(
Monodelphis domestica) Didelphimorphs are small to medium-sized marsupials that grow to the size of a house cat. They tend to be semi-
arboreal omnivores, although there are many exceptions. Most members of this order have long
snouts, a narrow
braincase, and a prominent
sagittal crest. The
dental formula is: teeth. 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, some opossums have
prehensile tails. Like most marsupials, many females have a pouch. The tail and parts of the feet bear
scutes. The
stomach is simple, with a small
cecum. 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
water opossum or 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. The extinct
Thylophorops, the largest known opossum at , was a macropredator. 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.
Metachirus nudicaudatus, found in the upper
Amazon basin, consumes fruit seeds, small
vertebrate creatures like birds and reptiles and
invertebrates like
crayfish and
snails, but seems to be mainly
insectivorous.
Reproduction and life cycle As marsupials, female opossums have a reproductive system that includes a bifurcated
vagina and a divided
uterus; many have a
pouch. The average estrous cycle of the
Virginia opossum is about 28 days. Opossums do possess a
placenta, but it is short-lived, simple in structure, and, unlike that of placental mammals, not fully functional. The young are therefore born at a very early stage, although the
gestation period is similar to that of many other small marsupials, at only 12 to 14 days. They give birth to litters of up to 20 young. Once born, the offspring must find their way into the marsupium, if present, to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys. Female opossums often give birth to very large numbers of young, most of which fail to attach to a
teat, although as many as 13 young can attach, Opossums are moderately
sexually dimorphic with males usually being larger, 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.
Behavior , or "playing possum" 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 a few minutes to four hours, a process that begins with a slight twitching of the ears. Some species of opossums have
prehensile tails, although dangling by the tail is more common among juveniles. An opossum may also use 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. The mother in return makes a clicking sound and waits for the baby to find her. If threatened, the baby will open its mouth and quietly hiss until the threat is gone. == Diet ==