Types of Mammals

In terms of reproductive behavior, mammals fall into three groups- the egg layers, the pouched mammals and the placentals. The egg layers or monotremes, show several reptilian features and are represented only by the platypus and the echidnas. The pouched mammals, or marsupials, are confined mainly to Australia and New Guinea. They are born at very early stage – the new born red kangaroo is about the size of a broad bean- and continue their development while feeding on milk in a pouch on the mother’s body. Placentals, which make up the vast majority of living mammals, give birth to their young at a much later stage of development. While inside the mother’s body the offspring are nourished by the placenta, through which food and oxygen pass from the mother’s blood stream to the foetal blood stream. Many placental babies are so well developed by the time of birth that they can run about almost as soon as they are born.

Man has domesticated many mammal species for both work and pleasure, and has also created many different new breeds. Small mammals, especially rodents such as mice, gerbils, hamsters and guinea pigs, make excellent pets for children. Among slightly larger animals, cats and rabbits make good pets. Dogs were the first animals to be domesticated, probably over 10,000 years. Their ancestors were almost certainly a race of small wolves that lived in Southwest Asia. There are now well over 400 recognized breeds of dogs, including a wide range of working and sporting dogs as well as the more familiar pets.

Horses were first domesticated more than 4,000 years ago, when at least two breeds of wild horses- the Tarpan and Prexewalski’s horse – roamed the steppes of central Asia. One or both of these may have contributed to the domestic horse. Many breeds of horse now exist, from tiny ponies to the great shire horses once common farm animals and now making a comeback as draught animals.

Mammals

       Mammals are generally considered to be the most advanced group of animals; this is the class that contains man and the ape. All mammals have relatively large brains and a greater capacity for learning than most other animals. Unlike that of a reptile, the lower jaw of a mammal consists of a single bone, the dentary, on each side. This of great help to paleontologists in identifying fossil skeletons. Mammals first appeared on the earth a little over 200 million years ago, the descendants of some small insect eating reptiles, but they did not come to prominence until the extinction of the dinosaurs about 65 million years ago.

        When the dinosaurs disappeared, mammals were able to investigate many different ways of life and evolve in different directions. Since then thousands of species have come and gone many of them weird experimental forms such as the shovel lipped elephant and the giant Baluchitherium. The latter looked like a cross between a rhinoceros and a giraffe and, with a weight of perhaps 30 tones, was the largest of all known land mammals. Today, however, there are only about 4,100 mammalian species. The mammal teeth vary according to the animal’s diet. Carnivores have sharp edged cheek teeth that are suitable for slicing through meat. Herbivores, in contrast, have flat grinding teeth. Rodents have chisel like front teeth.

Birds

Birds evolved from reptilian ancestors about 140 to 150 million years ago. The earliest known bird is the Archaeopteryx, several fossils of which have been found in Germany; about the size of a pigeons. Archaeopteryx had many reptilian features, including toothed jaws and lizard like tails, but it also had feathers and was undoubtedly a bird. Birds are the only animals with feathers, which clearly evolved from reptilian scales. As well as aiding birds in flight, feathers help them to maintain their bodies at a constant high temperature. Each feather has a horny shaft, a rachis and numerous branches, barbs. The barbs of the outer feathers are linked together by hooked branches, barbules. The wingtip feathers are primary or flight feathers; those nearer the body are secondary feathers.
 Modern birds have no teeth and with their front legs turned into wings, their bills or beaks have to collect and process their food. The bill of each species is admirably suited to its diet. Many birds have lost the power of flight during their evolution. The ostrich the world’s largest living bird- grew too big to fly. However, its size and fast running in short burst give it plenty of protection. Rheas in South America, emus in Australia and cassowaries in New Guinea have evolved in similar ways and have all come to look alike, although they are not closely related. Birds living on islands with no mammalian predators often became flightless. The kiwi of New Zealand is a familiar example. Some of these birds died out when man introduced cats and other mammals, and many more are now in danger of extinction. Penguins cannot fly in the air, but they use their powerful wings as flippers to fly underwater.

Fangs and Venom of Snakes

         Venomous snakes inject their venom with fangs. These are enlarged teeth – at either the front or the back of the mouth. The venom is produced in glands in the roof of the mouth. Rattle snakes and vipers have very long fangs that are folded back along the roof of the mouth when not in use. The longest fangs, up to 5cm long, belong to Africa’s gaboon viper. Snakes can swallow prey much fatter than their own bodies because they dislocate their jaws to give an enormous gape. Furthermore the ligaments stretched between bones are extremely elastic. Using their teeth like ratchets, they gradually work their jaws forward to engulf the prey. 

        Only a few venomous snakes are really dangerous to humans, but the most poisonous ones are not necessarily the most dangerous because they may be timid or not live in the places where they don’t be in the contact with the people. Many number of peoples were killed by the most dangerous snakes are primarily the Indian Cobra and saw scaled viper. Both kill thousands of people in Indian and Southeast Asia each year, although exact figures are impossible to obtain. The puff adder is probably the most dangerous of the African snakes, while the western diamond back rattlesnake kills more people than any other North American snake.

Reptiles

The reptiles came into being about 300 million years ago; descending from some kind of amphibian ancestor that gained a scaly waterproof skin and the ability to lay shelled eggs that could survive on land. Most living reptiles still lay eggs, although some give birth to active young. They live on land in fresh water and in the sea, and between them they eat almost every kind of food. There are about 6000 living species, of which some 3000 are lizards and 2700 are snakes. Turtles, tortoises, crocodiles and alligators make up the rest of the class.
Reptiles are said to be cold blooded because they have no internal mechanism for maintaining a constant temperature like birds and mammals. Many however can adjust their body temperatures by varying their behaviors. They bask in the morning sun to warm up, hideaway in the midday heat and then come out again in the evening.
The lizard like tuatara is the only living member of a lineage more ancient than the dinosaurs, but it has hardly changed since its relatives died out of over 200 million years ago. It lives only on a few off shore islands of New Zealand, having been exterminated elsewhere in the country by introduced predators.

Amphibians

The amphibians were the first back boned animals to make a go of life on land, but even then they had to return to the water to breed- just like most of today’s amphibians. They evolved from a group of air breathing fish whose fins became transformed into legs capable of supporting their bodies. With length of 4 meter and more, some of these early amphibians were very much larger than their present day descendants. Amphibians feed mainly on insects and other invertebrates, although the larger species also eat small vertebrates. Most frogs and toads catch their prey with a long sticky tongue, which is flicked out at high speed. The lungs are not very efficient and much of the animal’s oxygen is obtained through the thin skin, which is well supplied with blood vessels. The thin skin restricts the amphibians ot damp habitats.

The Amphibians are divided into numerous orders, but only three of these have living members. The gymnophiona contains the worm like caecilians of tropical areas. The Urodela contains the tailed amphibians the newts and the rather more terrestrial salamanders. The Anura contains the frogs and toads, which are tailless and mostly adapted for jumping. In Europe, the name frog is applied to amphibians with smooth skins, while toad is applied to those with warty, often drier skins. This distinction, however, is not always made elsewhere in the world.