Mammals are vertebrate animals characterized by hair, mammary glands, and the ability to internally regulate body temperature (endothermy). Among the thousands of mammal species, true, powered flight is an incredibly rare evolutionary development. This locomotion requires sustained, upward movement through the air, driven by muscular effort to generate both lift and thrust. Only one order of mammals has achieved this remarkable feat.
The Only Mammals Capable of Powered Flight
The sole group of mammals capable of powered flight belongs to the Order Chiroptera, commonly known as bats. Comprising approximately 20% of all known mammal species worldwide, bats are the second-largest order of mammals, surpassed only by rodents. With over 1,400 species, their ability to fly has allowed them to colonize nearly every terrestrial environment on Earth, except for the polar regions.
The order is traditionally divided into two main groups, reflecting their vast ecological differences. The Megachiroptera, or megabats (often called flying foxes), typically rely on vision and smell for navigation and feed primarily on fruit or nectar. Conversely, the Microchiroptera, or microbats, are generally smaller and utilize sophisticated echolocation to navigate and hunt insects.
Anatomical Adaptations for Sustained Flight
True flight in bats is made possible by modifications to the mammalian forelimb skeleton and musculature. The defining feature is the wing, which consists of the patagium, a double layer of highly elastic skin. This thin membrane stretches across the bat’s body, connecting the neck, the greatly elongated arm and finger bones, and the hind limbs.
The skeletal structure is radically reshaped to support this membrane and function as a wing. Unlike other mammals, a bat’s metacarpals and phalanges are extremely elongated, forming the primary struts that support and control the outer edges of the wing. This arrangement allows the bat to dynamically change the wing’s curvature and shape with each beat, providing exceptional maneuverability. The powerful downstroke necessary for lift and thrust is driven by significantly enlarged pectoral muscles.
To accommodate these large flight muscles, a bat has a pronounced sternal keel on its breastbone, similar to that found in flying birds. The muscles responsible for the powerful downstroke are many times stronger than those of a non-flying mammal. The bat’s physiological systems are also adapted for the high energy cost of flight, including a high metabolic rate and an enlarged heart and lung capacity to ensure a constant oxygen supply.
Gliding Mammals: The Difference Between True Flight and Passive Movement
The ability to glide is fundamentally different from true powered flight. Gliding mammals, such as flying squirrels, sugar gliders, and colugos (often called flying lemurs), use a similar, though less complex, membrane of skin called a patagium. This membrane stretches between their limbs and body, forming a parachute-like surface.
These animals launch themselves from a height and use the membrane to slow their rate of descent and direct their trajectory. This movement relies on gravity to provide the initial momentum and altitude, and is referred to as gravitational gliding. Gliding mammals are unable to generate the muscular power needed to actively flap their wings, create upward thrust, or gain altitude once they have begun their descent.
Their movement is a controlled fall, allowing them to travel great horizontal distances between trees, but they cannot sustain horizontal travel or gain altitude. The skeletal structure of these gliders, while adapted to support the membrane, lacks the extreme elongation of the finger bones and the specialized musculature found in bats. The anatomical requirements for active, sustained flapping are present only in the Order Chiroptera.