There are about 1,000 species of bats; it is not surprising to find that there is considerable room for variation within the group. Bats are considered as the only flying animals because of their specific anatomical features while the flying squirrel only glides. The unique features of body of the bat are designed to make it able to find prey, to communicate, and its wings to fly around. The body of the bat has special features like its wings and limb. Their wings have webbing in it. They come under the order Chiroptera its name (literally, "hand-wing").
Functional wings and true flights are characteristics of all bats. The origin of wings of bat is clearly revealed by their skeleton. These wings are modified forelimbs. The skeleton of the wing consists of humerus, radius and ulna, metacarpal and phalanges. Humerus is longer but thinner compared to the humerus of other mammals. The radius is also long and thin, ulna is greatly reduced and fused with the radius to turn it strong and support the wings. The wrist region is less flexible. It is specialized to support the particular motions associated with flying. In most of the bats only the thumb i.e. 1st digit presents a claw, which is useful for climbing, food handling, and fighting, but in some flying foxes the 2nd digit also has a small claw. The rest digits i.e. 2-5 supports the wing. The skeleton of each digit articulate proximally with metacarpal. The bones and muscles of the wing are so arranged that the wing is extended/opened and closed through the operation of only a single muscle for each action. The shape of the bones is such that lifting, or relaxing the humerus stretches a muscle attached to the radius pulling it out or in, and moving the radius has a similar effect on the carpals and metacarpals, thus the whole arm can be opened and closed very quickly and efficiently with a minimum of muscular efforts. The bones present in the wings that work like fingers with much flexibility allowing it to have a high range of movement without difficulty. Bat wings usually run from the shoulder region to the ankle, or to the digits. The wing joins the body from sides, in a few bats it arises from the middle of the back. Bat biologists use different names to refer to different parts of the wings.
· Propatagium: from shoulder to wrist.
· Plagiopatagium: from the body to the 5th digit.
· Dactylopatagium: membrane spanning digits 2-5.
Bats have a much larger double-thick membrane of skin (patagium) that joins their long fingers from the bases to the tips so fingers cannot be flexed independently, muscles found in the arm can open up the hand, producing a wing like an umbrella. The surface area can become larger by the main inner part of the membrane (plagiopatagium) joining to the side of the bat’s body and down to the legs. The wings are mainly made up of cartilage but it does not contain much calcium in it. The tips can bend without damaging the wings. There is a flat section in the middle that allows for plenty of support to be offered to the wings. This is also very flexible so it can be bent in all directions. Wing and tail membranes appear naked in most bats, but on close examination they can be seen to be covered with minute hairs, and in some species, with distinctive tufts and fringes of hairs. The function of these hairs may be to modify the aerodynamic properties of the surface of the wings, but this is speculative. A few species have a thick layer of fur on their tail membranes and some parts of their wings; these include tree roosting species such as members of the vespertilionid genus Lasiurus. This fur probably serves to insulate and camouflage the bat.
The wing bones are covered with skin. This skin is very elastic and stretchable. It is made up of external epidermis and internal dermis, which is supplied with criss-crossed tiny blood vessels (easily seen in a live bat when the wing is stretched in front of a light), oil secretory glands and muscles to control the wings during flight. The bat’s wings are thin in comparision to birds. The wings are very sensitive because of presence of Merkel cells in them. Their body has the ability to repair itself very fast if tear occurs in the wings.
