Squirrels, such as the common gray or fox species, are often described as monomorphic, meaning males and females appear physically similar in size and coloration. Distinguishing the sex of a squirrel from a distance is challenging because general appearance offers few reliable clues. Determining the sex requires close observation of specific anatomical and behavioral markers. These markers often change depending on the animal’s reproductive status and the time of year.
The Definitive Physical Markers
The most consistent method for determining a squirrel’s sex involves observing the anogenital region beneath the tail. This area features two distinct openings: the anus, located closest to the tail base, and the genital opening. The separation distance between these two points, known as the anogenital distance (AGD), is the permanent marker used for sex identification.
In female squirrels, the distance between the anus and the vulva is very short, with the two openings appearing almost merged or close together. The vulva presents as a small, vertical slit located immediately below the anus. This close proximity remains constant regardless of the female’s reproductive cycle.
Male squirrels possess a noticeably longer anogenital distance, separating the anus from the penile opening by approximately one centimeter in an adult. This greater space is a reliable indicator of the male sex. When viewed from underneath, the male’s two openings are clearly distinct and separated by a patch of fur, unlike the female’s nearly adjacent openings.
Seasonal Variation in Appearance
While the anogenital distance is a permanent feature, secondary sexual characteristics offer temporary visual cues during certain times of the year. For males, the visibility of the scrotum is highly dependent on the breeding season. Outside of breeding periods, the testes are often retracted into the body cavity, making the scrotum inconspicuous or invisible.
During the peak breeding season, the scrotum descends and enlarges, becoming prominent and sometimes dark in coloration, offering an unmistakable marker of a male. For females, the clearest seasonal sign is the appearance of the nipples, or mammary glands. Female squirrels typically have multiple pairs of nipples, but these are generally inconspicuous when the animal is not reproducing.
The nipples only become swollen and visible when the female is actively nursing a litter of young. If a squirrel is observed with a prominent row of teats along her belly, she is a female and is currently lactating. Size or general body mass are not dependable indicators of sex, as these fluctuate based on the individual’s age and local food availability.
Behavioral Cues and Context
Observing specific behaviors, particularly during the breeding season, provides strong contextual evidence for a squirrel’s sex when direct physical examination is impossible. The most recognizable sex-specific behavior is the “mating chase,” which occurs when a female is in estrus and ready to mate. This event involves one female being pursued by a group of one or more males.
The chase can be intense and lengthy, with the males following the female across trees and the ground for hours. The squirrel being pursued is the female, while the squirrels doing the chasing are the competing males. This pursuit is driven by the female’s pheromones, which signal her receptivity.
Another behavioral indicator relates to parental care, as male squirrels do not participate in raising the young. Any adult squirrel observed carrying nesting material into a drey or tree cavity during a nesting period is most likely a female preparing to give birth. A squirrel seen actively nursing, defending, or moving a litter of young is the mother. Females may continue to nurse their kits for 10 to 12 weeks, providing a long window for this maternal behavior.