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Living Habits of Reptiles (Part 5)

© Janat Khatoon

One factor involved in the distribution of animals is the distance to be covered and another is the time that has elapsed, a third consideration is the innate ability of the potential colonist to get across the ability of the animal to survive in the new place when it gets there. No matter how narrow the barrier, how long the time, or how great the dispersal ability of an animal, its spread will finally be limited by the amount of appropriate habitat. In trying to account for the distribution of the fauna of an area it is often not easy to distinguish between the effects of history and ecology.

The three major media of the world-earth, air and water-have all been repeatedly invaded and exploited by the reptiles in many different ways. Most reptiles live on the surface of the ground or in trees; but they can be found as neighbours of the earthworms, or gliding through the air, and, of course, their ancestors once achieved the art of flight. There is no modern reptile capable of sustained aerial locomotion. The most advanced aerial reptiles that ever lived, the pterosaurs of the Mesozoic, are all extinct. Though primarily able to soar and glide, they were in some cases surely capable of flapping flight, and may even have fed on the wing, like an albatross or a pelican. The other important-and more lasting-aerial venture during the age of Reptiles was made by the line that became birds.

Although true flight has been lost to the reptile line, there are many arboreal snakes and lizards in which body weight has been drastically cut down, making it possible for them to fall from great heights without injury. In fact, they often scurry straight way to the nearest tree and go up it, as if the incident had not even broken their train of thought. The light-bodiedness of such tree-climbing reptiles is itself an aerial adaptation that enhances survival by checking acceleration before a dangerous speed of fall is reached. From this to parachuting, by spreading flaps of skin, broadened feet and legs, or the rib-supported edges of the body wall to increase buoyancy, is another step along the same adaptive path, and one that has been taken by various modern lizards and snakes. In some of these-notably Chrysopelea, the oriental "flying snake", and the lizards of the genus Draco-the broad surfaces are so extensive that the fall actually becomes a long glide and the direction of the flight may even be partially controlled by the gliding animal. The whole line of development from the arboreal habitat to aerial locomotion seems such a clear one that the lack of any recent flying reptile appears as an incongruous gap.

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