One third of all of the species of mammals are bats. There are two major groups of bats: the megachiropterans (the largest of which have wingspans approaching 5 feet) and the more diverse group of microchiropterans. Microchiropterans evolved from megachiropterans. Bats have a number of characteristics (such as the unique innervation of the wing by the facial nerve) which indicate that they descended from a common ancestor.
Although they all share similarities, such as adaptations to flight, there are significant anatomical variations within the bats. Megachiropterans vary from 10 to 1,5000 grams in weight with forearm lengths which vary between 36 and 228 mm. Microchiropterans vary from 2 to 196 grams in weight with forearm lengths of 22 to 115 mm (Nowak, 1994). There is considerable variation in specific anatomical bat features, such as the size and shape of the cranium in bats. Fossil and modern species of bat form a clade and considerable skeletal differences can exist between species which are closely related, such as skull shape in the Old World leaf bats (Hand, 2003).
|There is also considerable variation in the faces of bats.|
|The fibula (a bone of the lower leg) is a strong bone in the subfamily Molossinae. It is a more slender bone in some bats, in other bats it is cartilaginous, and in Nycteris it is absent (Dobson, p. xiv). There are three distinctive forearm types in bats. The ulna degenerates to some degree in many bats (Adams, p. 279)|
In most bats (some Megachiropterans and almost all Microchiropterans) the second finger possesses a rudimentary bone (phalanx). Some Microchiropterans lack a second phalanx. (Dobson, p. xii). The first toe of Cheiromeles is thumblike in its appearance (Dobson, p. xvi). Microchiropterans possess a claw on their second finger while most megachiropterans retain it (Walker, p. 187).
Many bats possess a structure on their hind foot called the calcar. The calcar may be composed of cartilage, calcified cartilage, or bone. It is present in most, but not all, of bats in both suborders. In some Microchiropterans, it forms a basket for catching insects. It begins its development as a piece of cartilage near but separate from the calcaneus. Of the three fossil bats, Icaronycteris and Archeonycteris lacked a calcar while it was present in Archaeopteropus (Adams, p. 321-7).
|The greatest number of teeth in bats is 38 (2 incisors,1 canine,3 premolars,3 molars in the upper jaw/3,1,3,3 in the lower jaw) although many have fewer than this. In Desmodontes (the group that include vampire bats), the dental formula is 1,1,2,0/2,1,3,0 for a total of 20 teeth. (Dobson, ).|
EARS AND ECHOLOCATION
In bats, ears are more variable than in other group of mammals and can reach the largest size (relative to body size) in mammals. In most Microchiropterans, the ears are longer than the head. (Dobson, p. xix) Most bats have a prominent ear lobe called the tragus (Walker, p. 184). The cochlea (the structure of the inner ear which perceives sound) is variable in bats. The size of the cochlea of fossil bats is small and overlaps the megachiropterans. The number of cochlear turns is 1.75 in Pteropus and 3.5 in Rhinolophus (Adams, p. 143-5).
The oldest fossil bat, Icaronycteris, exhibits ear similarities to insectivores.
How do bats fly at night? Until the end of the 18th century, many assumed that the ability of bats to fly at night resulted from magical powers associated with the devil. We now know that bats produce high pitched sounds which, after being reflected off of objects around them, allow them to even hunt for insects. This use of sound for navigation, called echolocation, is not unique to bats. Echolocation has evolved separately in oilbirds (genus Steatornis related to whipporwhills), cave swiftlets (Collocalia, related to swifts), tenrecs, whales, and some gliding marsupials and rodents.
The Megachiropterans do not perform echolocation except for the genus Rousettus which uses tongue clicking. Although Rousettus does use echolocation for positional information while flying, it relies on vision and smell in order to find food (Nowak, 1994; Adams, p. 17).
Different bats emit different sounds. These sound pulses can vary in their length of duration, their pitch, the rate at which they change their pitch, and their volume (Kunz, 1982). Bats of the families Rhinolophidae and Hipposideridae release sound through their nostrils. Some insects (such as some moths and lacewings) respond to bat signals by dramatically changing their flight pattern or emitting confusing sounds of their own (Nowak, 1994; Kunz, 1982).
