Basic Body Plan
The body of bats is well designed for flight. However, they achieve flight differently from birds. Like birds, the bones of bats are light-weight and delicate. However, bats have a short neck compared to birds, and they lack a deeply keeled sternum, or breastbone, where the flight muscles attach in birds. Instead, three shallow pairs of muscles on the breast power the downstroke of the wing during bat flight, while the upstroke is provided by three pairs of muscles on the back. Because they do not have a well-developed breastbone, bats have a flat profile through the chest, and so they can squeeze through small openings and roost in narrow crevices.
The wing structure of birds and bats is also different. The skin and feathers of the wings birds are supported mainly by their second and third "fingers." In comparison, the wings of bats are formed by thin, elastic skin extending from the sides of the body to the tips of all four elongated "finger" bones. Their much-reduced thumb remains free of the wing membrane and is used to manipulate food, and as a hook when the bat climbs and clings to surfaces or vegetation. The wing membranes are also supported by the hind legs, and in species with a tail, it is entirely or partially enclosed by wing membrane stretching between the hind legs. The hind legs of bats are unique among mammals in being rotated 180°, so that the knees point backward, allowing the leg to flex in a reverse fashion. This is believed to assist in steering during flight, and in taking off from the characteristic head-down roosting position of bats.
The Megachiroptera have a dog-like face. However, the face of the Microchiroptera is often striking and weird-looking, with fleshy embellishments that form complicated dimples, wrinkles, and horseshoe- or leaf-like structures. Some species have tubular nostrils. Biologists have suggested that these facial embellishments function in the projection of sounds produced for echolocation, like megaphones or acoustic lenses. While Megachiroptera generally have simple ears, there is huge variation in the size, shape, and elaboration of ears among microchiropteran bats. Depending on the species, their ears may feature special folds and ridges that are thought to play a role in sound perception. For instance, many of these bats possess a large tragus, a fleshy projection on the bottom front edge of the ear opening, and believed to aid an echolocating bat in determining the horizontal position of a target.
Like many birds, bats pass the food they have eaten fairly quickly through their digestive tract, so as to reduce the amount of time they must carry the extra weight of undigested food. Total output time is as little as 20 minutes in some smaller bat species, which is similar to birds of the same size.