NEW WORLD OPOSSUMS: Didelphimorphia

PHYSICAL CHARACTERISTICS

The word "opossum," commonly used to refer to all species within the family Didelphidae, is derived from an Algonquian Indian word for the Virginia opossum, the only living marsupial species north of the U.S.-Mexico border. "Possum," without the first "O," refers to certain Old World marsupials in Australia and New Guinea.

Didelphidae are tiny to medium-sized animals, most tending toward the smaller end of the size spectrum. Males are larger than females. In most species, the tail is about the same length as the combined head-and-body length, or longer, scaly and only lightly furred, and is prehensile (able to grasp) to varying degrees among species. In the smallest species, adult head and body length runs 3.3 to 7.2 inches (8.5 to 18.5 centimeters) and tail length is 3.5 to 10 inches (9 to 25 centimeters). In the largest species, adult head and body length runs 13 to 19.5 inches (32.5 to 50 centimeters) and tail length is 10 to 21 inches
(25.5 to 53.5 centimeters). Adult weight in the larger species is usually between 4.5 and 12 pounds (2 and 5.5 kilograms).

The limbs of Didelphidae are short, except for the yapok (or water opossum), whose hind legs are a little longer than the forelegs. All four feet bear five digits and the hallux (HAL-lux; big toe) is opposable. All digits are clawed, except for some species in which the hallux lacks a claw. The muzzle (mouth area) is long and pointed, and the ears are prominent. The canine teeth are long and large.

The fur may be fine and velvety, thick and woolly, or somewhat coarse and stiff. Pelt colors, combinations, and patterns vary widely among genera (JEN-uh-rah) and species. The brown four-eyed opossum and the gray four-eyed opossum owe their common names to a colored spot of fur above each eye. In some species, there are dark brown or black patches around the eyes.

In most Didelphidae species, the back and sides of the body are dark, the underparts lighter. Upperparts may be gray, dark brown or reddish brown, the underparts white or yellowish. The thick-tailed opossum has an elaborate coloration that varies among individuals. The upper body fur may be yellow, yellow-brown, or dark brown, while the underparts are reddish-brown, light brown, or dark brown. The fur may have an unusual purple tinge. The face may show vague markings. The body shape of this species is also unusual, tending toward a long, low-slung, weasel-like form, with short but strong legs.

GEOGRAPHIC RANGE

In a very general sense, the Didelphidae can be said to inhabit both New World continents, from southeastern Canada to southern South America, but the common or Virginia opossum is the only marsupial making its home in the continental U.S. and Canada. All other species of Didelphidae range across Mexico, Central, and South America, from northern Mexico to southern Patagonia in South America, and on some of the Lesser Antilles Islands.

HABITAT

The Virginia opossum inhabits the widest range of habitats of any New World opossum, being found over most of the continental United States and southeastern Canada, in forest, grassland, and desert. The other species variously inhabit tropical and subtropical forests, and a few, like the Patagonian opossum, inhabit temperate grasslands in South America. The dryland mouse opossum prefers desert-like conditions in Central America.

DIET

Diet among Didelphidae is omnivorous, with some variation among species. Food sources include insects, small reptiles, small mammals, especially rodents, birds' eggs, fruits, seeds, snails, freshwater crustaceans, earthworms, and carrion. One species is skilled at subduing scorpions. The yapok, or water opossum, hunts and eats freshwater fish. Some species store fat in the bases of their tails to carry them through the lean months.

"NEW WORLD" MARSUPIALS?

When you hear or read the word "marsupial," you probably think of kangaroos, koalas, and Australia. Maybe you think of New Guinea, the big tropical island just north of Australia, and its hordes of tree kangaroos and other marsupial types, or the Virginia opossum, the only wild marsupial in North America north of Mexico. South and Central America might not even come to mind, but an extraordinary seventy-five species of marsupial mammals live today on those landmasses, from the deserts of northern Mexico through the forests of Central and South America, and across the grasslands of Patagonia, almost to the southern tip of South America. How did they get there, on the other side of the Pacific Ocean from Australia?

Eighty million years ago, small, early mammals, including marsupials, were flourishing. Today's southern continents were united in a supercontinent called Gondwana, which split from the northern supercontinent, Laurasia, made up of the present-day northern continents, around 160 million years ago. The two giant continents continued to split apart into the continents of the present day. The southern continents of Australia, Antarctica, and South America remained joined into a great landmass that allowed early animals to wander freely back and forth across the landmass. Ninety million years ago, Antarctica separated from South America, isolating South America (which had lost its connection with Laurasia 160 million years ago), and isolating the ancestors of the Australian marsupials and monotremes in what would become the present-day island continent of Australia and its large satellite island, New Guinea. South America, like Australasia (Australia and nearby islands), became a continent-sized refuge for early marsupial types, although these would be sharing the continent with placental mammals. By forty million years ago, marsupials had become extinct in North America, Africa, Asia, and Antarctica but flourished in Australasia and South America, where they continued to evolve and diversify.

