Woodpeckers have developed the practical habit of clinging to vertical surfaces and bracing themselves by means of their tail feathers, which in turn have become stiffened and thereby definitely modified for the purpose.
In the birds, the tail is formed internally by a short bony support, the pygostyle, and by the surrounding muscles, and externally by dermal elements, particularly feathers. It is in the feathers that the most obvious and interesting avian variation exists.
Perhaps in no group of animals have the tails become more specialized for purposes of ornamentation or display than in the birds, particularly in the males. In this regard, it is only necessary to mention such striking examples as the lyre bird, the bird of paradise, and the peacock. On the other hand, woodpeckers and chimney-swifts have developed the more practical habit of clinging to vertical surfaces and bracing themselves by means of their tail feathers, which in turn have become stiffened and thereby definitely modified for the purpose.
The wide variations shown by the tails of mammals are thoroughly interesting. For instance monkeys of the New World, belonging to the family Cebidae, nearly all have prehensile or grasping tails, but monkeys of the Old World, curiously enough, have only ordinary, non-prehensile ones, which may, perhaps, be considered decorative, but which perform no useful arboreal or gymnastic duties.
Then, there is the very short tail of the "cottontail" rabbit, which is covered by thick, soft, white, downy fur, as the common flame implies, and is consequently incapable of any monkey-like uses, but for all that it is still useful enough. Since the back and sides are brownish and much darker in color, this fluffy little ball is very conspicuous when the rabbit is running. Therefore, an enemy in pursuit of "bunny," be it man or beast, usually finds his attention fixed upon the "cottontail" rather than upon the prospective victim as a whole. This frequently results in the rabbit’s escape, for when it suddenly ceases its zigzag run and as suddenly squats to hide the tail from view, the pursuer frequently finds himself completely baffled, for with the disappearance of the "white spot" the whole animal seems to disappear.
The flying squirrel has a large, flattened, hairy tail, which serves as a balancer in his soaring jumps from tree to tree, but the opossum has a long hairless, prehensile tail that could not possibly aid its owner in the same manner.
In contrast to the rabbit’s type, the tail of the jumping mouse is sparsely haired and of an inconspicuous color. Moreover, instead of being short, and rounded, it is very elongate, measuring about twice the length of the body.
The flying squirrel has a large, flattened, hairy tail, which serves as a balancer in his soaring jumps from branch to branch and tree to tree, but the opossum has a long, hairless, prehensile tail that could not possibly aid its owner in the same manner. Instead, the opossum often grasps objects by winding its tail about them. This trait appears early in life, for each baby opossum thus clings tightly to "mamma" opossum’s tail as she swings it over her back and goes for a quiet evening stroll; while later, in the adult, the entire weight of the body may be easily supported from some convenient limb by this remarkable appendage alone.