Bats are mammals in the order Chiroptera which have adapted their forelimbs into wings, making them the only known mammals capable of true flight (there are a handful of other mammal species that can glide). They are more agile in flight than birds, and are among the most numerous and diverse of all mammals; there are than 1,400 species of bats known to science, comprising about twenty percent of all named mammal species. They are found in nearly all terrestrial environments, absent only from polar deserts. Bats range widely in size, from Kitti’s hog-nosed bat with its six-inch wingspan (it is probably the smallest known mammal) to the giant golden-crowned flying fox, which can attain wingspans of five and a half feet. Fossils of bats are rare due to their fragility, so almost nothing is known about their evolution aside from the fact that they existed during the Eocene epoch 48 million years ago.
