Which group contains the only mammals capable of true, sustained, powered flight?

Answer

Flying squirrels, sugar gliders and colugos only glide. Bats flap their wings to fly.

Context

The bat wing is a modified hand. The arm bones and four long, slender fingers form a frame, and a thin double layer of skin stretches across it, threaded with blood vessels, nerves and tendons. The thumb stays free and clawed for clinging, and a spur called the calcar helps support the membrane between the legs and tail. The scientific group name, Chiroptera, means hand-wing.

Many movable joints let a bat change the wing's shape during every beat, which explains its agility when it chases insects in the dark. The flight muscles attach to the shoulder blades, and the skeleton is light, with flattened ribs and fused vertebral joints. Flapping produces both lift and thrust, which is what separates powered flight from gliding.

Gliding mammals take a different route. Flying squirrels, sugar gliders and colugos (often called flying lemurs, although they are not lemurs) stretch a skin fold between their limbs and descend along a controlled path. They can steer, but they cannot climb by flapping, so they must launch from a height.

Bats are also not rare oddities. Smithsonian fact sheets count more than 900 species, a total exceeded among mammals only by rodents. Two persistent myths are wrong: bats are not blind, since all species can see, and they are not rodents or closely related to them.