General characteristics

Alligators, crocodiles, turtles, lizards, and snakes are reptiles. A reptile’s skin is covered with scales or with tough, horny plates that keep the reptile from drying out. Since a reptile loses little water through its skin, some reptiles are able to live in very dry places. Some even live in the desert.

Though reptiles no longer rule the land, they still play the major ecological role in most of the world’s biological communities. This is especially true in the tropics, deserts and warm grasslands.

Modern reptiles have a waterproof skin and produce amniotic eggs. All reptiles also have strong, bony skeletons and well-developed lungs. Most of them have two pairs of limbs of amphibians. Reptilian feet have toes with claws that are used for digging or for climbing on trees and rocks.

The nervous system of reptiles is similar to that of amphibians. The brain is small in relation to the body. Despite the small brain, reptiles have shown complex behavioral patterns, including elaborate courtship rituals.

The excretory system of reptiles is modified to minimize water loss. Water is absorbed into the body through the intestine, bladder, a pair of kidneys, and cloaca.

Like amphibians, most reptiles have a three-chambered heart. However, crocodiles and their relatives have four-chambered heart, a structure more efficient at separating oxygenated and deoxygenated blood.

Like many other kinds of animals, reptiles lay egg, which is called amniotic egg. Because most reptiles live on land, their eggs and sperm are not left in water as those of most amphibians are. Instead, the male deposits sperm inside the female’s body. There, the sperm combine with eggs. A protective covering, or shell, forms around the eggs. Then, in time, the eggs are laid in some kind of nest.

The shell is important to the baby reptile that is developing inside. The shell keeps the reptile from drying out. It also protects the reptile from other animals during the time before hatching. Another part of the egg – the yolk – is also important to the baby reptile. The yolk is the food for the reptile before the egg hatches.

When a reptile egg hatches, the baby reptile looks like a tiny adult. That is, a reptile does not go through metamorphosis as it becomes an adult.

Class Reptilia is divided into several orders, and among them (the most famous) are:

Crocodilia (the crocodiles, alligators, and related species adapted for life in shallow water).

Chelonia (turtles and tortoises). Their bodies are enclosed within two bony shells.

Squamata (lizards and snakes, reptiles with long slender bodies).

The order Crocodilia includes about 250 species of alligators, crocodiles, and their smaller relatives, caimans and gavials. Crocodilians are active water-dwelling carnivores.

Alligators and crocodiles have large jaws and a powerful tail. Like most reptiles, they eat other animals. Small animals may even be swallowed whole. The crocodile group is made up of reptiles that live in water and are good swimmers. The crocodilian’s eyes and nostrils rise above the rest of its head. This adaptation allows a crocodilian to remain submerged and still see and breathe. A special valve between its mouth and nose passage keep water from entering the breathing passage even when the animal opens its mouth to feed under water. Crocodilians differ from one another mainly in the structure of their head and teeth. The alligator has a broad head with a rounded snout. The crocodile has a triangular head with a pointed snout. In alligators, the teeth of the lower jaw fit inside the teeth of the upper jaw. In crocodiles, most of the upper and lower teeth mesh together. Many of these reptiles, such as crocodiles drift quietly in the water with their nostrils and eyes sticking out. In this way, they hunt their prey which includes fish, swimming birds and turtles. Large alligators and crocodiles can be quite dangerous to people and to other large animals. The alligator or crocodile usually attacks other animals in the water. The alligator or crocodile will pull the other animal under the water and hold it under until the animal drowns. While holding the other animal under, the alligator or crocodile keeps its own nostrils above the water.

The female lays eggs in piles of rotting plant material or in holes in the ground. She stays near the nest until the eggs hatch. Then she stays with young animals until they can take care of themselves. Other kinds of reptiles do not stay with their young in this way.

Alligators and crocodiles often make dens in riverbanks. The entrance to the den is underwater but the «living room» of the den is above the water level.

Turtlesandtortoises make up the second group of reptiles. These reptiles live in armour-like shells that provide protection. Some turtles, such as a box turtle, have a shell that covers their whole body. The shell of some other kinds of turtles covers only part of their body. They also have strong, sharp beaks without teeth. The limbs of sea turtles are flippers. The land turtles – tortoises – are especially well-known for their long life spans. Some species have lived for over 150 years in captivity.

Most turtles move rather easily in water. However, most turtles cannot move very fast on land. Their shell is often just too large and heavy.

You may remember that amphibians go to the water or to a place that is wet to lay eggs. But turtles that live in the sea do just the opposite. They return to the land to lay eggs. One such turtle is the large green turtle. Its legs are more like paddles than legs, so it moves very slowly on land. Its eggs are laid in beach sands above the usual high-tide levels.

A female turtle leaves her eggs to hatch in the warm sand. After the eggs hatch, the baby turtles must make their way down to the sea by themselves. The shell of a very young turtle is not hard enough to protect the turtle. Many of them are caught and eaten by birds and other animals before they get to the sea.

Members of the order Squamate are the most successful of the modern reptiles. The two major kinds of squamates are lizards and snakes.

Lizards. Most of these reptiles have four legs with five clawed toes on each foot. Lizards range in size from five centimeters to about three meters. Monitor lizards, such as Komodo dragons, are the largest lizards. They can weigh 140 kilograms.

