Behaviour ( reptile )
Defense
Avoidance and noise
Avoidance, the commonest form of defense in the animal kingdom, is also the commonest one in reptiles. At the first recognition of danger, most snakes and lizards crawl or scamper away into the undergrowth; turtles and crocodilians plunge into water and sink out of sight. But should the danger arise so suddenly and so close at hand that flight may be hazardous, other expedients are adopted. Crocodiles, some lizards, turtles, and some snakes hiss loudly when confronted by an enemy. Rattlesnakes rapidly vibrate the tip of the tail, which consists of loose, dry, horny rings. A few snakes without rattles (e.g., the fox snake, Elaphe vulpina, of the United States) vibrate the ends of their tails rapidly, and if, as often happens, the tail hits dry leaves, it makes a sound deceptively like the rattle of a rattlesnake.
Body form and posturing
Change in body form, which is relatively common in snakes, usually involves spreading the neck, as in cobras (family Elapidae), or the whole body, as in the harmless hognose snakes (Heterodon) and DeKay’s snake (Storeria dekayi) of the United States. Some snakes inflate the forward parts of their bodies; inflation is one of the defensive actions of the large South American tree snake Spilotes and of the African boomslang (Dispholidus). Threatening postures may be assumed by snakes as they change their body form. A cobra raises the forepart of its body and spreads its hood when endangered. The typical defensive posture of a viper is with the body coiled and the neck held in an S-curve, the head poised to strike. Some lizards flatten their bodies, puff out their throats, and turn broadside to the enemy. The helmeted iguanids (Corythophanes) of Central America and the chameleons of Africa increase their apparent size in this way when approached by snakes. The Australian bearded lizard (Amphibolurus barbatus) spreads its throat downward and outward. The Australian frilled lizard (Chlamydosaurus kingi) suddenly raises a wide membrane, or frill, which extends backward from the throat. Many lizards and snakes open their mouths when threatened, but do not strike. A common African lizard, Agama atricollis, faces an enemy with head held high and mouth open to show the brilliant orange interior.
Display of colour
Display of colour in Agama atricollis may not be part of a threatening mechanism, but it is so in the instances of certain red- or yellow-bellied snakes that turn over or curl up their tails, exposing the brightly coloured undersurface. This behaviour, known in harmless (e.g., the American ring-necked snake, Diadophis) as well as venomous snakes (e.g., the coral snake, Micrurus frontalis), is displayed only by snakes having red, orange, or yellow undersides. These colours must have some significance, as yet not fully understood, to predacious animals, for they are also the common colours in insects having warning coloration. The defense mechanism of camouflage involving form and colour is common. Many arboreal snakes and lizards (e.g., chameleons) are green; some of the green snakes (e.g., the vine snakes of South America, Oxybelis, and of southern Asia, Ahaetulla) are very slender, resembling plants common in the habitat. Lizards of semi-arid and rocky country frequently are pale in colour and blotched in pebble fashion e.g., the leopard lizard (Crotaphytus wislizeni) of the southwestern United States. Mimicry of dangerous species by harmless snakes is a passive defense. Its validity as an actual mechanism of defense is, however, sometimes challenged. The venomous coral snakes (Micrurus) of the Western Hemisphere are ringed with bright red, yellow, and black. A series of relatively harmless snakes, such as Erythrolamprus and Anilius of South America and the scarlet king snake, Lampropeltis triangulum doliata, of southeastern United States, have similar colours and patterns that may confer some protection against predators.
Striking and biting
If a threatening posture does not succeed in driving off an enemy, many reptiles become more aggressive. Some snakes (e.g., DeKay’s snake) strike, but with their mouths closed. Others (e.g., the hognose snakes) strike with their mouth open but do not bite. Still others strike and bite viciously. Among the nonvenomous snakes of North America, few are as quick to bite as the water snakes (Natrix). The sole danger from the bites of these snakes is infection of the wound. Most of the dangerously venomous snakes (vipers, pit vipers, and cobras) bite in self-defense. Vipers and pit vipers usually strike from a horizontally coiled posture. From this position the head can be shot forward, stab the enemy, and be as rapidly pulled back in readiness for the next strike. From the typical raised posture a cobra sweeps its head forward and downward to bite. To strike again it raises its head and neck once more; such aggressive, defensive movements of cobras are slower than those of pit vipers. Many lizards, regardless of family and size, bite in defense. Gekko gecko of Southeast Asia bites if sufficiently threatened. Although small lizards have a bite effective against only the smallest predators, a large monitor lizard (Varanus) can inflict a painful wound with its large teeth and strong jaws. Some turtles, particularly the soft-shelled turtles (Trionyx), bite frequently, vigorously, and effectively.
