Bats are nocturnal, coming out at night. They are the only mammals capable of true (meaning, flapping) flight, because the other so-called flying mammals (for example, squirrels, lemurs, and sugar gliders) glide, they do not fly. Today, the diversity of bats is astonishing, with more than 1,000 species making them second only to rodents as the most diverse group of mammals. Although some bats have remarkable faces and behavior, wings are the most conspicuous features of the flying bats. Upon landing, bats immediately fold their wings so they appear to shrink in size. Small size is another distinctive feature of bats. While a few species weigh more than 3.5 oz (100 g) as adults, most weigh less than 1.7 oz (less than 50 g), and the majority less than 0.9 oz (less than 25 g). The smallest bats in the world (hog-nosed bats, Craseonycteris thong-longyai, family Craseonycteridae from Thailand and Myanmar) weigh 0.07 oz (2 g) as adults. The largest, the Indian flying foxes (Pteropus giganteus) from India, Pakistan, and Southeast Asia, weigh 52.9 oz (1,500 g).
Chiroptera means hand (cheiro) and wing (ptera). Bat wings are folds of skin supported by elongated arm, hand, and finger bones, and attached to the sides of the body. In most cases, bats' thumbs are relatively free of the wing membranes and bear claws, which are absent from the fingers. Flying foxes and their allies (family Pteropodidae) usually have claws on their second fingers, but some species (dawn bats, genus Eonycteris, and naked-backed fruit bats, genus Dobsonia) lack these claws. Second fingers with claws occur in some fossil bats.
Measurements made with Doppler radar indicate that bats fly about 6.5-49.2 ft (2-15 m) per second. Bats use nine pairs of muscles to power flight. Muscles that power the down-stroke are located in the chest, and those responsible for the upstroke are located in the back. Although some bats have quite muscular forearms, their wing bones tend to be lightly muscled. In most bats, the folds of skin (wings) enclose some connective tissue, nerves, and blood vessels. Some free-tailed bats (Molossidae) also have sheets of muscle in the wing membranes. In mechanics and aerodynamics, the flight of bats is very similar to that of birds. The passage of air over the airfoil section of the wings generates lift. Movements of the wing tips generate propulsive thrust.
Bats show considerable variation in wing shape and flying abilities. In many species, the wings are relatively broad, providing good lift. Shorter-winged species tend to be more ma-neuverable than longer-winged ones. Many species of flower-visiting bats and some species that take animal prey from the ground can hover. While some species of bats can take off from the ground, those with longer, narrower wings cannot.
Although both can fly, bats differ from birds. While living birds such as ostriches, emus, and penguins, and fossils like elephant birds have lost the ability to fly, there are no living or fossil flightless bats. Bats have teeth, while living birds do not. Bats give birth to live young, while birds lay eggs. Since teeth are heavy, having them at the front end of a flying animal could create aerodynamic problems. Additionally, laying eggs could be more efficient and less costly for a flying animal than bearing live young. The diversity of birds (more than 8,000 species) suggests that their approach is more successful than that of bats (1,000 species).
In birds, the wings do not involve the hind limbs as they do in bats. The hind limbs of bats tend to be spindly and poorly muscled, and many bats are not mobile on the ground. Most birds are much more mobile on the ground, and their robust hind legs reflect this reality. Mobility in the air, on the
ground, and beyond (as for penguins) may partly account for differences in diversity between bats and birds.
In addition to the obvious (i.e., bird wings are made of feathers, while bat wings are folds of skin), there are other differences between these animals. In bats, the division of power generation across nine pairs of muscles means that they are thin in profile through the chest. In birds, two pairs of muscles power flight: one the down-stroke, the other the upstroke. Both sets of muscles are located on birds' chests and the upstroke muscles operate by a pulley system. A thin profile through the chest allows bats to squeeze into narrow cracks and crevices that serve as places to spend the day (day roosts). While a bird typically has a prominent keel on its breastbone (sternum), this feature is absent in most bats and never as well developed as it is in birds. Birds have wishbones (furcula), bats do not.
In most bats, the thumbs are free of the wings, appearing there as claws. In some bats, notably thumbless ones (family Furipteridae) and some ghost bats (genus Diclidurus, family Emballonuridae), the thumbs are greatly reduced in size and may lack claws. At the other extreme are the very long thumbs of vampire bats (Desmodus rotundus). Vampire bats' thumbs act like "throwing sticks," giving the bats extra leverage for taking off from the ground.
Bats' wing membranes usually join along the side of the body, but in two groups they meet in mid-back. The naked-backed bats belong to species in two families, Old World fruit bats (genus Dobsonia, family Pteropodidae) and naked-backed moustached bats (two species in the genus Pteronotus, family Mormoopidae). The function of the naked back remains unknown.
Wing membranes may attach to the bats' hind legs, extending as far down as the fifth toe of the hind feet. In other species, they attach higher up, at the ankle or knee. Carnivorous bats typically have an interfemoral membrane (between the hind legs) that encloses all or part of the tail. On the tail side, the calcar, a cartilaginous structure protruding from the ankle, supports the back edge of the interfemoral membrane. In flight, the interfemoral membrane acts as a rudder and also reduces oscillations of the body through each wing-beat cycle. But interfemoral membranes are not essential to bats: some plant-visiting and blood-feeding species have very narrow interfemoral membranes or lack them completely.
Most bats have tails. But while tails of some (e.g., mouse-tailed bats, Rhinopomatidae, and some Pteropodidae) are long and slender, others (e.g., free-tailed bats, Molossidae) are short and thick. Tails may or may not extend to the end of the interfemoral membrane. Some bats lack obvious tails.
The diversity of bats is reflected by the variety of trophic (ecological) roles they fill in ecosystems. While most species are mainly insectivorous, others eat plant products (fruit, leaves, seeds, nectar, or pollen). Other bats eat animals such as fish, frogs, birds, and even other bats. The most infamous of bats are the blood-feeding vampires, arguably the most remarkable of mammals. The ecological diversity of bats is reflected in their anatomy and behavior. The cheek teeth (molars and premolars) of carnivorous bats are quite different to those of frugivorous (fruit-eating) bats. Nectar- and pollen-feeding bats have teeth more specialized for crushing food, while the vampires have scalpel-sharp razors.
The facial features of bats reflect their remarkable diversity. Bats also are diverse in their selection of roosts, places they spend the day, and by the social systems that develop in these places.
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