Flight in mammals is unique to bats. Moreover, flight is rare in the animal kingdom generally. Among invertebrates (those animals without backbones) only insects can fly. Birds’ fly, and once pterosaurs (winged reptiles) did. Bats are the only other, and the newest, entry into this exclusive club. The ability to fly has constrained bats in structure and locomotion. For example, no bats effectively run or swim, and none is capable of burrowing. Further, flight has required major physiological challenges, not the least of which is the huge surface for heat loss created by the wing membrane. Flight, however, has also allowed bats to diversify into many different ecological roles.
Birds evolved flight independently of bats, and although both are flying vertebrates, they differ considerably in terms of locomotor anatomy. Unlike birds, which use only chest muscles to power flight, the wings of bats are powered by both chest and back muscles. The down stroke is driven by the chest muscles and produces the force keeping the bat airborne, whereas the force created by the back muscles produces forward movement.
Unlike upright, perching birds, most bats drop into flapping flight from a hanging position. Some, however, crawl to an edge of a rock face and hop into flight. When landing, bats usually survey the area using echolocation or vision, and then perform a flip in the air, hooking their claws onto the roost substrate to stop.
Their flight prowess is made possible by their ability to alter each wing's movement independently of the other, allowing them to roll or slip to one side, literally on a dime. In addition, because the second through fifth fingers traverse the wing membrane from the leading to the trailing edge, bats can change the camber (cross-sectional curvature) of the flight membrane by bending or unbending their fingers. This allows for precise control of the lift produced by the wing.
Flight is not cheap. Quite the contrary, flight is a costly mode of locomotion. Heart rates of flying bats reach as high as 1,200 beats per minute. However, although the metabolic cost of flight is high, transit time is shorter than in terrestrial commuting (as most of us have experienced from flying in airplanes). In fact, although flying bats consume energy as much as three times as fast as the highest recorded rates for terrestrial mammals of comparable size, flight is so efficient that a bat uses less than one-fourth the energy used by a terrestrial mammal to travel a given distance. Thus, the advantages of flight are numerous. Flight gives bats the mobility to travel far and wide for feeding and roosting areas and is responsible for their nearly cosmopolitan distribution.
Differences among bats in wing anatomy produce an array of flight styles. Although all bats are capable of powered flight, each species has distinctive flight characteristics, which allow and determine ecology (including diet, habitat and pattern of echolocation). For example, size and shape of the wings determine flight speed and turning ability, and thus reflect foraging strategy. Bats with shorter, broader wings have lower wing-loading (body weight divided by area of the wing) and are capable of slow, maneuverable flight through dense vegetation. These species tend to be gleaning insectivores or nectar feeders, which require quick directional changes and hovering. Although insectivorous species that use aerial pursuit techniques to capture insects have variable wing sizes and shapes, most have higher wing-loadings than nectarivores or gleaners to enhance speed.
The daily patterns of activity for many bats have been well documented. Time of emergence from day roosts to forage is variable, depending on species, but most begin to arouse about an hour before leaving to forage. Foraging usually begins at dusk, and several individuals may depart together from their day roost. A specialized outflight behavior can be observed in Brazilian free-tailed bats in the San Luis Valley. The sight of the colony emerging from an abandoned mine shaft at dusk is breath-taking. Undulating for miles over the valley floor, the spectacle has been described as resembling an enormous, flying serpent. Peggy Svoboda, a wildlife biologist who spent several summers documenting daily activities, population size, morphology and reproduction of this colony said, "It looks like a river, and it sounds like the wind."
Foraging distances depend on species and prey density. Many insectivorous bats range several miles to feed. Most, however, travel only as far as necessary to fill their stomachs. After feeding, individuals of most species retire to a night roost, usually beneath an open overhang. There they digest that night's bounty. In some cases they forage again in the early morning before returning to the comfort and protection of the day roost. As sunset approaches, the cycle is initiated again.
BATS IN FLIGHT
Bats are highly adept at negotiating in extremely tight spaces. In one laboratory experiment, a Dent's horseshoe bat -- with a wingspan of about 10 inches --was released into a box measuring approximately 3 feet by 3 feet by 1 foot. Calling it "one of the most impressive demonstrations I have ever witnessed," Dr. Brock Fenton reported that the bat "flew at length without touching the walls, ceiling or floor."