Spitting
The spitting of venom by certain African cobras, the ringhals (Hemachatus haemachatus), and the black-necked cobra (Naja nigricollis) is a purely defensive act directed against large enemies. A fine stream of venom is forced out of each fang, which, instead of having a straight canal ending in a long opening near the tip as in most cobras, has a canal that turns sharply forward to a small round opening on the front surface well away from the tip. At the moment of ejection the mouth is opened slightly, and venom is forced out of the fangs by contraction of the muscle enveloping the poison gland. Usually a spitting cobra raises its head and the forepart of its body in the characteristic cobra defensive posture prior to spitting, but venom can be ejected from any position. The effect on skin is negligible; the eyes, however, may be severely damaged, and blindness can result unless the venom is washed out quickly.
Use of the tail
A few lizards, representing different families, have in common thick tails covered by large, hard, spiny scales. Such a tail swung vigorously from side to side is an effective defense against snakes, especially when the head and body of the lizard are in a burrow or wedged between rocks. Lizards’ tails are useful in defense in another way. When captured, many lizards voluntarily shed their tails, which wriggle violently, temporarily confusing the predator and allowing the lizard to escape. Each vertebra of the tails of lizards with this capacity has a fracture line and can be split on that line when tail muscles contract violently. Simultaneous stimulation of the nerves in the severed portion keeps it twitching for a few seconds after separation. Usually the tail is broken in only one place, but a few lizards, particularly the so-called glass snakes (Ophisaurus), break their tails into several pieces. The stump heals quickly, and a new tail grows; often, however, the regenerated tail is not so long as the original and has simpler scales. Snakes, turtles, and crocodiles may have their tails bitten off by predators, but they cannot break them voluntarily or regenerate them. Some snakes use their tails in diversionary tactics by raising them and moving them slowly. Species with this habit commonly have thick, blunt, brightly coloured tails. The small African python Calabaria and the Oriental venomous snake Maticora wave their tails in the air as they move slowly away from a threat.
Balling
Many snakes, both harmless and venomous, attempt to hide their heads under coils of their bodies. The body may be coiled loosely, as it is in most species with this habit, or tightly so that it forms a compact ball with the head in the centre. Balling, as the latter habit is called, is a characteristic response of Calabaria and another African python, Python regius. The African armadillo lizard (Cordylus cataphractus), a species with heavy scales on its head and hard spiny scales covering its body and tail, rolls on its back and grasps its tail in its mouth. It thus presents a ring of hard spines to a predator.
Odours
Some reptiles use musk-secreting glands when other defensive measures fail. The water snakes (Natrix), the garter snakes (Thamnophis), the alligator lizards (Gerrhonotus), and the musk turtles (Sternotherus) emit a foul-smelling substance from anal glands.
Feeding habits
With few exceptions, modern reptiles feed on some form of animal life: insects, mollusks, birds, frogs, mammals, fishes, or other reptiles. Land tortoises are vegetarians, eating leaves, grass, and in some cases even cactus. The big green iguana (Iguana iguana) of Central and South America, its relative the chuckwalla (Sauromalus obesus) of southwestern United States and northern Mexico, and the spiny-tailed agamids (Uromastix) of North Africa and southwestern Asia also are herbivorous. The marine iguana (Amblyrhynchus cristatus) of the Galápagos Islands dives into the sea for seaweed. The majority of carnivorous reptiles have nonspecialized diets, feeding on a variety of animals. In general, the smaller the reptile, the smaller is its prey. Only the very largest of living snakes the reticulated python (Python reticulatus), the Indian python (P. molurus), and the anaconda (Eunectes murinus) are capable of eating large mammals such as small pigs and deer. Among crocodiles the largest species the Nile crocodile (Crocodylus niloticus), the East Indian saltwater crocodile (C. porosus), and the Orinoco crocodile (C. intermedius) have been known to attack and to eat men. Presumably, even larger prey was devoured by the great carnivorous dinosaurs such as Allosaurus and Tyrannosaurus, which were almost certainly capable of killing the largest of their herbivorous contemporaries.
