Order Chiroptera—Bats

The order Chiroptera is the second largest, in terms of species, of the mammalian orders. Traditionally, two suborders (Megachiroptera and Microchiroptera) have been recognized, but with the advent of molecular data, the higher classification of bats is in disarray. The suborder Yinpterochiroptera is recognized by some, for example, and relationships above the level of family badly need clarification. Under the traditional taxonomy, only the suborder Microchiroptera is represented in the New World; both the Microchiroptera and the smaller suborder Megachiroptera occur in the Old World.

The most notable feature of bats, of course, is that all of them are capable of true flight. Among all known animals, only the bats, birds, pterosaurs, and insects have attained true flight. Among mammals, the bats rule the night skys as thoroughly as the birds rule those of the day. Although birds and bats both are flying machines, the mechanisms of flight are very different. In birds, the wing flight surfaces are composed of feathers supported by light-weight bones that, in the case of the manus (hand), are reduced in number and size with considerable fusion of elements. In bats, the wing flight surfaces are composed of skin stretched between the elements of a greatly enlarged hand (in fact, "chiroptera" means "hand wing"). This membrane, together with other membranous surfaces, forms the patagium. The patagium also extends from the wing to the vicinity of the ankle (forming the plagiopatagium), and many bats also have a membrane (the uropatagium) stretched between the tail and legs. A membrane lying in front of the elbow joint is the propatagium. The patagium is under control of muscles, as well as bony support; it consists largely of skin with a thin layer of connective tissue and blood vessels (muscular control large is by connections from without the membrane). Although seemingly fragile, the membrane is tough and repairs tears or holes rapidly.

Fig. 1 (above right). Right wing of a bat (Myotis) with major bones labeled.

Lift is provided not only by the chiropatagium, but also by the other membranes and, apparently, to a degree by the body and ears in some forms. Unlike the situation in birds, the legs normally play a large role in flight and in directional control.

The number of digits is retained at five, with digits II through V enveloped by the wing membrane (the chiropatagium), but the thumb free and, compared to the other digits, very short and with a well developed claw. Generally, only the proximal two phalanges of the flight digits are ossified. There are a number of specialized osteological features, including some fusion of vertebrae, flattened ribs, a well developed clavicle,  and a small keel on the sternum. In regard to the latter, keep in mind that basic wing musculature is different between birds and bats; whereas birds primarily use ventral muscles for both raising and the downward power stroke of the wings, bats distribute muscular activity among dorsal and ventral muscle groups. Thus the huge sternal keel seen in flighted birds is not necessary. Some other specialized osteological features are shown in the class handouts.

Another feature of the Microchiroptera (but not of the Megachiroptera, except for one genus) is the utilization of echolocation as the major orientation sense. High-frequency sounds, are emitted through the mouth or nose; the echo from objects in the vicinity allows "visualization" of those objects, including distance, direction, direction of movement, speed of movement, and nature of the object's surface. Many aspects of ear shape and size and of facial ornamentation are believed to be involved in echolocation.

The question as to when echolocation evolved in relation to flight has never been satisfactorily resolved. However, recent fossil findings suggest flight evolved before echolocation. Simmons et al. (2008) describe a new family, genus, and species of bat from the Fossil Butte Member of the Green River Formation of Wyoming (ca. 52.5 mya). This is the same member that produced Icaronycteris, widely considered the earliest and most primitive bat, but apparently was basically modern in almost all features. Although obviously adapted to powered flight, Onychonycteris finneyi shows several primitive features. The cochlear of the inner ear is smaller than in most echolocating bats; in laryngeal echolocating bats, cochlear size is correlated with echolocation. It thus appears that flight preceded echolocation. Other primitive features include claws on all fingers and limb proportions intermediate between other bats and forelimb-dominated, non-volant, climbing mammals.

Within the Chiroptera, there is great diversity in ecologic niches, though all include flight. In terms of diet, there are frugivorous (fruit-eating), insectivorous, carnivorous, sanguivorous (blood-eating), piscivorous (fish-eating), and nectivorous (nectar- and pollen-eating) forms. Even within the larger niches, there often are smaller—for example, some species of Myotis take prey in the open whereas others are gleaners, picking prey from leaves, etc. In terms of utilization of habitat for resting, there are cave forms, crevice users (rock crevices, spaces between bark and tree trunk, etc.), tree bats, and, since man, taxa that utilize human-built structures (including houses and bridges).

Although bats occur as far north and south as the tree lines, their center of abundance is the tropics, and in some tropical areas the number of kinds outnumber all other mammalian species put together. In general, the farther from the tropics, the fewer number of species. There are about 25 species within our region, belonging to four families.

Last Update: 22 Jan 2008