BEHAVIOR AND REPRODUCTION

New World opossums are marsupials, mammals that give birth to tiny, only partly developed young that crawl into the mother's pouch, latch their jaws tightly onto a milk nipple, and finish their development. Most mammals are placental, meaning that they carry their young in the womb for longer periods before birthing them, and these are born in a more completely developed state. "Marsupial" comes from "marsupium," the Latin word for pouch or bag, and names that special feature of marsupials.

Not all species have females with complete, functional pouches. In species without pouches, newborn young just cling with their jaws onto the mother's nipples and grasp her fur, remaining so until weaning, or stopping breastfeeding, and clinging to the mother wherever she goes. Some of the non-pouched opossums have partial pouches that cover only the rows of nipples on either side, and run the length of the underbelly. Females may have from five to as many as twenty-five nipples. In the common large opossum species, a typical female has a functional, snug, fur-lined pouch and thirteen nipples inside, arranged in a circle, with one nipple in the center, although the number of nipples may vary among species and even among individual females within a species.

American opossums may have definite mating seasons in more temperate regions, or may breed anytime of the year in the tropics. Litter sizes generally run between four and nine young. As many as sixteen young, or a record fifty-two for the Virginia opossum, may be born in a single litter. In such large litters, some of the young are likely to die before weaning, depending on the number of nipples the mother has. The gestation period is short, about two weeks, followed by up to ten weeks of pouch life. When leaving the pouch, the young may still nurse and ride on their mother's back for another month before striking off on their own. Individuals reach reproductive age at four months to one year. Lifespans among Didelphidae species are short, only one to five years.

For shelter, some American opossum species build nests of twigs and leaves, or of grasses; others dig their own burrows or use burrows abandoned by other animals, abandoned birds' nests, or shelter in hollow logs and among rocks.

All but a few species are nocturnal (nighttime) foragers, and as far as anyone knows, all are solitary, breaking that rule only during mating times. Outside of the mating season, same-sex individuals of a species, upon meeting, ignore or threaten each other. During the breeding season, a male and female may stay together for several days. Some species are mainly arboreal (spending most of their time in trees), others forage on the ground, and some do both. The Patagonian opossum is an excellent swimmer in freshwater, where it hunts for fish, even though it is not as specialized as the water opossum.

NEW WORLD OPOSSUMS AND PEOPLE

As a whole, the Didelphidae are no threat or bother to humans. People hunt and eat some species and use their fur for clothing and parts of clothing. The gray short-tailed opossum frequents houses in South America, where it is welcome because it hunts and eats rodents and insects infesting the houses.

Brown four-eyed opossums, gray four-eyed opossums, woolly opossums, and common mouse opossums occasionally raid fruit and corn crops. The southern opossum, and the white-eared opossum sometimes kill poultry.

CONSERVATION STATUS

Out of all the Didelphidae species, the IUCN lists three as Critically Endangered (facing an extremely high risk of extinction), three as Endangered (facing a very high risk of extinction), fifteen as Vulnerable (facing a high risk of extinction), and eighteen as Near Threatened (close to becoming threatened with extinction).

VIRGINIA OPOSSUM (Didelphis virginiana): SPECIES ACCOUNTS

Physical characteristics: The Virginia opossum is one marsupial that a majority of Americans have surely seen, if only as roadkill. These opossums have low-slung, vaguely rat-shaped bodies that in adults can weigh up to 14 pounds (6.4 kilograms). Males are larger than females. Adult head and body length can reach 20 inches (50 centimeters), and the tail length can reach 18 inches (47 centimeters). The body fur is light to dark grayish, due to a coat of white fur with black tips under a longer coat of pale guard hairs. The head is white and elongated, and studded with long whiskers. In some individuals, the gray coat may extend in a stripe across the crown, tapering to an end between the eyes. The eyes are black and shiny. The long, strong tail is scaly, colored whitish or pinkish, and nearly hairless, much like a rat's, and is prehensile, able to grasp tree branches and carry nesting materials. The ears, nostrils, forepaws, and hind-paws are pinkish and only sparsely furred. Each paw has five digits, and the hallux (HAL-lux; big toe) is opposable, allowing the opos-sum to grasp branches.

Geographic range: The Virginia opossum is one of the few marsupials, in Australasia or the Americas, that is at home in temperate regions with cold winters. Its range extends as far north as Ontario, Canada, and as far south as Costa Rica in Central America. Virginia opossums are found in North America, from Central America and Mexico in the south, through the United States east of the Rocky Mountains and north into southwestern Ontario. Opossums are also found along the west coast of the United States.

