The Virginia opossum (Didelphis virginiana) is the only marsupial species native to North America. Unlike placental mammals, opossums rely on an external pouch for extended infant development. This specialized biology often prompts curiosity regarding the animal’s reproductive anatomy.
The Variable Number of Nipples
The number of nipples a female opossum possesses directly constrains her reproductive capacity. While the count can vary slightly, the Virginia opossum most frequently has 13 nipples. This odd number results from their unique arrangement: a single central nipple surrounded by a ring of 12 others.
This fixed count establishes the maximum number of offspring that can be successfully raised from a single litter. Although the count generally ranges between 11 and 13, the number of available nipples dictates the maximum number of young that can survive the initial development phase.
The Unique Marsupial Reproductive Strategy
All of the female opossum’s nipples are located inside the marsupium, the abdominal pouch characteristic of marsupials. Opossums have an extremely short gestation period of only 11 to 13 days, resulting in an embryonic birth. Newborns are born in a highly underdeveloped state, each one being no larger than a honeybee.
Immediately following birth, these tiny, blind infants must make an unassisted journey from the birth canal to the pouch. Once inside, the young locate and firmly attach to a nipple. The nipple then swells inside the infant’s mouth, securing the connection for approximately 50 to 70 days while the young continue development and nursing.
Why the Number Matters for Opossum Survival
The short gestation and high frequency of breeding allow the opossum to maintain a high reproductive rate, compensating for a high infant mortality rate. Although the average litter size is between six and nine young, a female can give birth to more than 20 infants at once. This large birth number frequently exceeds the 13 available nipples.
Survival is determined by immediate competition, where only the strongest and fastest newborns secure one of the limited nipples. Offspring born in excess of the available nipple count cannot feed and will not survive. This mechanism ensures the mother only invests energy in nursing the number of young she can physically support.