Sone bats prefer to roost in barns, attics, caves or abandoned mines, those shelters providing safety from predators, protection from fluctuations in weather, and seclusion for rearing the young. Other species select hollow trees or rock crevices as their daytime resting site, while certain ones are known to roost in exposed locations, clinging to tree trunks or hanging upside down from tree branches. To survive in colder regions, bats either hibernate in shelters with high humidity and temperatures above freezing, or migrate to warmer areas where food is available. Based on similarities of bones and teeth, most authorities agree the bat's ancestors were probably insect eating placental mammals, possibly living in trees, and likely the same group that gave rise to shrews and moles. Bats are not rodents and are not even closely related to that group of mammals.
Bats are unique.
The structure of the wing membrane, the arrangement of the bones supporting it, and the positioning of the muscles provide the bat with the lightness and maneuverability necessary for catching insects, hovering above flowers, or quickly avoiding obstacles. Extending from the sides of the body and incorporating the hands, legs, and tail are the two thin layers of skin containing blood vessels, nerves, and tendons that essentially make up the wing membrane. The bones of the arm and four of the fingers are light, slender, and lengthened to support, spread, and manipulate the membrane. The thumb, usually with a sharp claw, is not attached but remains free for clinging to various surfaces. Unlike other mammals, the hind limbs are positioned so that the knees, when bent, point backward while the bottom of the feet face forward.