Habitat: Virginia opossums prefer living in forest, farmland, and suburbia with possible denning sites and a water source close at hand, but this adaptable species can survive and thrive almost anywhere, including grassland and near-desert conditions. These opossums are nomadic, seldom staying in one foraging area for more than a year. Individuals may sleep during the day in whatever temporary shelters they find, or build nests, lined with leaves. Refuges include woodpiles, thickets, rock crevices, and in various human-made structures such as under porches and raised houses, and in barns, drainpipes, and sheds.

Diet: The Virginia opossum is truly omnivorous, eating almost anything that can be considered food. A partial list of dietary preferences includes rats, mice, moles, slugs, snails, shrews, worms, beetles, ants, grasshoppers, crickets, frogs, garbage, fruit, corn, berries, and carrion. An even more unusual source of food is poisonous snakes, to whose venoms the opossums are immune. This includes copperheads, water moccasins, and rattlesnakes.

Behavior and reproduction: Like most opossums, Virginia opossums live and forage, search for food, solitarily. They forage mostly at night, but sometimes during the day. If male individuals meet, they avoid each other or sound off with threat displays, with hissings, growlings, and screechings, often going on to one-on-one combat. Males fight one another ferociously during mating seasons. On the other hand, if a male and female meet during the breeding season, they will mate and then stay together for several days.

Mating seasons vary according to how far north individual opossums live. Virginia opossums begin mating in December in the southern states, in March in the northernmost states and Canada, and in January and February for areas between. In Canada and in the north and central states, females usually bear only one litter per year. Two or even three litters are common in the southern states and further south.

Young are born thirteen days after mating. Litters can range in numbers of up to twenty, with a record of fifty-two, but since the mother has only thirteen nipples, only a maximum of thirteen in a litter can survive. Newborns are scarcely bigger than rice grains. The young spend up to 100 days, or slightly over three months, in the pouch. By seventy-five to eighty-five days, the young are weaned and leave the pouch, but remain with the mother for another two or three months before leaving to live on their own. Until they leave, the mother carries the young on her back. Young males reach sexual maturity at eight months, females at six. The longest recorded lifespan in the wild for the Virginia opossum is three years, although captive individuals have lived as long as ten years.

When threatened by a predator, a Virginia opossum may react in any of several ways. Escape is always the optimal choice, and includes climbing trees and swimming. If escape proves impossible, the opossum may use its variation of the basic mammalian threat response, opening its jaws wide, baring its fifty-five teeth, and hissing at its foe. It may also discharge a foul-smelling, greenish fluid from anal glands. Or, the opossum may use its "drooling" display, building up its saliva content, drooling from its mouth and blowing froth and bubbles from its nostrils, in hopes of convincing a predator that the opossum is seriously diseased and therefore dangerous to eat.

The opossum's final defensive recourse is either fighting back or performing its most famous behavior, "playing possum." The animal collapses, the eyes glaze, the jaws open, the tongue lolls, the teeth are partly bared, and the stinky anal fluid release adds the final carrion touch. The deathlike state is a form of catatonia, in which the animal lies limp, does not react to touch or prodding, and cannot be roused by any method. The muscles become limp, basic functions slow. Predators of opossums, among them coyotes, dogs, bobcats, and birds of prey, will reject the seemingly dead opossum and leave it untouched. From one minute to six hours after the predator has left the scene, the opossum rouses itself and moves off.

Throughout its range in Canada and in parts of the United States that have long, cold winters, Virginia opossums feed to build up extra body fat in the fall in preparation for the lean winter months. The species doesn't hibernate, but in especially cold weather, individuals may stay quietly in their shelters for a few days. Otherwise, they're outside and hiking across the snow to forage.

Virginia opossums and people: Virginia opossums sometimes help themselves to human garbage, but cause far less mess and destruction than do raccoons. Virginia opossums, like most mammals, can carry and transmit rabies. Virginia opossums have been, and still are, hunted for food and for their pelts.

Their ability to eat almost anything organic puts Virginia opossums in the front ranks of living nature's cleaning crews. They eat pest insects like cockroaches, garden pests like snails and slugs, pest mammals like roof rats and mice, and they eat all varieties of carrion.

Conservation status: Virginia opossums have adapted to humans successfully, are in no danger of extinction, and have even extended their ranges in some areas. ∎

WATER OPOSSUM (Chironectes minimus): SPECIES ACCOUNTS

Physical characteristics: Unlike most of the New World opossums, the yapok, or water opossum, is specialized for an aquatic lifestyle. It is the only living aquatic marsupial species. In general terms, the yapok can be thought of as a sort of marsupial otter. The name "yapok" is derived from the Oyapock River in northern South America.

