The teeth are rooted and sharp. The canines are large. The last premolar is preceded by a multicuspidate, molariform, deciduous tooth, and the molars are tricuspidate. The facial part of the skull is long and pointed, but the cranial part is small. Didelphids are active mainly in the evening and at night.
Opossums are insectivorous, carnivorous, or, more commonly, omnivorous.
As in other marsupials, the gestation period is short and the developmental period is long. The gestation period of Didelphis , about 12.5-13 days, is among the shortest found in mammals. The hind legs of opossums at birth are much smaller than the forelegs and practically useless. In young opossums, the passage from the nasal chamber to the larynx is so separated from the passage to the esophagus that the baby can swallow and breathe at the same time. The pouch young of opossums and probably other pouch marsupials breathe and rebreathe air that contains 8-20 times the normal content of carbon dioxide.