Squirrels are a diverse group of small to medium-sized rodents that belong to the family Sciuridae, which contains over 270 species globally. The family is native to the Americas, Eurasia, and Africa, inhabiting nearly every type of terrestrial environment from tropical rainforests to arctic tundra. Squirrels play an important ecological role as seed dispersers and prey for various predators, influencing the composition of forests and other ecosystems. While they share characteristics like strong gnawing incisors, the members of the Sciuridae family exhibit differences in physical form and behavior, which allow for a functional classification into three primary groups based on their preferred habitat.
The Arboreal Squirrels
Arboreal squirrels, commonly known as tree squirrels, are the most recognized members of the family, spending the majority of their lives in the forest canopy. They possess specialized anatomical features, including long, muscular limbs and ankles that can rotate 180 degrees for swift descent. They also have large, sharp, curved claws that provide a strong grip for maneuvering on vertical surfaces and thin branches.
Their most defining feature is the long, bushy tail, which functions as a dynamic tool for balance, a parachute for controlling falls, and a rudder for steering during jumps. Arboreal squirrels, such as the Eastern Gray Squirrel (Sciurus carolinensis) and the Fox Squirrel (Sciurus niger), are typically active during the day, making them frequently observed. The Eastern Gray Squirrel is often smaller and more agile in the treetops, while the Fox Squirrel is larger, sometimes weighing over 750 grams, and spends more time foraging on the ground.
They construct nests called dreys, which are spherical structures made from twigs and leaves, often built high in the forks of trees or tucked inside hollow tree cavities. These squirrels are scatter-hoarders, burying nuts and seeds over a wide area for later retrieval, a behavior that contributes to forest regeneration. They rely heavily on tree nuts for their diet, using their powerful jaws to crack hard shells.
The Terrestrial and Burrowing Squirrels
Terrestrial and burrowing squirrels, often referred to as ground squirrels, include many species not immediately associated with the name “squirrel.” They are characterized by their life on or under the ground. They typically have shorter, sturdier forelimbs and robust claws optimized for digging extensive burrow systems, which serve as protection from predators and temperature extremes.
Their tails are generally shorter and less bushy than their arboreal relatives, though they remain fully furred. This group encompasses a wide range of body sizes, from the small, striped Chipmunks (Tamias) to the larger Marmots (Marmota), which can weigh up to eight kilograms. Prairie Dogs (Cynomys) also fall into this category, known for their highly social, colonial structures and complex underground towns.
Most ground squirrels are diurnal, but unlike tree squirrels, many species in temperate regions enter prolonged periods of torpor or true hibernation during winter. Their burrow systems can be elaborate, featuring multiple entrances and chambers for nesting and food storage, sometimes extending over nine meters deep. These species are omnivorous, consuming a diet of seeds, fungi, insects, and small vertebrates, often utilizing internal cheek pouches to transport forage back to their burrows.
The Gliding Squirrels
The gliding squirrels, or flying squirrels, consist of approximately 50 species defined by their specialized mode of locomotion. They do not fly like birds or bats, but rather glide between trees using a furred membrane of skin called the patagium. This membrane stretches from the wrist of the front limb to the ankle of the hind limb, creating an aerodynamic surface when the animal leaps and spreads its limbs.
Their long tail functions as a stabilizer and rudder, allowing them to steer and control their descent path over distances that can exceed 90 meters. A small, cartilaginous strut extending from the wrist helps to stiffen the leading edge of the patagium, giving them fine-tuned maneuverability in mid-air. Gliding squirrels are the only group within the Sciuridae family that is primarily nocturnal, which helps them avoid diurnal predators like raptors.
To navigate in low light, they possess large eyes that are highly adapted for nocturnal vision. These small animals typically nest in tree cavities, sometimes aggregating in groups during colder months to conserve heat and energy.