No mammal besides the bat can truly fly. Bats are the only mammals capable of powered flight, meaning they generate thrust by flapping their wings. However, 64 species of mammals can glide through the air, sometimes covering impressive distances. These gliding mammals are often called “flying” squirrels, “flying” lemurs, or “flying” possums, but none of them actually fly. They launch themselves from high points and use stretched membranes of skin to slow their descent and travel between trees.
Why Bats Are Truly Alone
Powered flight requires a specific set of anatomical equipment: a flight stroke that produces thrust, strong shoulder muscles, a keeled breastbone, and elongated wing structures that can be flapped. Bats have all of this. Their wings are made of thin skin stretched between extremely long finger bones, and they contain specialized muscles that allow precise control during flapping.
Gliding mammals have a simpler setup. They use a patagium, a broad flap of skin stretched between their forelimbs and hindlimbs. When they leap from a tree and spread their limbs, this membrane catches air and slows their fall, letting them travel forward. But the primary force moving them is gravity, not muscle-powered thrust. They can steer by adjusting their body position and membrane shape, but they cannot gain altitude or sustain themselves in the air the way a bat can.
Colugos: The Best Gliders
Colugos, sometimes called “flying lemurs,” are neither lemurs nor flyers. They belong to their own order, Dermoptera, and only two species exist, both living in Southeast Asia and the Philippines. They are the most specialized gliding mammals alive, with a membrane that stretches from their neck to their fingertips, down to their toes, and even between their tail and hind legs. This gives them the largest gliding surface relative to body size of any mammal.
That enormous membrane pays off. Colugos can cover up to 150 meters in a single glide, though the average is closer to 30 meters. They hang upside down in trees and launch themselves to search for fruit, moving between trees without ever touching the ground. Interestingly, colugos may be the closest living relatives of bats, and bats likely evolved from gliding ancestors that resembled something like them.
Flying Squirrels: The Largest Family
Flying squirrels account for the vast majority of gliding mammals. There are 48 species spread across 15 genera, found throughout Asia, Europe, and North America. If you live in the eastern United States or Canada, there’s a good chance flying squirrels live in the woods near you, though you’d rarely see them since they’re nocturnal.
Their patagium connects their forelegs to their hind legs, and they have a special piece of cartilage at the wrist that extends the membrane even further, giving them extra surface area. Some species can glide about 46 meters between trees. They steer by adjusting the tension in their membrane and using their flat tail as a rudder. When they approach a landing spot, they pitch upward to increase drag and slow down, reducing the impact.
A separate group of rodents in Africa, called scaly-tailed flying squirrels, also glide. Despite the name, they aren’t closely related to true flying squirrels. These six species belong to the family Anomaluridae and evolved gliding independently.
Gliding Marsupials of Australia
Australia and its surrounding regions are home to eight species of gliding marsupials. The most familiar is the sugar glider, a small possum-sized animal that has become popular as an exotic pet. Sugar gliders have a membrane running from their forelimbs to their hindlimbs, though it’s less extensive than a colugo’s. Their membrane attaches at the wrist rather than the fingertips, which leaves their hands fully mobile for climbing and gripping branches.
Sugar gliders play a meaningful role in forest ecosystems. They feed heavily on nectar, making them active pollinators, and they also eat insects that damage trees. In the wild, they glide at angles of roughly 30 degrees, using gravity and air resistance to travel between feeding sites. The greater glider, a larger marsupial relative, and the tiny feathertail glider round out the Australian group.
Why Gliding Evolved So Many Times
Gliding has evolved independently in at least six separate lineages of living mammals: colugos, two groups of rodents (true flying squirrels and scaly-tailed squirrels), and three families of marsupials. This makes it one of the most striking examples of convergent evolution in the animal kingdom. Each group independently developed skin membranes, lighter skeletons, and the ability to control their descent, all without sharing a common gliding ancestor.
The adaptation goes back far longer than any of these living groups. Fossils of a creature called Volaticotherium, discovered in China, show that mammals were gliding as far back as the Jurassic Period, roughly 160 million years ago. This animal was about the size of a modern flying squirrel, around 5 to 6 inches long, and had a patagium stretched between its limbs. It belonged to an entirely extinct group of early mammals called eutriconodonts that lived alongside dinosaurs for over 100 million years.
Despite gliding evolving repeatedly, none of these lineages have shown any signs of developing powered flight. The leap from passive gliding to active flapping requires dramatic changes to the skeleton, musculature, and metabolism. Bats made that transition once in evolutionary history, and no other mammal has come close.