Many mammals, specially living in forest areas, have become modified to live on trees. Their mode of living is named as arboreal. Arboreal mammals are able to climb the trees and use their branches as the highways. There are three possible modes of aerial locomotion or flight-gliding, soaring and flapping.
Gliding is the rudimentary flight displayed by certain fishes (Exocoetus), amphibians (Rhachophorus), lizards (Draco), and among some mammals by some phalangers, flying lemurs and squirrels. Soaring and flapping are more efficient and need more profound morphological and physiological adaptations. Soaring flight is attained by certain birds and flapping flight is accomplished both by birds and bats (Fig.33.9).
I. Gliding Mammals and Adaptations:
1. Gliding Mammals:
Common examples are:
(i) Order Marsupialia:
Flying phalangers (Petaurus) and feather tails (Acrobates).
(ii) Order Dermoptera:
Flying lemurs (Cynocephalus).
(iii) Order Rodentia:
Flying squirrels. Kashmir wooly flying squirrel (Eupetaurus cinereus) and brown flying squirrel (Petaurista phillipensis).
2. Gliding Adaptations:
In gliding mammals, body is elongated, flattened and streamlined. Limbs are long and equal. Tail is long and gradually tapering. There is a double fold of furred skin, called patagium or parachute membrane stretched on either lateral side between neck, limbs, body and tail. It is sometimes also provided with a cartilaginous rod springing from the elbow or wrist. When the animal is at rest, the parachute is scarcely visible as it remains folded close to the body by its own elasticity.
3. Nature of Gliding Flight:
Gliding is not a true continuous flight but merely a prologed aerial leap, covering 10 to 20 metres at the most. This type of movement involves no propulsion other than the initial force of jumping. Gliding is characterised by leaping or jumping from a high point and held up by some sustaining organs, then to glide to lower level. Thus, there is no locomotive force other than gravity.
The gliding mammals glides from one tree to another, smoothly and swiftly downwards, supported by the outstretched parachute and limbs. Aerial progress is in a straight line, rapidly losing height and with little manoeuvrability. However, the flight can be guided to some degree by changing the position of the limbs, by twisting the parachute membrane, and by using the tail. Before alighting the animal raises the front body part to check speed and to soften its impact on the landing target.
II. Flapping or Flying Mammals and Adaptations:
Bats belong to the order Chiroptera. They are the only mammals with true and sustained flight. This flight is effected by the flapping or up and down strokes of wings. Bats are built for flight and compete very well with birds.
Flight Adaptations of Bats:
Bats possess many adaptations which are infinitely more profound. The modifications are not so conspicuous in their external features. But radical changes have taken place internally in their skeleton and musculature. The skull of bats has been altered in the most exaggerated manner for special purpose.
Wings or patagia of bats are thin like a paper and elastic membranes. These are extensions of leathery skin from the lateral sides of body, legs and tail. Forearm is greatly elongated, carrying a hand with 5 fingers. In small insectivorous Microchiroptera, the first finger or pollex is short, free and sharply clawed.
The other 4 fingers are clawless, enormously lengthened and embedded in the web of wing to support it. Like the ribs of an umbrella, the fingers open and close the wing and keep it taut when expanded. In large fruit-eating bats or Megachiroptera, the 2nd finger also ends in a claw. The 3rd finger is the largest, corresponding to the leading edge of the wing membrane.
In most bats, an inter-femoral membrane, which also enclose the tail wholly or partially between the hindlegs. It is also supported by a spur of bone, the calcar, projecting from the tarsus of each foot. A similar ante-brachial membrane connects the neck with the humerus bone of upper arm. Thus, there is a continuous and uninterrupted parachute of skin around the body of bat.
The muscles and other body structures are specially designed in bats to support and operate the wings. Bat possess a capacious thorax containing a remarkably large heart and lungs. The keeled sternum offers space for the attachment of great pectoral muscles which sustain the arms in flight.
In bats, though converted into wings for flight, the arms and hands are also used for walking, climbing, holding food and killing prey in flight.
In bats, the hindlegs are small, weak and with sharp curved claws on toes used for suspending the bat head down from a tree branch or perch, while resting or sleeping. Knee-joints are directed backwards, instead of forwards, which helps in maximum spread of wing membranes but is of little help in other movements.
In bats, the tail is variable in size. It may be large, small or scarcely visible. When well developed, the tail supports the inter-femoral membrane which can act as a brake to flight, used as a large pouch for holding prey or food, or even as an aerial cradle for the reception of a new born baby.
In bats, the milk dentition is curious in adaptation to flight. The young are often born with small hook-like or needle-like teeth, similar and sometimes forked. These are supposed to serve the young for gripping firmly the maternal teats while the mother is flying.
Bats are extremely modified for nocturnal flight. They are able to fly even in total darkness with ease and swiftness avoiding obstacles. They possess special sense organs which allow them to perceive prey and surrounding obstacles at a distance even in total darkness.
Bats are supersensitive to sound and possess a natural echo-apparatus or radar system of their own. In flight bats keep their mouth open and continually emit tremendous bursts of supersonic or ultrasonic sounds which are quite inaudible to human ear.
The warning echos reflected back enable bats to locate and evade obstacles in their path. In numerous insectivorous bats sensitive accessory lobes on ears called tragus, help to pick up warning echos. Besides certain bats have skin flaps encircling nose, called nose-leaf.
The exact use of nose-leaf is not well-understood, but probably it acts as an antenna for the perception of air vibrations. The wing webs are abundantly supplied with scattered nerves and blood vessels and probably act as tactile organs which also help in avoiding the obstacles in their course during nocturnal flights Bats are so incredibly sensitive that thousands of them may swirl around for hours in total darkness in a cave without a single collision.