Locomotion
Walking and crawling
The majority of reptilian orders are quadrupedal i.e., four-legged. Among the land vertebrates, the limbs gradually shifted from a lateral to a ventral position. In most amphibious reptiles the limbs projected out to the side and then bent downward to the ground at the knee and elbow. With few exceptions, the quadrupedal reptiles have the same awkward position. With such an orientation, the centre of gravity of the body is not in the same axis as the hands and feet, resulting in a sideways as well as a forward component of thrust when the animal walks. The typical reptile throws its body into a slight horizontal curve to progress straight forward. In mammals the limbs are directly underneath the body, the centre of gravity is in the axis of the limbs, and all of the thrust of the limbs is directed forward. The latter position and type of motion are more efficient. The lateral orientation of the limbs in amphibians and reptiles also makes it more difficult to raise the body off the ground. Despite the awkwardness of the orientation of their limbs, some reptiles are (and many extinct forms probably were) capable of moderate speeds. Crocodilians raise their bodies off the ground and make short, fast rushes. Short-bodied lizards also can move fast for short distances; longer bodied lizards have greater difficulty in raising their bodies. They usually have short legs and proceed in a serpentine fashion, with the body, thrown in horizontal curves, doing much of the work. A snake moves by pushing backward against rocks, sticks, or any relatively fixed point a lump of earth or a small depression in uneven ground with the rear surface of the horizontal curves of its body. Each joint of the body passes through the same curves, pressing against the same object and thrusting the forepart of the body forward. Heavy-bodied snakes such as pythons and certain rattlesnakes can move forward without throwing their bodies into curves. This rectilinear movement depends on the ability of snakes to stretch or contract their bodies in the longitudinal axis. By raising a part of its belly, stretching that part forward, lowering it to the ground, and repeating the process alternately with other parts of the body, a heavy snake moves forward smoothly in a straight line. Some modern lizards have adopted semi-bipedal locomotion. The collared lizard (Crotaphytus collaris) of the United States and the frilled lizard (Chlamydosaurus kingi) of Australia show the early stages of bipedalism, a phenomenon widespread among the dinosaurs and therefore important in reptilian history. These lizards run on their long hindlegs with the forward parts of their bodies at an angle of about 60° off the horizontal. Presumably, bipedalism among the dinosaurs began as it did among modern lizards, as an occasional means of obtaining bursts of speed. Because the centre of gravity is in front of the hips, modern bipedal lizards must move forward continuously in order to maintain a semi-erect posture; they can stand still in that position only for very short periods. The awkward sideways orientation of the limbs forces bipedal lizards to swing each leg outward as it is brought forward and to push the body sideways and forward when each leg thrusts backward against the ground. Bipedal dinosaurs eliminated this inefficient rocking motion, for during the course of evolution their hind limbs were rotated forward so that they were directly under their bodies. Thus, they delivered their full force in the forward direction. So successful was this mode of locomotion that dinosaurs utilizing it dominated terrestrial life for millions of years.