Adult head and body length runs 10.5 to 16 inches (27 to 40 centimeters); tail length, 12 to 17 inches (31 to 43 centimeters); and adult body weight, 1.3 to 1.7 pounds (0.6 to 0.8 kilograms). The animal is covered with short, dense, water-repellent fur, unique among the Didelphidae. The sides and upper body are black, with three pairs of prominent, grayish bands that run vertically from the light gray underbelly almost to the spine. The head is blunter and wider than is common among Didelphidae species. The upper part of the head, including the eye area, is black. A dark gay bar runs the length of the snout from the nostrils to the crown. The lower part of the head is grayish. A prominent white stripe runs from above each eye to the ear. The eyes are large and black. The prominent, nearly furless ears are oval in shape. Conspicuous tufts of long, white or gray whiskers are mounted on each side of the head near the nostrils and over the eyes.

The hindfeet are webbed and the yapok uses them as its main propulsion organs when swimming. The hallux (first toe), usually shorter than the other toes in mammals, is elongated in the yapok, making the foot shape symmetrical and thus able to push more efficiently against the water. The forefeet are not webbed, and have elongated, furless fingers with reduced claws, which are furnished with a well-developed tactile sense.

Among the yapok's many peculiarities is that both females and males carry well-developed pouches that open toward the rear. The female uses a muscle to close her pouch when carrying young, which can survive without oxygen for several-minute intervals. The male uses his pouch to hold and protect the scrotum, drawing it up and into the pouch when he swims.

Geographic range: Yapoks are found in Central and South America, from southern Mexico and Belize through all of Central America, and into Colombia, Venezuela, the Guianas, Ecuador, Peru, Paraguay, Brazil, and northern Argentina.

Habitat: Yapoks live along streams, rivers, and lakes in tropical and subtropical rainforests of Central and South America, from sea level to 6,000 feet (1,830 meters) or more above sea level.

Diet: The yapok eats crayfish, shrimp, fish, and some water plants.

Behavior and reproduction: Females are polyestrous, meaning that they come into heat and become receptive to mating more than once a year. A breeding pair stays together for several days, the male following and circling the female until actual mating. A typical litter contains one to five young.

Yapok young have the fastest rate of development among all the Didelphidae species. After about forty days in the pouch, the young have grown body fur, pigmentation and the various markings, and opened their eyes. At about fifty days, the young begin to let go of the nipples and leave the pouch, but continue to suckle and stay with the mother, sometimes riding on her back.

Individual water opossums are solitary and hostile toward others of their species, except during mating times. An individual hunts and forages in freshwater streams, between rest periods, throughout the night. During the day, the animal rests in a temporary ground nest that it constructs from leaves and grass in a shady area. Close by is a more permanent underground burrow, which the yapok excavates in the stream bank, with its entrance a few inches above the water line. The entrance tunnel is about 2 feet (0.6 meters) long, and leads to a den lined with leaves or grasses. Individuals use their prehensile tails to carry nesting materials.

A yapok fishes and forages underwater, propelling itself with alternate strokes of its powerful hind legs and webbed feet. The animal shuts its eyes and ears and depends partly on its whiskers to detect motion, while its fingers, acutely sensitive to touch, are used to contact, check the texture of, and grasp prey.

The longest known lifespan for a captive yapok is three years.

Water opossums and people: Water opossums, confined to forests and riversides by their specialized lifestyles, are no threat or bother to humanity. Humans hunt them for their waterproof pelts, to be made into garments and accessories.

Conservation status: The yapok is listed as Near Threatened, not currently threatened, by the IUCN. ∎

FOR MORE INFORMATION

Books:

Nowak, Ronald. M. Walker's Mammals of the World. Baltimore: Johns Hopkins University Press, 1995.

Szalay, Frederick. Evolutionary History of the Marsupials and an Analysis of Osteological Characters. Oxford, U.K.: Cambridge University Press. 1994.

Periodicals:

Cifelli, R. L., and Brian M. Davis. "Marsupial Origins (Paleontology)." Science 302, no. 5652 (December 12, 2003): 1934.

de Muizon, C., and R. L. Cifelli. "A New Basal Didelphoid (Marsupialia, Mammalia) from the Early Paleocene of Tiupampa (Bolivia)." Journal of Vertebrate Paleontology 21, no. 1 (2001): 8–97.

de Muizon, C., R. L. Cifelli, and R. C. Paz. "The Origin of the Dog-like Borhyaenoid Marsupials of South America." Nature 389, no. 6650 (Oct 2, 1997): 486–489.

Goin, F. J., et al. "New Discoveries of 'Opossum-like' Marsupials from Antarctica (Seymour Island, Medial Eocene)." Journal of Mammalian Evolution 6, no. 4 (1999): 335–365.

Hamrick, M. W. "Morphological Diversity in Digital Skin Microstructure of Didelphid Marsupials." Journal of Anatomy 198 (2001): 683–688.