The North American opossum (Didelphis virginiana) is the only marsupial found north of Mexico. Recognized for its prehensile tail and defensive feigning of death, the female opossum’s life cycle is defined by a high reproductive output. This begins with a surprisingly large number of young born in a single litter.
The Initial Count: Litter Size and Variation
The number of young born in a single litter can range from four to 25 offspring, showing significant variability. While the potential maximum is high, the average number of young successfully carried in the pouch is typically eight or nine.
The mother’s health and geographic location influence the final count. Females in warmer southern regions often produce up to three litters per year, compared to a single annual litter in northern climates. Litter size can also vary throughout the breeding season, which runs from midwinter to late autumn, with the second litter often being smaller than the first.
The Marsupial Paradox: Why So Many Are Born
The opossum’s large litter size results from its unique marsupial reproductive strategy. The gestation period is incredibly brief, lasting only 12 to 14 days. This short internal development means the young are born in an extremely altricial, or underdeveloped, state.
Newborns are minuscule, often described as being about the size of a jellybean, and weighing only 0.13 to 0.20 grams. They are blind and hairless, lacking the full development seen in placental newborns. This underdeveloped state necessitates that the majority of growth occurs outside the mother’s body, specifically within her marsupium, or pouch.
The Journey to the Pouch and Survival Rate
Birth begins a competitive journey for the newborns. Despite their underdeveloped state, they possess muscular forelimbs and clawed front feet. The tiny opossums must crawl unaided from the birth canal, up the mother’s fur, and into the pouch.
The mother does not assist in this process, though she may lick the fur to create a path. This unassisted crawl results in a high mortality rate, as many young fail to complete the journey. The final limiting factor on the litter’s size is the number of nipples inside the pouch, which are typically 13.
Only the young that successfully reach the pouch and secure a nipple will survive. The nipple swells inside their mouth, anchoring them firmly in place for weeks. If more than 13 young survive the crawl, the excess will perish due to the lack of an attachment point. This bottleneck ensures the number of young raised aligns with the mother’s capacity to nurse them.
Growth, Weaning, and Independence
The surviving young remain attached to a nipple in the pouch for 50 to 70 days to complete their development. During this time, they grow rapidly. Their eyes typically open around 62 to 80 days after birth.
Around 70 to 80 days of age, the young begin to emerge from the pouch because they are too large to fit comfortably. They transition to riding on the mother’s back while she forages. The young also begin to eat solid food around 85 days old, supplementing the milk they still receive.
Weaning occurs between 93 and 105 days of age, or roughly three to four months after birth. At this point, the young are considered independent and disperse to find their own territory. The functional litter size that reaches independence is significantly smaller than the initial birth count, corresponding to the mother’s limited number of nipples.