While birds and insects are commonly associated with flight, the idea of aerial locomotion in mammals seems less common. However, the biological world holds surprises: one group of mammals has evolved genuine, sustained flight, while others have developed an impressive form of aerial movement.
The True Flyers: Bats
Bats are the only mammals capable of true flight, a feat they achieve through unique anatomical specializations. Their wings are highly modified forelimbs, where the bones of the fingers (digits two through five) are greatly elongated and support a thin, elastic membrane called the patagium. This membrane, composed of skin, connective tissue, and blood vessels, stretches between their elongated fingers, body, and hind limbs, providing a flexible airfoil.
Bats manipulate their wings with numerous joints and strong, specialized muscles, including large pectoral muscles attached to a keeled sternum, allowing for powerful downstrokes and precise control over their flight path. This intricate wing structure enables bats to perform complex aerial maneuvers, generating both lift and thrust through active flapping. Many bat species also utilize echolocation, emitting high-frequency sounds and interpreting the returning echoes to navigate and hunt in darkness, further enhancing their aerial capabilities.
The Gliding Mammals
While bats exhibit true flight, other mammals have evolved a different method of aerial locomotion known as gliding. These gliding mammals, including flying squirrels, sugar gliders, and colugos, possess specialized skin membranes that allow them to move through the air without powered flapping. The most prominent adaptation for gliding is the patagium, a furry membrane of skin that extends between their limbs, sometimes even to the neck or tail.
For instance, flying squirrels and sugar gliders have a patagium stretching from their wrists to their ankles, while colugos have an extensive patagium nearly encompassing their entire body. These mammals launch from elevated positions, spreading their patagium to create an aerodynamic surface. They steer and control their descent by adjusting the membrane’s tension and shape, allowing them to travel considerable horizontal distances.
Distinguishing True Flight from Gliding
The fundamental difference between true flight and gliding lies in how lift and thrust are generated. True flight, as seen in bats, involves the active generation of both lift (the upward force opposing gravity) and thrust (the forward force opposing drag) through the continuous, muscular flapping of wings. This active propulsion allows true flyers to take off, gain altitude, hover, and maintain sustained flight, requiring significant energy expenditure.
In contrast, gliding is a passive form of aerial locomotion that relies on gravity and aerodynamic forces acting on a fixed membrane. Gliding mammals launch from a higher point and fall through the air in a controlled manner, using their patagium to increase air resistance and generate enough lift to slow their descent and achieve horizontal distance. They cannot gain altitude or generate their own forward momentum once airborne; their movement is a controlled descent. This distinction highlights the unique evolutionary pathway taken by bats to conquer the skies through active, dynamic flight.