat.  In fact, in baited traps in the Smokies, more opossum were caught with decomposed bait, rather than fresh bait.   Can also eat rattlesnakes, copperheads and water moccasins due to their apparent immunity to pit viper venom.  They have even been known to eat American toads, and have been observed eating road kill wood frogs.  One Maryland study (1952) found a diet composed of 14% plant material and 86% animal material.  Similar studies from other states showed 75% of the diet being animal material (a 3’ 8” black racer was found in one opossum).  Opossum are known to kill rabbits and chickens and take eggs and nestlings.  Opossum put on brown fat in late fall and early winter to help get through winter (as much as 31% body weight).   They also like cat food (see remarks).
Life Style: Two litters with up to 13 per litter per year are normal (a third brood can occur if an earlier brood is lost in the pouch stage).  Breeds from January through November with two periods of mating, usually in January/February and May/June.   Females are polyestrous, having an estrus cycle of 28 days. Estrus lasts for only two days.  Usually about 21 young are born after only a 13-day gestation (although up to 56 have been recorded), but the number of nipples limits the brood, with normally only an average of eight to nine in the Appalachian region (six or seven to the south) leaving the pouch (after 50 to 65 days on the teat).  Additionally, not all mammae are functional.  Newborn (rather, living embryos), all generally born within 12 minutes, are smaller than a honeybee.  Young are weaned after 90 – 100 days and are independent at to 3 ½ months (Forsyth says 2 to 3). Males tend to disperse further away than females. Female young often breed in the first fall at six months, males the following spring.  Adults are often between 50 to 60% males, due to the males higher mortality associated with larger ranges and need for more food/feeding (especially in winter).  Life span of 2 - 3 years (Forsyth says 7; Shedd says 1 – 2 years).  Petrides computed an average life expectancy of 1.33 years with a 4.8-year turnover in population.
Social Structure: Solitary, but are tolerant of neighboring individuals.  Family groups break up rapidly following weaning.  Males are usually aggressive toward other males but rarely toward females.  Females are tolerant of each other (except when in heat), but very intolerant of males, except when sexually receptive.  Occasionally a male and female may share the same den.                                                                
Dens/Nest: Opossum are nomadic, finding different daytime dens, as food resources deem necessary. Often, this means a new site every night, although in winter, opossum will tend to be more sedentary.   Daytime den sites include hollow trees, fallen logs, brush piles and ground burrows.  A truly unique mammal, even the pregnant female will not keep a regular den; an evolutionary adaptation enabled by the young traveling with the mother in her pouch.  Nesting material is carried into the den in it’s prehensile tail.  They are known to enlarge crow or squirrel nests by adding grass, leaves, and corn husks.  They do not dig their own winter den, rather, they use abandoned woodchuck and skunk burrows, especially those enlarged by fox.  Opossum have been known to den communally.  Has even been known to share a winter den with rabbits, skunks, raccoons, and woodchucks.  One extensive burrow system in Michigan was simultaneously occupied by an opossum, a woodchuck, a raccoon, and a striped skunk.  
Tracks: Front feet are 2" long; rear 3" long with five toes showing in a star-like pattern.  Straddle of 4", stride of 7".   Tracks are easy to recognize by the opposing rear thumb.
Scat: Not distinctive, varying with diet. Usually semi-liquid, decomposing quickly.

Remarks: Opossum is an Algonquin name meaning, “white animal”.  At the time of European arrival in North America, opossum were limited to south of the Potomac River to Harpers Ferry, and on a line to Wheeling, then northwest including most of Ohio, Indiana and Illinois.  With the removal of natural predators, they have moved north since that time and now reach Canada.  The northern limits seem determined by climate, availability of den sites and winter food.  Being a relatively newly translocated tropical species, it hasn’t evolved adaptations such as food-storage techniques and hibernation.  However, it does have the ability to store massive amounts of body fat; up to 30% of their body weight, and can lose up to 45% of their body weight over the winter season.

The opossum is unique in it's resistance to envenomation by poisonous snakes.  Anesthetized opossums have been subjected to bites from diamondback and timber rattlesnakes, cottonmouth moccasins, Russell's viper and common Asiatic cobra, with no tissue reaction other than the fang punctures.  (Unfortunately, for some of the test opossums, they were then killed for autopsy purposes.)  Other testings by injections with higher doses from pit vipers, cobras, vipers, coral snakes, puff adder, and sea snake, resulted in death of all opossum within 24 hours, with the exception of those give crotalid venom (pit vipers).  Thus, it can be said that opossum are unaffected by the bites of pit vipers (copperheads, rattlesnakes, and water moccasins), but are sensitive to venoms of other poisonous snakes.  As mentioned in diets, opossum do prey on these crotalids.

Opossum are shy and secretive marsupial omnivores. Although they can climb well, assisted by their prehensile tail, and swim, they spend most of their time on the ground.  They are also known to be strong swimmers.  Females have a fur-lined pouch on the abdomen, while males do not. 

Known for feigning death (playing possum), which includes lying on its side, drooling, defecating and generally making itself most unattractive to the predator. It is interesting to realize that this catatonic state is an instinctive behavior, which may last for minutes or hours before the opossum resumes its normal activity.  It is commonly believed that, although catatonia seems partly under the conscious control of the animal, physiological changes suggest a state analogous to fainting in humans.  During this catatonic state, the heart rate has been recorded to decrease by 46 percent, with a reduction of respiration of 30 percent.  Rue reports that his experiences show the opossum will recover as soon as the danger has departed.  He also states that only about 10% of opossum exhibit this display.  He further says the shock that initiates the "death feign" will have less effect the next time the shock is applied, and by the third or fourth trial, the opossum will show no signs of impact whatsoever.  

Researchers do agree, more likely than feigning death, the opossum will hiss, growl, lunge at the predator or run away.

Opossum have 50 teeth, more than twice that of most placental mammals.

Males have a forked penis, which matches the paired lateral vaginae of the female. In other more advanced mammals, the female reproductive tube fuses in the middle to form a single canal. In fact, the opossums produce paired sperm.  As a pair, the sperm swim in a straight line, but if separated, they swim in circles.  Copulation involves the male grabbing the female by the nape of the neck and both falling over to the right side. On occasion when they may fall to the left, or remain upright, copulation is likely to be unsuccessful. After mating, a temporary plug forms in the female's vagina, preventing her from being mated with other males. The plug lasts for about 36 hours, at which time the 36-hour estrus period is over. The 1/2" young are born with the mother sitting up and they then climb up to the mammae located in the pouch where they stay attached for the next two months.

Older opossum often show signs of frostbitten ears and tail.

One story is related (Shedd) about a kind lady who would put out cat food for stray cats.  She would leave the porch door open to allow the cats to come in to eat.  One day, among the cats eating from her food bowls, was an opossum.  The cats and opossum seem not to be disturbed by each other’s presence, and the opossum became a frequent visitor to her cat feeding station.

For it’s size, the opossum has one of the shortest life spans of all mammals.

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