Hindlimbs: The hindlimbs can rotate with180º, so while walking on the ground its knees stick up into the air. The bones of the pelvic girdle (ilium, ischium and pubis) are more strongly fused than in other mammals. The lower section of the hindlimb is composed almost entirely of the tibia; the fibula is vestigial, and fused with tibia. The whole limb can rotate through a wide angle allowing a hanging bat to swivel its body through a complete circle. The toes of the hindlimbs all have strong, laterally compressed, claws and an automatic locking system involving a tendon that passes through a sheath of cartilaginous rings attached to the phalanx that constrain its movement. This tendon is so attached that the bat’s own weight keeps it tight. This allows the bat to sleep without falling from its roost. (Birds also have a locking mechanism on their claws to stop them from falling off their perch, but their system is quite different.) The hind legs of many bats are partially or completely joined by a membrane, the uropatagium, which also may enclose the bony tail. It is supported by the legs, the tail skeleton, and by the calcar, a special cartilaginous extension of the ankle. The calcar sometimes bears a distinctive projection called a keel. In some species, both the uropatagium and calcar are absent
Head: The skull is highly variable in its shape; this variation is dependent on the animal’s diet, with nectar feeding bats having long thin skulls while many insectivorous species have relatively short blunt skulls. All bats have very tiny teeth that are razor sharp. They can easily bite through the skin of fruits or prey. They also have a very long tongue that they use for eating, drinking, and pollination. They can roll that tongue up around their rib cage too when not using it. The eyes of the bat will vary depending on which species you happen to be talking about. This category of microbats is very small and they aren’t well developed. They do have some vision though so myth that “bats are blind” is false. Some species of bats have quality vision, and they are able to detect ultraviolet lighting. They are able to rely on excellent hearing and smell to make up for vision pitfalls. They have a small dot like nose but they have an excellent ability to smell with it. The hearing and communication mechanism of the bat is found in the inner ear. Not all of the bat species have it but most of them do. This is called echolation and it allows them to hear and to communication through vibrations. Other animals with this ability include the dolphin. This echolation system provides the bat with detailed information about what is going on around them in their environment. They can find prey in complete darkness due to this ability. Therefore even bats that don’t see well are still able to find their prey and to survival without difficulty.
Vertebral column: In general bats have: 7 cervical (neck) vertebrae; 11 thoracic (chest) vertebrae; 4 lumbar (abdominal) vertebrae and between 0 and 10 caudal (tail) vertebrae. In some species the last cervical and first thoracic vertebrae are fused. The megachiroptera have no caudal vertebrae, and hence no tail.
Blood vascular system: Bats have one way valves in their arteries so prevent the blood from flowing backwards. This is why they are able to hang upside down with the blood rushing to their heads.
Specific Behavioural features:
* Since bats are the only mammals that can fly, with wingspans ranging from 6" to 6′, they failed to make great strides in the walking-about, or in the standing departments. Their legs and feet are simply not strong enough to support these functions for great lengths of time. Bats are not capable of taking-off like most birds as their wings are not able to generate sufficient lift. They cannot even boost their energy by running as their hind legs are too tiny and therefore, inadequate. So what do bats do? First, they use their front claws to climb their way up to a high spot and when it is necessary for them to fly, they drop themselves from that height and take-off from there. That’s why bats, when in their bat caves, or roosting in trees, take the weight off of their tired limbs by hanging upside down. Their breast-feeding babes, born live, as are all mammal offspring, have no choice but to hang in limbo with mom. Together, they reap the benefits of downward gravitational pull.
* Bats also invert our common beliefs regarding their eyesight. The bat, a nocturnal creature, hunts its prey at night, and rests up (or down, as is the case) for the evening’s events by day. One would be incorrect in assuming that the bat has keen night vision, to assist it in its feeding frenzy. Not so. When a predator enters the cave which they sleep in, they fly around trying to scare the predator away. For a plan b, they stay very silent at the top of the cave so the predator won’t hear them. Bats also have no scent so the predators won’t find them in the dark.
* The bat employs an echo system, whereby it tosses out a high-frequency sound; one inaudible to the human ear, which in turn strikes a target, and then bounces back to the bat, thereby letting it know what lies ahead. Were it not for the bat’s radar, we would hear even more bumps in the night than we already do.
* Once the bat has found a spot to hang, it uses its muscles to pull its claws open and then relaxes its body to hold on to the surface. The weight of the upper body of the bat forces the tendons connected to the talons to clench. That essentially means that the bat does not have to do anything to hang! It just has to exert energy to get a grip on a surface. If a bat dies while roosting it actually carries on to hang there! The bats continue to rest in peace until something comes along and jostle it loose. Bat has developed a special kind of foot joint that lets it lock its foot in a tightly clasped position, with the weight of the body keeping the foot locked. By hanging from a safe and secure perch, and locking this joint, the bat can drift off into torpor without worry of falling. Since it does not need to use any muscle to hold its grip the bat can "sleep" for as long as its body fats can sustain it.
“An Anatomical Perspective”
Dr. Yogita Pandey, Dr. Rakhi Vaish and Dr. Nidhi Gupta
College of Vety. Sc. & Animal Husbandry, Jabalpur