Didelphis marsupialis

The Common Opossum is Costa Rica's largest and most frequently encountered marsupial. It is a mammal that has adapted to life in disturbed habitats and coexists well among humans. They are similar to the Virginia Opossum (Didelphis virginiana), which ranges into North America.

Like other opossums, these animals are strictly nocturnal. They are omnivores and are known to feed on a great variety of items. This includes the occasional dumpster dive when they find themselves loitering residential areas.

Common Opossums are basically solitary animals and only interact with members of the opposite sex during their breeding season.

After mating, females are only pregnant for about two weeks. They can give birth to as many as 20 tiny "larvae", each one measuring about 1 centimeter. For these little creatures, natural selection begins at the moment of birth.

The larvae must find their way into the mother's pouch and attach themselves to one of her mammary glands. The mother will lick a path for them to follow on her stomach, leading her young to the pouch. Scientists believe that the tiny opossums find their way mostly by using their sense of smell.

Once they arrive inside the pouch, they must urgently secure one of their mother's mammary glands. Because female Common Opossums only have about nine mammary glands, only half her offspring has a chance of living. Once they attach themselves to the nipple, the tip of it swells in the larvae's mouth. This prevents them from falling off.

The young opossums will develop in their mothers pouch for about 60 days before they are ready to leave it. Once they do, they will usually remain in a den during the day as well as when their mother is out foraging. The young are fully weaned at 100 days of age.

It is estimated that Common Opossums only live about 2 years in the wild. In captivity they may live twice that long. Their range spans from eastern Mexico to northeastern Argentina.