Many vertebrates have tails. Though seemingly trivial, the tail is a rudimentary structure with a long evolutionary history. Although in some cases (such as humans) lacking a tail is not a problem, the tail is irreplaceable to others.
The fifth hand
The spider monkey (Ateles sp.) may be the “tail expert” among all primates. This monkey has a black, hairy body and tail, except that the tip of the tail is hairless - which increases the friction between the tail and surfaces, allowing the monkey to hold tight to any object with its sensitive tail. In fact, the tail is said to be “the fifth hand” of the monkey - its tail can act as the pivotal point as it climbs the tree, or even support the whole body of the monkey and free its limbs for picking fruits as it swings among tree branches. The tail, which is longer than the limbs, is strong, and well suits the spider monkeys with their arboreal life in the rainforest.
The third leg
In Australia, the star animal - kangaroo - also has an extraordinarily powerful tail. Scientists previously thought that the main function of the tail is for balancing the body, especially when male kangaroos are fighting and kicking their rivals. Later research discovered that the tail is actually critical in the kangaroo's daily movement. It was found that in hopping, the tail reaches the ground slightly before the hind legs, and is involved in propelling the kangaroo forward besides balancing, performing the role of “a third leg” that allows the kangaroo to swiftly evade predators.
Bird's tail - formed by feathers - functions like the tail of an aircraft to control the direction of flight. To raptors that are active in forest, such as Crested Goshawk (Accipiter trivirgatus), the tail is all the more important as they need to frequently change speed and direction to avoid obstacles as they swiftly navigate, including to pursue prey. This is particularly so in situations when they cannot fully extend their wings, but must rely on the tail to manoeuvre in the narrow spaces between trees.
The tail fin of dolphins is the engine that provides the power and direction as they swim. Unlike fishes that sway their bodies sideways to propel themselves, dolphins move their muscular tail fins up and down when swimming. The tail fin determines how fast a dolpin swims, or how high it leaps above the sea surface. The fastest moving marine mammal is the Common Dolphin (Delphinus delphis), which can reach speeds of up to 60 kilometres per hour with its powerful tail fin.