Gliding, according to the article “Gliding Mammals” from http://mapoflife.org, is the “descent through the air at an angle of less than 45 (degrees) to the horizontal.” Over time, squirrels, possums, and lemurs have evolved specialized body parts to facilitate this motion.
There are 14 genera and 43 species of flying squirrels, as they are called. They have developed a membrane of skin and cartilage called a patagium, which extends from the wrists of their forelimbs to the ankles of their hind limbs, and flat, wide tails. These features allow the nocturnal mammals to glide over some 656 feet from tree to tree. Flying squirrels inhabit North America, Europe, and Asia.
Gliding possums consist of phalangers with six species, feather-tailed possums with two species, and the greater glider with one species. Their muscular patagium opens and expands, and their flat, wide tails act as stabilizers during the glide. They can jump as much as 164 feet between trees. Phalangers are native to Borneo, Australia, and New Guinea. The feather-tailed possums, named for the rigid, feather-like hairs on the sides of their broad, long tails, include the smallest gliding mammal yet discovered. They are found in Australia and New Guinea. The greater gliders have a different type of patagium. According to the Map of Life article, it extends from their fifth finger to the knee of their hind limbs. These marsupials inhabit the Australian continent.
Gliding lemurs, colugos, and sifakas (also lemurs) live in the rainforest of Southeast Asia and Madagascar. The colugos have the largest patagium on their long, slender bodies. It stretches from their shoulder to the ends of their forelimbs and back to the ends of their toes and tail. They also have webbing between all of their digits, and extremely keen eyesight. The entire setup makes them rather proficient gliders. They jump about 229 feet between trees. The evolved features of the sifakas are more restrictive. Their patagium is found only under the forelimbs. These limbs also have thick hairs which may create drag.
For these modern-day canopy mammals, gliding between the trees provides much more protection from large predators than scurrying about on the ground.
“Gliding Mammals.” http://mapoflife.org.