Lizards live in brush piles, near fallen trees, in rocky places, in burrows, or in any other place where they can hide from their enemies. Lizards have a keen sense of sight and smell to help them find food. They eat insects, spiders, snails, worms, mice, young birds, and even other lizards. They have teeth for grabbing and holding the animals they eat, but they do not use their teeth for chewing. Instead, they swallow their food whole. Some animals that eat lizards are other reptiles, foxes, coyotes, and birds.

Lizards are protected by their alertness and their speed in seeking cover. They may also protect themselves by fooling their enemies. Some lizards have a tail that breaks off easily. An animal trying to catch a lizard would be most likely to grab the tail. If the tail breaks off, the lizard might be able to escape. Then the lizard will grow a new tail.

Snakes make up the fourth main group of reptiles. Snakes are like lizards in some ways. They tend to live in the same kinds of places and eat the same kinds of food. But there are many differences between snakes and lizards. Snakes do not have legs as lizards do. Snakes have jaws that open very wide and have a single row of scales along their belly. Lizards do not. Snakes and lizards occasionally shed their outer skin coverings. Snakes are also unusual in that they do not have limbs or ear openings.

All snakes molt. That is, they lose their outer layer of skin. Even the coverings of their eyes come off. The skin first becomes loose near the snake’s head. After the snake gets its head out of the old skin, the snake crawls out of the rest of the skin. Sometimes the skin turns inside out as the snake crawls out. A snake may molt more than three times a year. Generally, young snakes molt more often than old snakes because young snakes grow more rapidly.

Some snakes are poisonous. That is, they put poison into whatever they bite. However, most snakes are not poisonous. Of about 2,700 kinds of snakes in the world, only about 270 kinds are poisonous. Snakes are helpful to people because they eat harmful animals such as rats, mice, and certain insects.

Snakes can swallow animals larger than themselves. Their lower jaw is not attached to the upper jaw in the same way that it is in other reptiles. This allows a snake to open its jaws very wide. Snakes have unusual jaws that enable them to swallow prey much larger than the diameter of their own heads.

Poisonous snakes have two long, hollow teeth called fangs. Fangs are like hollow needles through which poison flows into a «bite». Most poisonous snakes in the USA are pit vipers. They are called pit vipers because of a depression, or pit, that they have between each eye and the nostrils. The fangs of these snakes are usually folded back against the roof of their mouth. Only when the snake wants to bite are the fangs out. Rattlesnakes, copperheads, and cottonmouths are pit vipers.

Another kind of poisonous snake is the coral snake. Its fangs are not folded back against the roof of the mouth. The fangs stay in one position. A coral snake has bright bands of yellow, red and black around its body.

Snakes do not like to be around people. All snakes avoid people if at all possible. However, snakes will strike when they are cornered, stepped on or surprised.

Throughout the world deaths from snakebites may number as many as 30,000 a year. The number of deaths from snakebites is greatest in India and countries in South-East Asia.

Most life scientists believe that, at one time, there were much more reptile groups. Fossil remains suggest that the huge dinosaurs that once roamed the Earth were reptiles. Other reptile groups probably included ocean-dwelling and flying species. Scientists are still not completely sure what caused these organisms to die out.

5.9.5. Class Aves: Birds

Birds are warm–blooded animals with complete double circulation and a skin covered with feathers. The forelimbs are wings. Flight is not a feature that makes birds unique, however. Birds share this ability with bats and insects. Birds, like the reptiles, fertilize their eggs internally and lay eggs with shells that do not dry out on land. Their embryo is enclosed in an eggshell with protective membrane. About 8.590 species are known. In their skeleton structure birds are essentially reptiles highly specialized for flight. They have feathers, however, one of the distinctions of this class, and unlike the reptiles, they maintain a high and constant body temperature, which may in part permit the high-energy output required for flight. Their bodies are lightened by air sacs and also by hollow bones.

The most massive bone in the bird skeleton is the breastbone, or sternum, which provides the keel for the attachment of the huge muscles that operate the wings. Flying birds have jettisoned all extra weight, the female reproductive system has been trimmed down to a single ovary, and this becomes large enough to be functional only in the mating season. The pectoralis muscle lowers the wing and the supracoracoideus raises it. A group of vertebrae, called sacral vertebrae, are located at the lower end of the spine. The femur is the upper leg bone, the lower leg bone is the tibia.

Most of the common nonflying birds such as the penguin and the ostrich are believed to have evolved secondary from flying types.

Characteristics of birds:

· The body is divided into neck, head, trunk and tail;

· Bones are lightweight and filled with air;

· Air sacs are found throughout the body;

· Birds lack teeth; jaws are covered by horny beak;

· Front limbs are modified into wings; hind limbs are adapted for perching, hopping, swimming or other similar activity;

· Body temperature remains constant;

· The heart has four chambers;

· Reproduction involves the production of an amniotic egg most species incubate in a nest.

One big difference between birds and all the other animals you have studies so far is that birds are warm-blooded. The body temperature of cold-blooded animals is about the same as their surroundings. So, cold-blooded animals can not live where the air temperature stays below the freezing point of water for long periods of time. The body temperature of the warm-blooded animals does not depend on their surroundings. Their body temperature always stays about the same. A temperature is about 41 degrees of C is normal for birds. Birds keep this high temperature because they have a high rate of metabolism. That is they turn food into energy rapidly. But birds also need a lot of energy because they are active. They also need a lot of energy to keep their body temperature up – especially in cold weather. Because in their high rate of metabolism birds eat a very large amount of food for their size. In fact, some birds eat their own weight in food each day.