Opossums are marsupials which are mammals that have a pouch in which their young nurse after birth. The opossum is the only marsupial in North America. The female usually gives birth to 18 to 25 babies, each smaller than a honey bee. The mother has only 13 nipples so if a baby doesn’t latch on to one it will not survive. Once the baby finds a nipple it will hang on tight and will not let go for six weeks. After six weeks, the baby will climb out of the pouch and cling on the back of the mother where it will stay for three to five months. They are seven to nine inches from nose to rump when they set out on their own. Mortality of young in the pouch is 10 to 25%, and of those surviving through weaning fewer than 10% live longer than a year. It is one of the shortest lived mammals for its size.
Opossums have 50 teeth, the highest number found in any land mammal. The fore feet act like fingers and their hind foot has a thumb like a big toe. The tail can be used like a fifth hand when the opossum is moving from tree to tree. A young opossum can hang by its tail. Their unspecialized biology, flexible diet, and reproductive strategy make them successful colonizers and survivors. Originally native to the eastern United States, the Virginia opossum was intentionally introduced into the west during the Great Depression, probably as a source of food. Fossils of opossums can be found dating back 70 million years ago.