The noses of certain families of bats (Rhinolopidae, Nycteridae, and Phyllostomidae) are unique among mammals. They are composed both of an elaborate nasal integument and glandular structures. Among other functions, they are very sensitive to touch (Dobson, p. xix).
Bat noses and ears have been modified to conduct and receive sound, respectively.
There is considerable variation in the brains of bats. Megachiropterans have larger cerebullums and cerebrums, both with gyri and sulci (folds and grooves) while Microchiropterans have smaller and smooth cerebellums and cerebrums. While the telencephalon (which includes the cerebrum) is proportionately larger in the Megachiropterans, the medulla and midbrain are proportionately larger in the Microchiropterans. The encephalization quotient (which measures the size of the brain relative to body size) varies from .52 to 1.23 in Microchiropterans and .95 to 1.37 in Megachiropterans.
In Megachiroptera, the pons is larger and the there is a clearer division between the medulla and spinal cord. In Microchiropterans, the paraflocculus dorsalis is evident and the inferior colliculus may be very large. The midbrain is exposed in Microchiropterans, unlike Megachiropterans.
The olfactory bulb is large in most Megachiropterans but small in most Microchiropterans. The accessory olfactory bulb is absent in Megachiropterans but is present in some Microchiropterans. Some bats retain their vomeronasal organ and members of the family Phyllostomidae possess a vomeronasal nerve. In Desmodus, the hypoglossal nerve is very large.
There are variations in brain structure between families of bats. For example, the anterior rhinal and hippocampal sulci are more developed in Phyllostomidae, the medial geniculate nucleus in Noctilionidae, the rostral and sylvian sulci in Miniopterinae, the dorsal cochlear nucleus in Megadermatidae. (Adams, p. 106-114). The brains of two bats are compared below.
The eyes of bats are well developed but vision only supplements the information gathered by other senses rather than serving as a primary sense. Only megachiropterans have the ability to see in color. Sequence studies of opsin genes indicate that vision serves an important role in nocturnal bats (Zhao, 2009).
There are variations in the reproductive
system of bats. In males, a baculum may or may not be present and in females, the uterus
may of the simplex type, two horned, or an intermediate shape. Some female bats store sperm over winter before
ovulating in the spring while fetal development is delayed over winter
in other species (
Some bats species are solitary while others roost in colonies. These colonies vary in size in different species from several individuals to more than 20 million (Nowak, 1994). Most are nocturnal, becoming active around sunset (although each species can differ in how close to sunset activity begins). A few are active during the day, such as island-dwelling megachiropterans. One colony of Mexican Free-Tailed bats consists of 50 million individuals and is estimated to eat almost 7 tons of insects per year (Richarz, 1993).
No other group of mammals is as diverse in its feeding habits as bats. Some have adapted to feeding on fruit, leaves, nectar, fish, and even blood. Most microchiropterans are insectivores, including all those species which do not live close to the equator. Bats are the primary nocturnal predators of insects and they can ingest an amount of insects equal to their body weight during one night’s feeding. While most can feed on diverse insects, some specialize on one specific organism. For example, the North American bat Plecotus townsendii feeds primarily on moths and the Australian bat Kerivoula papuensis specializes on orb weaver spiders. Some bats are valuable to plants for their roles in pollination and seed dispersal (Kunz, 1982). Raccoons and skunks occasionally feed on bats. Bobcats, bullfrogs, and trout may sometimes feed on bats. Barn owls are a major predator of bats (Richarz, 1993).
Bats spend about half their lives in their roosts and it is here that they mate, hibernate, and raise their young. While some bats will roost in a variety of settings and will use certain sites, such as caves, when they are available, other species are restricted to certain types of roosts. Rock crevices, holes in tree trunks, and spaces under tree bark are often used as roost sites as well. Some tropical bats possess adhesive disks on their feet (such as in Thyroptera discifera and T. tricolor) which adapt them (and also limit them) to roosting in unfurled leaves of certain plants. Bats are most likely to return to the same roost night after night if that type of roost (such as a cave) is not commonly found. Bats are unusual among mammals in that they can reduce their body temperature while roosting to save energy (Kunz, 1982).