While birds and insects commonly navigate the skies, the ability to fly among mammals is exceedingly rare. This unique biological achievement is rare among mammals. Exploring which mammals have taken to the air reveals insights into evolutionary pathways and how life adapts to its environment.
The Mammal of True Flight
Among all mammals, bats are the sole group capable of true, sustained powered flight. Their classification, Chiroptera, literally means “hand-wing,” a fitting description of their unique anatomy. A bat’s wing is a highly modified forelimb, featuring dramatically elongated finger bones that support a thin, elastic membrane of skin, known as the patagium. This membrane stretches between the elongated digits, extending to the hind limbs and sometimes enclosing the tail.
This specialized wing structure allows bats to generate both lift and thrust through active flapping, distinguishing their movement from gliding. The flexibility of their wing bones and the multiple joints within the wing enable bats to manipulate their wing shape with precision, achieving great agility and maneuverability in the air. Bats are diverse, with over 1,400 known species, making them the second-largest order of mammals globally. They are found across six continents, occupying a wide range of ecological niches from insectivores to fruit-eaters.
Masters of Gliding
While bats command the skies with powered flight, several other mammalian groups have independently evolved the ability to glide. Gliding differs from true flight because it does not involve continuous thrust or active flapping to generate lift; instead, it is a controlled descent from a higher point. These mammalian gliders utilize a specialized skin membrane called a patagium, which acts like a parachute or a built-in wingsuit. When they leap from an elevated position, they extend their limbs, stretching this membrane to catch the air and slow their descent while moving horizontally.
Prominent examples include flying squirrels, sugar gliders, and colugos, often referred to as “flying lemurs.” Flying squirrels possess a furred patagium stretching from wrist to ankle, with flattened tails aiding stability and steering. Sugar gliders also have a patagium extending from their forelegs to their hindlegs. Colugos, considered the most efficient gliders among mammals, have an extensive patagium that stretches from their neck to their fingertips, toes, and even their tail. These gliding adaptations, though visually similar, arose through convergent evolution, meaning different species developed similar traits independently to solve the challenge of arboreal locomotion.
Why Flight is Rare Among Mammals
The evolution of powered flight is a complex and energetically demanding process, which explains its rarity in the mammalian class. True flight requires substantial physiological and anatomical modifications that are energetically costly. Animals capable of sustained flight must develop a lightweight yet strong skeletal structure, along with powerful muscles to generate the necessary lift and thrust. While bird bones are often hollow, bat bones are slender and have a lower density compared to non-flying mammals of similar size, reducing overall weight.
The high metabolic rate needed to fuel continuous muscle contractions during flight places significant demands on cardiovascular and respiratory systems. These systems must efficiently deliver oxygen to the muscles and remove waste products. In contrast, gliding, while providing aerial locomotion, is a less energy-intensive strategy as it relies on gravity and air resistance for movement, not sustained muscle power. The unique evolutionary pathway of bats, originating from an arboreal gliding ancestor, involved changes that transformed a hand into a wing. This combination of high energy requirements, specialized skeletal and muscular adaptations, and unique evolutionary history has limited true flight to only bats.