Arboreal animals are animals that spend most or all of their time in trees. Many of them exist, and some are quite famous -- the koala, lemur, flying squirrel, New World porcupine, tree sloth, spider monkey, tarsier, leopard, orangutan, chameleon, gecko, fruit bat, and many tree frogs, snakes, birds, and lizards. Arboreal animals live in all the forests of the world, but are the most common in tropical forests, where the lush foliage and the canopy level creates a veritable "floor" of trees and leaves. In the nooks and crannies of trees, water collects in small pools, providing a source of moisture for a whole mini-ecosystem.

To climb in trees consistently and without falling, arboreal animals display a wide variety of adaptations, many of them shared between them. These include lithe bodies, clawed or sticky feet, and prehensile (independently movable) tails. Some arboreal animals, like tree sloths, have huge claws that let them hang from trees without expending any energy whatsoever. Some tree sloths cling so tenaciously to trees that they continue hanging for days after death.

The primary biodiversity hotspots for arboreal animals are the world's four largest rainforests -- the Amazon, Congo, Madagascar, and Southeast Asia. The reason for the evolution of the arboreal lifestyle is obvious -- trees are rich in animals and fruits, and allow their occupants to avoid predators on the ground. In fact, some animals, such as sloths, are so fearful of the ground that if their offspring accidentally falls, they will avoid going down to recover them. In rainforests, thick tree branches often rise 100 ft (30 m) or more above the ground, providing ample room to live and eat. Some animals spend their entire lives jumping from tree to tree, never touching the ground.

One of the most interesting adaptations displayed by arboreal animals are stretchy membranes between their legs or toes that allow for extensive gliding. While flight has only evolved independently four times in the history of nature (insects, pterosaurs, birds, and bats), gliding has evolved dozens of times. Some gliding arboreal animals include the flying squirrel (found across Eurasia and North America, American species rarely seen due to their nocturnal lifestyle), flying frogs (a trait which has evolved independently in more than 3,400 species), and Draco lizards, which can glide for up to 100 m (328 ft) under optimal conditions.