New World Opossums: Didelphimorphia

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.