Clinging and climbing
Associated with arboreal life are groups of anatomical features mainly concerned with clinging. The commonest clinging structures in vertebrates are claws; they seem to be the only arboreal adaptations of some lizards, such as the common iguana (Iguana iguana). Similar structures appear in many lizards of the family Gekkonidae, in the anoles (Anolis) of the family Iguanidae, and in some skinks of the family Scincidae. Pads on the feet consist of wide plates or scale under the fingers and toes. The outer layer of each plate or scale is composed of innumerable tiny hooks formed by the free, bent tips of cells. These minute hooks catch in the slightest irregularities and enable geckos to run up apparently smooth walls and even upside down on plaster ceilings. Because the hooklike cells are bent downward and to the rear, a gecko curls its toes upward to disengage them. Thus, when walking or running up a tree or wall, a gecko must curl and uncurl its toes at every step. The giant Solomon Islands skink (Corucia), true chameleons (Chamaeleontidae), arboreal vipers, boas, and pythons use prehensile tails that is, tails capable of supporting most of the weight of the animal or used habitually for grasping for clinging to their aerial supports. For this purpose, however, true chameleons rely mainly on a tonglike arrangement of their digits, which are united into two opposed bundles on each foot three on the inside and two on the outside of the front foot, and two on the inside and three on the outside of the hindfoot. Slender vine snakes of several genera of the family Colubridae are capable of extending half the body length in a horizontal plane without support; they do so habitually in bridging the gap between branches. Most snakes can reach across an open space, but all except the vine snakes can extend only a short length of the body, and that portion invariably sags like a cable. The vine snakes bridge an open space like an I-beam. This ability is based partly on reduced body weight and partly on deepened and strengthened vertebrae.
Swimming
In water, of course, neither bipedal nor quadrupedal locomotion is very effective. Aquatic reptiles, with few exceptions, use the same means of propulsion as do fish and whales—that is, powerful beats of the tail. Crocodilians and aquatic lizards such as some monitors (Varanidae) lash their tails from side to side while holding the limbs against the body. The same method was used by the ancient mesosaurs (Mesosauria) and ichthyosaurs (Ichthyosauria). The marine ichthyosaurs, which were the reptilian counterpart of the porpoises, may have used their very short limbs for steering. A fishlike method of swimming requires a flexible body and at least a moderately long tail. Turtles propel themselves by using their feet as paddles the hindfeet, which have webbed toes, in the case of freshwater turtles, and the forefeet, which are modified into large paddles, in the case of marine turtles. The extinct marine plesiosaurs (suborder Plesiosauria), with their short bodies and tails and their large paddle-like limbs, swam the way marine turtles do, although they may have used their hindlimbs for more than just steering. Both pelvic and pectoral (shoulder) girdles were modified in the plesiosaurs into structures having small upper portions and very large lower portions. As the upper element, especially that of the pelvic girdle, has the important function of transferring the weight of the body to the limbs, it is likely that the limbs of plesiosaurs could not support the body weight on land and that the plesiosaurs never came out of water. Most plesiosaurs had long necks. By moving toward their prey with the neck curved, they probably could strike suddenly. The heavy trunk would provide the inertia against which the neck could move, thus preventing a significant backward shift of the animal as the head shot forward. The modern sea snakes (Hydrophidae) show the same adaptation. Though they swim with an eel-like undulation of the body, the sea snakes have relatively small heads, slender necks, and very heavy middle and rear sections. With most of the body mass concentrated in the second half of the animal, almost all of the force of the strike is used to drive the head forward.
Flying
Three groups of reptiles have experimented with flight. Thecodontia, a group of Archosauria (the so-called ruling reptiles, which included dinosaurs and crocodilians), became highly successful at this means of locomotion and evolved into birds. A second group of archosaurs, the Pterosauria, developed wings that were supported along the front margin by the arm and an extremely elongated finger. The pterosaur wing was made of skin; since it lacked both internal supports and feathers, it probably lacked the flexibility or durability of a bird wing. Flight of the pterosaurs presumably amounted to soaring and gliding. It is not understood how they moved when not flying and how they managed to take off if they happened to land on level ground. Since most remains have been found in marine deposits, it is assumed that they lived along ocean shores, probably roosting on cliffs from which takeoff would have been easy. The third experiment with flight was made by a group of modern lizards (Draco). The “wing†of these small lizards consists of skin supported by five or six elongated ribs between the arm and leg. At rest the ribs and the wings are folded against the sides of the body. In flight the wings form broad semicircles from arm to leg on each side. These flying lizards, which live in the forested country of Southeast Asia and the East Indies, are capable only of gliding. A flying lizard launches itself from a tree into the air and glides toward another tree, turning upward sharply just before lighting on the new perch. Since the arms and legs are not modified, this lizard is capable of scampering about like any strictly arboreal lizard.
[tags]behaviour, reptile, animal, species[tags]
