Flying squirrels (tribe Pteromyini) are definitively classified as mammals. They are placed within the Class Mammalia and the Order Rodentia, making them close relatives of tree squirrels and chipmunks. Confusion often arises because their distinctive aerial ability suggests movement usually associated with birds or bats. This unique adaptation does not override the fundamental biological characteristics that define them as mammals.
Defining Features of Mammals
The classification of flying squirrels as mammals is based on biological requirements shared by all members of the Class Mammalia. A defining trait is endothermy, the ability to generate internal heat to maintain a near-constant body temperature regardless of the external environment. Their thick fur provides the insulation needed to regulate this temperature, which is important since many species are nocturnal and active in cooler nighttime air.
Flying squirrels possess true hair or fur, which covers their body and the gliding membrane itself. Like all mammals, they reproduce through internal fertilization and give birth to live young. The mother nourishes her offspring through specialized mammary glands that produce milk, a non-negotiable trait for classification as a mammal.
Flying squirrels share internal anatomy with other mammals, including a four-chambered heart that efficiently separates oxygenated and deoxygenated blood, supporting their high metabolic rate. Their teeth are specialized, reflecting their omnivorous diet of nuts, seeds, fungi, insects, and bird eggs. These physiological and anatomical markers confirm their place within the Mammalia class.
Gliding, Not Flying: The Patagium Explained
The unique aerial movement of flying squirrels is gliding, which is fundamentally different from the powered flight used by birds and bats. True flight involves continuous flapping to generate lift and forward thrust. Gliding is a controlled descent that trades height for horizontal distance, enabled by the patagium, a furry membrane of skin that extends laterally along the body.
The patagium stretches tautly from the wrist of the forelimb to the ankle of the hindlimb, creating a large aerodynamic surface when the squirrel spreads its limbs. Specialized cartilaginous rods, known as styliform elements, support the structure and project from the wrist to provide rigidity. By adjusting the tension and curvature of the patagium, the squirrel can alter the lift and drag forces acting upon its body.
Maneuverability is achieved through precise limb and body movements, allowing the squirrel to steer around obstacles and change direction mid-air. The flattened, bushy tail acts as an airfoil, functioning as a stabilizer and a powerful air brake before landing. When approaching the target tree, the squirrel pulls its body upward, causing the glide path to curve and slow its velocity, allowing it to land gently and vertically.
This gliding ability is an example of convergent evolution, where unrelated species develop similar features to adapt to environmental pressures. Like the Australian sugar glider and the colugo, the flying squirrel evolved the patagium to travel efficiently and avoid predators within a dense, arboreal habitat. Despite covering distances exceeding 150 feet, this specialized mechanism remains a passive form of movement, distinguishing the squirrel’s controlled descent from the active propulsion of true flight.