The squirrel family (Sciuridae) is a diverse group of rodents that includes tree, ground, and flying squirrels. A common question is whether males and females differ in size, a trait known as sexual dimorphism. The answer is not straightforward, as size comparison depends heavily on the specific species and its ecological niche.
Understanding Sexual Dimorphism in Squirrels
Sexual dimorphism describes any consistent difference in size, color, or shape between the male and female of a species. In most common squirrel species, this difference in body size is not prominent, meaning the sexes are considered largely monomorphic.
For example, the Eastern Gray Squirrel (Sciurus carolinensis) and the Eurasian Red Squirrel (Sciurus vulgaris) show no significant difference in body size or weight. When size differences do exist in the Sciuridae family, they follow two distinct patterns. The size relationship is highly dependent on whether the species primarily lives in trees or on the ground.
How Size Varies Across Different Species
The size relationship is best understood by looking at the two major ecological groups of squirrels. Among tree squirrels, males and females are generally the same size, or females may be slightly larger than males. This minimal difference is referred to as female-biased dimorphism and is observed in various arboreal species, including flying squirrels.
In contrast, ground squirrels, which include species like marmots and chipmunks, show a trend toward male-biased dimorphism. The males of these species tend to be noticeably larger than the females, a pattern evident in the Arctic Ground Squirrel. For ground-dwelling species, males can be up to 14% larger than females in specific structural measurements.
Evolutionary Reasons for Size Differences
The divergent size patterns between tree and ground squirrels are driven by distinct selection pressures. In species where males are larger, such as the ground squirrels, the primary driver is intense sexual selection. Larger body size offers a physical advantage during intrasexual competition, where males engage in combat to secure access to receptive females.
The physical strength afforded by a larger body allows males to dominate rivals, leading to greater mating success and ultimately perpetuating the trait. This advantage is amplified in highly social species that live in colonies, where competition for mates is concentrated.
The general monomorphism or slight female-biased dimorphism seen in many tree squirrels is instead linked to reproductive investment. Females may be equal to or slightly larger due to the demands of gestation and successfully raising a litter, a concept known as fecundity selection.
The pressure is on the female to allocate resources effectively to produce and nourish young, which can favor a slightly larger female body mass. Furthermore, having similar body sizes in both sexes of tree squirrels may also provide equal benefits for shared activities like territory defense.