The smallest bat, and arguably the smallest extant mammal

Bats are mammals of the order Chiroptera.[a] With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more manoeuvrable than birds, flying with their very long spread-out digits covered with a thin membrane or patagium. The smallest bat, and arguably the smallest extant mammal, is Kitti's hog-nosed bat, which is 29–34 millimetres (1 1⁄8–1 3⁄8 inches) in length, 150 mm (6 in) across the wings and 2–2.6 g (1⁄16–3⁄32 oz) in mass. The largest bats are the flying foxes and the giant golden-crowned flying fox, Acerodon jubatus, which can weigh 1.6 kg (3 1⁄2 lb) and have a wingspan of 1.7 m (5 ft 7 in).

The second largest order of mammals after rodents, bats comprise about 20% of all classified mammal species worldwide, with over 1,400 species. These were traditionally divided into two suborders: the largely fruit-eating megabats, and the echolocating microbats. But more recent evidence has supported dividing the order into Yinpterochiroptera and Yangochiroptera, with megabats as members of the former along with several species of microbats. Many bats are insectivores, and most of the rest are frugivores (fruit-eaters) or nectarivores (nectar-eaters). A few species feed on animals other than insects; for example, the vampire bats feed on blood. Most bats are nocturnal, and many roost in caves or other refuges; it is uncertain whether bats have these behaviours to escape predators. Bats are present throughout the world, with the exception of extremely cold regions. They are important in their ecosystems for pollinating flowers and dispersing seeds; many tropical plants depend entirely on bats for these services.

Bats provide humans with some direct benefits, at the cost of some disadvantages. On the benefits side, bat dung has been and in many places still is mined as guano from caves and used as fertiliser. Bats consume insect pests, reducing the need for pesticides and other insect management measures. They are sometimes numerous enough and close enough to human settlements to serve as tourist attractions, and they are used as food across Asia and the Pacific Rim. On the disadvantages side, fruit bats are frequently considered pests by fruit growers. Due to their physiology, bats are one type of animal that acts as a natural reservoir of many pathogens, such as rabies; and since they are highly mobile, social, and long-lived, they can readily spread disease among themselves. If humans interact with bats, these traits become potentially dangerous to humans.

Depending on the culture, bats may be symbolically associated with positive traits, such as protection from certain diseases or risks, rebirth, or long life, but in the West, bats are popularly associated with darkness, malevolence, witchcraft, vampires, and death.

In the 1980s, a hypothesis based on morphological evidence stated the Megachiroptera evolved flight separately from the Microchiroptera. The flying primate hypothesis proposed that, when adaptations to flight are removed, the Megachiroptera are allied to primates by anatomical features not shared with Microchiroptera. For example, the brains of megabats have advanced characteristics. Although recent genetic studies strongly support the monophyly of bats,[7] debate continues about the meaning of the genetic and morphological evidence.[19]

The 2003 discovery of an early fossil bat from the 52-million-year-old Green River Formation, Onychonycteris finneyi, indicates that flight evolved before echolocative abilities.[20][21] Onychonycteris had claws on all five of its fingers, whereas modern bats have at most two claws on two digits of each hand. It also had longer hind legs and shorter forearms, similar to climbing mammals that hang under branches, such as sloths and gibbons. This palm-sized bat had short, broad wings, suggesting that it could not fly as fast or as far as later bat species. Instead of flapping its wings continuously while flying, Onychonycteris probably alternated between flaps and glides in the air.[7] This suggests that this bat did not fly as much as modern bats, but flew from tree to tree and spent most of its time climbing or hanging on branches.[22] The distinctive features of the Onychonycteris fossil also support the hypothesis that mammalian flight most likely evolved in arboreal locomotors, rather than terrestrial runners. This model of flight development, commonly known as the "trees-down" theory, holds that bats first flew by taking advantage of height and gravity to drop down on to prey, rather than running fast enough for a ground-level take off.[23][24]

The molecular phylogeny was controversial, as it pointed to microbats not having a unique common ancestry, which implied that some seemingly unlikely transformations occurred. The first is that laryngeal echolocation evolved twice in bats, once in Yangochiroptera and once in the rhinolophoids.[25] The second is that laryngeal echolocation had a single origin in Chiroptera, was subsequently lost in the family Pteropodidae (all megabats), and later evolved as a system of tongue-clicking in the genus Rousettus.[26] Analyses of the sequence of the vocalization gene FoxP2 were inconclusive on whether laryngeal echolocation was lost in the pteropodids or gained in the echolocating lineages.[27] Echolocation probably first derived in bats from communicative calls. The Eocene bats Icaronycteris (52 million years ago) and Palaeochiropteryx had cranial adaptations suggesting an ability to detect ultrasound. This may have been used at first mainly to forage on the ground for insects and map out their surroundings in their gliding phase, or for communicative purposes. After the adaptation of flight was established, it may have been refined to target flying prey by echolocation.[22] Bats may have evolved echolocation through a shared common ancestor, in which case it was then lost in the Old World megabats, only to be regained in the horseshoe bats; or, echolocation evolved independently in both the Yinpterochiroptera and Yangochiroptera lineages.[28] Analyses of the hearing gene Prestin seem to favour the idea that echolocation developed independently at least twice, rather than being lost secondarily in the pteropodids,[29] but ontogenic analysis of the cochlea supports that laryngeal echolocation evolved only once.[30]
The head and teeth shape of bats can vary by species. In general, megabats have longer snouts, larger eye sockets and smaller ears, giving them a more dog-like appearance, which is the source of their nickname of "flying foxes".[40] Among microbats, longer snouts are associated with nectar-feeding.[41] while vampire bats have reduced snouts to accommodate large incisors and canines.[42]

