Answer to the following questions.

1. How many species of animals live on the earth?

2. How many groups do scientists divide living things?

3. What animals belong to the group of land animals?

4. What is the main difference between tame and wild animals?

5. What animals spend part of their lives on land and part in water?

6. What animals are called warm-blooded / cold-blooded?

7. What is the classification of animals connected with?

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MAMMALS

Mammal is a vertebrate (backboned animal) that feeds its young on the mother's milk. There are about 4,000 kinds of mammals. Cats and dogs are mammals. So are such farm animals as cattle, goats, hogs and horses. Mammals also include such fascinating animals as anteaters, apes, giraffes, hippopotamuses and kangaroos. And people, too, are mammals.

Mammals live almost everywhere. Such mammals as monkeys and elephants dwell in tropical regions. Arctic foxes, polar bears and many other mammals make their home near the North Pole. Only one group of mammals, the bat, can fly.

Mammals differ from all other animals in five major ways. Mammals nurse their babies - that is, they feed them on the mother's milk. No other animals do this. Most mammals give their young more protection and training than do other animals. Only mammals have hair. All mammals have hair at some time in their life, though in certain whales it is present only before birth. Mammals are warm­blooded - that is, their body temperature remains about the same all the time, even though the temperature of their surroundings may change. Birds are also warm-blooded, but nearly all other animals are not. Mammals have a larger, better-developed brain than do other animals. Some mammals, such as chimpanzees, dolphins and especially human beings, are highly intelligent.

Pouched mammals or 'marsupials' like kangaroo and wallaby and the modern 'placental' mammals give birth to their young. The main placental groups include bats, monkeys, apes, rodents, wild dogs and cats, sea lions, elephants, horses, rhinos, antelopes and deer.

Since the earliest times, human beings have hunted other mammals. Prehistoric people ate the flesh of wild mammals, used their skin for clothing, and made tools and ornaments from their bones, teeth, horns and hoofs.

About 10,000 years ago people learned they could domesticate (tame and raise) certain useful mammals. Although domestic mammals provide many products, people still hunt wild mammals such as antelopes, deer, rabbits and squirrels for their flesh and hides. Whales are killed for their meat and oil. Beavers, muskrats, otter and other wild mammals that have thick coats are trapped for their fur. Elephants, hippopotamuses and walruses are killed for their tusks,'which consist of ivory.

Mammals have many ways of life, and each species has a body adapted to its particular way of life. However, all mammals share some basic body characteristics. These characteristics include certain features of their skin and hair, skeleton and internal organ systems.

Mammals can be divided into three groups according to the way in which the fertilized egg develops into a new individual. These groups are placentals, marsupials and monotremes. Placentals give birth to a fairly well developed offspring. Marsupials give birth to a very tiny, poorly developed offspring. Monotremes do not give birth to the live young. They lay eggs.

Answer to the following questions.

1. What is a mammal?

2. What basic characteristics do mammals share?

3. What domesticated mammals do you know?

4. How do mammals differ from all other animals?

5. What did prehistoric people use wild mammals for?

6. Where do mammals live?

7. What groups can mammals be divided into according to the way in which the fertilized egg develops into a new individual?

Text for annotation WHAT IS MEANT BY “BIOLOGICAL CLOCKS”, AND HOW DO THEY WORK?

Sometimes you don't have to look at a clock. When you get hungry, you know its dinner time. When you get sleepy, you know its bedtime.

There is a cyclic change inside you that makes you feel hungry every so often and sleepy every so often. These changes are quite regular, so that you can measure time (rather roughly) by these cycles. Such cycles are an example of "biological clocks".

There are steady cycles in the world outside the organism. The most noticeable one is the alternation between the light of day and the dark­ness of night, but there is also the twice-daily rhythm of the tides which varies in amplitude with the monthly phase change of the moon, and there is the temperature cycle which varies with the day-night period and with the annual period of the seasons.

It is useful for an organism to respond to these changes. If its food is to be found by night or only in the warm season, it might as well sleep during the day or hibernate during the winter. If it is going to lay its eggs on the shore, it can do it best at the highest high tide that comes with the full moon. Even plants respond to these rhythms so that leaves curl at sunset, flowers or fruit come at particular seasons, and so on.

We can't suppose that living organisms do all this consciously. They don't say "It's nighttime, I shall sleep," or "The days are growing short­er, I shall drop my leaves." There are, rather, automatic cycles within the organism that match the astronomic cycles in the world outside. This match is produced by natural selection. Animals or plants that possess a good match do better and have a chance at more off-spring than those with a poor match, so that generation after generation the matches improve.

The inner cycles exist even on the molecular level. Body temperature shift up and down regularly, so do the concentration of certain consti­tuents of the blood, the susceptibility of the body to certain drugs, and so on. Most of these cycles take about a day for the completion of one up-and-down movement, and these are called "circadian rhythms," from a Latin word meaning" about a day."

Is the inner cycle controlled by the environmental rhythms? Not entirely. If an animal or plant is placed in an artificial environment in which the outside rhythm is removed — where there is constant light or constant temperature — the rhythms go on anyway. They may be less marked and may vary somewhat from a strict 24-hour cycle, but they are there. The environmental rhythms act as no more than a "fine control".

Men and women who jet across great distances find themselves in a radically different time zone, and their internal rhythms no longer match the day-night period. This gives rise to many uncomfortable symptoms until the biological clock is reset.

As to how the biological clock works I can tell you in two words: Nobody knows!

Is it some sort of periodic chemical reaction in the body? If so, the clock should vary with temperature or with drugs, and it doesn't. Is it something that is keyed to very subtle rhythms in the outer world that persist even when we wipe out light and temperature variations? Maybe, but if so we have not yet discovered the nature of those rhythms.

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