The family Sciuridae encompasses a diverse group of small-to-medium-sized rodents, commonly known as squirrels. These animals are found across nearly every continent, with the exception of Antarctica and Australia, inhabiting environments from tropical rainforests to arctic tundras. This widespread distribution and adaptability have resulted in approximately 280 distinct species recognized globally. The variety of habitats and lifestyles within this family makes understanding the term “squirrel” more complex than simply picturing the bushy-tailed animal seen in a backyard.
Defining the Major Categories
The squirrel family is organized into three primary categories based on habitat and physical characteristics: Tree Squirrels, Ground Squirrels, and Gliding Squirrels. Differences in morphology, or physical form, relate directly to where each type spends the majority of its life. Tree Squirrels are adapted for an arboreal existence, while Ground Squirrels exhibit traits suited for terrestrial life and burrowing. Gliding Squirrels are set apart by a specific anatomical feature that enables their unique form of locomotion.
Tree-Dwelling Squirrels
Tree-dwelling squirrels spend most of their time in forest canopies and wooded areas. Their anatomy is suited for an arboreal existence, featuring long, muscular legs, sharp claws for gripping bark, and a bushy tail. The tail serves a crucial function in maintaining balance while leaping between branches, and is also used for communication and as a thermal blanket.
A common North American example is the Eastern Gray Squirrel (Sciurus carolinensis), which thrives in natural woodlands and urban parks. These squirrels are scatter-hoarders, meaning they bury individual nuts and seeds in many different locations for later retrieval, a behavior that aids in forest regeneration. The Fox Squirrel (Sciurus niger) is another widespread species, which is larger and often found in more open woodlands than its gray counterpart.
The American Red Squirrel (Tamiasciurus hudsonicus) is smaller and prefers conifer forests. Unlike the scatter-hoarding of larger species, the red squirrel creates large, centralized caches of pine cones and other food sources. Tree squirrels do not hibernate, but endure cold periods by relying on stored food and remaining sheltered in nests called dreys or tree cavities.
Ground-Dwelling Squirrels
Ground-dwelling squirrels are adapted for terrestrial life and are often more robust in build than tree squirrels. These species construct elaborate, multi-chambered burrows that serve as shelter, food storage, and refuge from predators. Their forelimbs are short and sturdy, evolved for digging, which is a defining characteristic of their lifestyle.
The tails of ground squirrels are generally less bushy and shorter than those of tree squirrels, reflecting a reduced need for balance. This category includes social Prairie Dogs (Cynomys), which live in complex colonies called “towns,” and large, solitary Marmots (Marmota), which inhabit mountainous regions. Chipmunks (Tamias) are also classified as ground squirrels, recognizable by their stripes and use of cheek pouches to transport food.
Many ground-dwelling species in colder climates, such as the Thirteen-lined Ground Squirrel (Ictidomys tridecemlineatus), enter a deep state of hibernation during the winter. This contrasts with tree squirrels, which remain active year-round. Ground squirrels are diurnal, meaning they are active during the day, and often stand upright on their hind legs to survey their surroundings before retreating into their burrows.
Gliding Squirrels
Gliding squirrels represent a distinct group, sometimes incorrectly referred to as “flying” squirrels, despite their inability to achieve true powered flight. Their unique method of movement is gliding, which is facilitated by a specialized membrane of furred skin known as the patagium. This flap extends laterally from the wrist of the forelimb to the ankle of the hindlimb, creating an airfoil when the limbs are outstretched.
The patagium allows these animals to launch themselves from a high vantage point and smoothly glide between trees, sometimes covering distances over 150 feet. They steer and control their trajectory mid-air by adjusting the patagium’s tension and using their flattened tail as a stabilizer and rudder.
The American species, such as the Southern Flying Squirrel (Glaucomys volans), are the only squirrels that are strictly nocturnal. Their nocturnal nature and preference for mature forests mean they are rarely seen, despite often sharing habitats with diurnal relatives. Their large, dark eyes are an adaptation for navigating and foraging in low-light conditions. The Siberian Flying Squirrel (Pteromys volans) demonstrates this unique gliding adaptation across Eurasia.