Small insect-eating bats can have as many as 38 teeth, while vampire bats have only 20. Bats that feed on hard-shelled insects have fewer but larger teeth with longer canines and more robust lower jaws than species that prey on softer bodied insects. In nectar-feeding bats, the canines are long while the cheek-teeth are reduced. In fruit-eating bats, the cusps of the cheek teeth are adapted for crushing.[41] The upper incisors of vampire bats lack enamel, which keeps them razor-sharp.[42] The bite force of small bats is generated through mechanical advantage, allowing them to bite through the hardened armour of insects or the skin of fruit.[43]
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Wings and flight
Main articles: Bat flight and Bat wing development
Bats are the only mammals capable of sustained flight, as opposed to gliding, as in the flying squirrel.[44] The fastest bat, the Mexican free-tailed bat (Tadarida brasiliensis), can achieve a ground speed of 160 km/h (100 mph).[45]

File:Flapping-Tail-Membrane-in-Bats-Produces-Potentially-Important-Thrust-during-Horizontal-Takeoffs-and-pone.0032074.s004.ogv
Little brown bat take off and flight
The finger bones of bats are much more flexible than those of other mammals, owing to their flattened cross-section and to low levels of calcium near their tips. The elongation of bat digits, a key feature required for wing development, is due to the upregulation of bone morphogenetic proteins (Bmps). During embryonic development, the gene controlling Bmp signalling, Bmp2, is subjected to increased expression in bat forelimbs – resulting in the extension of the manual digits. This crucial genetic alteration helps create the specialised limbs required for powered flight. The relative proportion of extant bat forelimb digits compared with those of Eocene fossil bats have no significant differences, suggesting that bat wing morphology has been conserved for over fifty million years.[46] During flight, the bones undergo bending and shearing stress; the bending stresses felt are smaller than in terrestrial mammals, but the shearing stress is larger. The wing bones of bats have a slightly lower breaking stress point than those of birds.[47]

As in other mammals, and unlike in birds, the radius is the main component of the forearm. Bats have five elongated digits, which all radiate around the wrist. The thumb points forward and supports the leading edge of the wing, and the other digits support the tension held in the wing membrane. The second and third digits go along the wing tip, allowing the wing to be pulled forward against aerodynamic drag, without having to be thick as in pterosaur wings. The fourth and fifth digits go from the wrist to the trailing edge, and repel the bending force caused by air pushing up against the stiff membrane.[48] Due to their flexible joints, bats are more manoeuvrable and more dexterous than gliding mammals.[49]

Wing membranes (patagia) of Townsend's big-eared bat, Corynorhinus townsendii
The wings of bats are much thinner and consist of more bones than the wings of birds, allowing bats to manoeuvre more accurately than the latter, and fly with more lift and less drag.[50] By folding the wings in toward their bodies on the upstroke, they save 35 percent energy during flight.[51] The membranes are delicate, tearing easily,[52] but can regrow, and small tears heal quickly.[52][53] The surface of the wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the centre, making it even more sensitive and allowing the bat to detect and adapt to changing airflow; the primary use is to judge the most efficient speed at which to fly, and possibly also to avoid stalls.[54] Insectivorous bats may also use tactile hairs to help perform complex manoeuvres to capture prey in flight.[49]

The patagium is the wing membrane; it is stretched between the arm and finger bones, and down the side of the body to the hind limbs and tail. This skin membrane consists of connective tissue, elastic fibres, nerves, muscles, and blood vessels. The muscles keep the membrane taut during flight.[55] The extent to which the tail of a bat is attached to a patagium can vary by species, with some having completely free tails or even no tails.[41] The skin on the body of the bat, which has one layer of epidermis and dermis, as well as hair follicles, sweat glands and a fatty subcutaneous layer, is very different from the skin of the wing membrane. The patagium is an extremely thin double layer of epidermis; these layers are separated by a connective tissue centre, rich with collagen and elastic fibres. The membrane has no hair follicles or sweat glands, except between the fingers.[54][56] For bat embryos, apoptosis (cell death) affects only the hindlimbs, while the forelimbs retain webbing between the digits that forms into the wing membranes.[57]