Yes: the bat is the only mammal capable of active flight, that is to say beating its wings to stay in the air and propel itself. “Flying” squirrels and other gliders just let themselves be carried from one tree to another.
A hand becomes a wing
The bat's wing is a transformed hand: four disproportionately elongated fingers, connected by a thin membrane of skin — the patagium — which extends to the rear legs. Where the bird flaps a stiff, feather-covered limb, the bat adjusts each finger separately, giving it greater agility in flight than most birds.
This organizational plan dates back more than fifty million years and has made bats one of the most successful groups of mammals: around 1,400 species, or nearly one in five mammals.
Hovering is not flying
The gliding flight of flying squirrels or colugo is a slowed fall: the animal loses altitude from one point to another, without ever being able to rise again. Active flight requires profoundly altered muscles, skeletons, and metabolism—a milestone only one group of mammals has reached.
