A bat is a mammal in the order Chiroptera. The forelimbs of all bats are developed as wings, making them the only mammals naturally capable of flight (though other mammals, such as flying squirrels, flying possums and colugos, can glide for limited distances). The word Chiroptera comes from the Greek words cheir (χειρ) "hand" and pteron (πτερον) "wing," as the structure of the open wing is very similar to an outspread human hand with a membrane (patagium) between the fingers that also stretches between hand and body.

A measure of the success of bats is their estimated total of about 1,100 species worldwide, accounting for about 20 percent of all mammal species. About 70 percent of bats are insectivores. Most of the rest are frugivores, with a few species being carnivorous. Bats are present throughout most of the world. Bats perform a vital ecological role by pollinating flowers, and also serve an important role in seed dispersal. Many tropical plants are entirely dependent on bats.

Bats range in size from Kitti's Hog-nosed Bat measuring 29–33 millimetres (1.14–1.30 in) in length and 2 grams (0.07 oz) in mass, to the Giant golden-crowned flying fox which has a wing span of 1.5 m (5 ft) and weighs approximately 1.2 kg (3 lbs).

Fossil bats

Since bats are terrestrial and light-boned, there are few fossilized remains. An Early Eocene bat, Onychonycteris finneyi, was found in the 52-million-year-old Green River Formation in Wyoming (US) in 2003. The new genus was placed in a new family when it was published in Nature, February 2008. It was clearly a flier, but the well-articulated skeleton showed underdeveloped cochlea of the inner ear, which provide echolocation capabilities in all modern true bats, demonstrating at last that flight in bats was developed before echolocation. The team realized Onychonycteris finneyi was different when they noticed that the species lacked the ear and throat features present in all living, echolocating bats today, and even in other ancient species.

The bats of 52.5 million years ago flew differently than the bats of today, and had a vastly different appearance. Onychonycteris had claws on all five of its fingers, whereas modern bats have - at most - claws for only two digits on each hand. It also had longer hind legs, and shorter forearms, similar to those of climbing mammals that hang under branches (such as sloths or gibbons). This palm-sized animal had broad, short wings that suggest that it could not fly as fast or as far as those that evolved later. Instead of flapping its wings continuously while flying, it would likely have alternated flapping and gliding while airborne. These physical characteristics suggest that this species also did not fly as much as modern bats do; perhaps just flying to get from tree to tree, spending most of their waking day just climbing or hanging.