The North American Opossum, Didelphis virginiana, is the only marsupial found north of Mexico. Its evolutionary history is distinct from placental mammals, which greatly influences its reproductive strategy. The opossum’s breeding cycle involves two dramatically different phases: a remarkably short internal development followed by a prolonged period of external growth. Understanding this process requires shifting the focus away from the typical definition of pregnancy.
The Brief Gestation Period
The period of internal development, known as gestation, is one of the shortest among all mammals. For the opossum, this phase lasts only 12 to 14 days from conception to birth. This rapid timeline is a direct consequence of the animal’s marsupial physiology. The female possesses a transient, non-invasive placenta that cannot sustain the embryo for a prolonged period. Consequently, the young must be born while still in an embryonic state, making the opossum’s gestation the briefest recorded for any mammal on the continent.
The Journey From Womb to Pouch
Newborn opossums are born in an extremely undeveloped, altricial state. They resemble tiny pink embryos, about the size of a grain of rice, and weigh approximately 0.13 grams. They are blind, hairless, and have only rudimentary hind limbs, but they possess disproportionately strong forelimbs equipped with claws.
Immediately after birth, these young instinctively begin an arduous climb known as the “birth crawl.” They must travel from the birth canal, across the mother’s abdomen, and into the marsupium, or pouch. The mother may assist by licking the fur to create a clearer path, but the crawl itself is an independent, muscular feat driven by the precocious forelimbs.
This journey results in a high mortality rate for the litter. Success is determined by reaching the pouch and securing a grip on one of the mother’s nipples, which are usually limited to thirteen. Any young that fail to attach to an available teat will not survive.
Development and Growth in the Marsupium
Once a young opossum successfully attaches to a nipple inside the pouch, the true period of growth and development begins. The nipple swells inside the infant’s mouth, forming a firm connection that keeps the neonate secured for 50 to 70 days. This long attachment period allows for the development that would otherwise occur internally in placental mammals.
During the first two months in the marsupium, the young, often called joeys, undergo rapid transformation. They increase their weight ten-fold within the first week alone. Their eyes open between 60 and 70 days of age, and they begin to develop their characteristic coarse, grayish fur.
The young start to temporarily detach from the nipple and venture out of the pouch around 75 to 85 days old. They often ride on the mother’s back, clinging tightly to her fur, as they gain strength and learn survival skills.
They begin to sample solid food around 85 days old and are fully weaned and independent when they are 100 to 120 days old. This external phase of development is a defining characteristic of the opossum’s reproductive investment.
Reproductive Seasonality and Frequency
The opossum’s reproductive cycle is extended, allowing for multiple litters within a single year, particularly in warmer climates. The breeding season can begin as early as January and may last through October. The exact timeframe is dependent on geographic latitude; northern females may produce only one litter per year, while southern regions commonly have two or three litters.
Though a female may give birth to a large number of offspring, sometimes exceeding 20, the average number that successfully attach and survive in the pouch is eight or nine. The female’s estrous cycle is 28 days, and she is receptive for a short window of about 36 hours. The ability to quickly resume the cycle and produce multiple litters is a factor in the opossum’s population dynamics.