The North American Opossum, Didelphis virginiana, presents a unique challenge for determining its sex because external differences are not easily visible from a distance. Unlike many mammals with obvious external genitalia, the opossum’s anatomy is subtle, making casual observation unreliable. This difficulty is compounded by their nocturnal and solitary nature, which limits opportunities for close inspection. Successfully distinguishing between sexes requires a combination of comparative observation and close examination of specific anatomical features.
The Defining Feature of Females
The most definitive marker of a female opossum is the marsupium, or abdominal pouch, a characteristic feature of all marsupials. This pouch is located on the lower abdomen and serves as an incubator where the underdeveloped young complete their initial growth phase. Newborn opossums, which are tiny and embryonic at birth, crawl into the pouch and attach firmly to a nipple inside.
The pouch provides a safe, warm environment for the young to nurse and develop for several weeks. When a female is actively nursing a litter, the pouch is noticeably enlarged and often contains visible movement. However, when the female is not breeding, the pouch is much less pronounced, appearing as a slight fold or shallow depression.
Observable Differences in Physical Size
While the pouch is a direct indicator, a more general sign of sex can be found in the animal’s overall physical size, a phenomenon known as sexual dimorphism. Adult male opossums are typically larger and heavier than adult females of the same age and geographic location. Males often weigh between 2.1 and 2.8 kilograms, whereas females generally range from 1.9 to 2.1 kilograms.
Differences are also apparent in the head and neck structure. Males tend to have broader heads and thicker necks compared to the more slender profile of females. This size difference is comparative, meaning it is only useful when comparing two or more individuals side-by-side, and it is not as reliable as a direct anatomical inspection.
Identifying Anatomical Features
For a definitive determination, close examination of the ventral area is necessary, which is typically only feasible for professional handlers or wildlife rehabilitators. Female opossums possess a specific anatomical arrangement of nipples near their vent. They typically have 13 nipples, which are arranged in a distinctive oval or horseshoe pattern of 12 surrounding a central single nipple.
The female’s vulva, or genito-urinary opening, is also present near the vent. In the male, the most distinguishing feature is the scrotum, which holds the testes and is located on the lower abdomen, positioned cranially (in front of) the cloaca. The scrotum is often covered in white or pale fur, but its presence is a clear sign of a male.