Mammals are the highly evolved animals on the planet. As per evolution theory, they are originated from the reptiles like the birds. But they have some interesting facts.
1. Give birth to young ones: Mammals unlike birds, reptiles etc, do not lay eggs. They give birth to fully formed young ones. The young ones are developed in the womb of the mother and are delivered when fully formed. The only exception to this is the platypus and spiny anteater (echidna).
These mammals lay their eggs and once the babies are out, they nourish them with milk.
2. Mammary glands: Mammals are named so due to the presence of mammary glands. The female mammals produce milk through these glands to nourish their newly born offspring.
3. Ability to fly: Mammals and birds are said to be evolved from reptiles. But unlike birds mammals cannot fly. The only exception to this rule is the Bat or flying fox.
They have wings which enable them to fly.
4. Socialize well: Most animals like fish, amphibians, molluscs etc. do not socialize much. But mammals socialize very well. This is for the reasons like safety, rearing of offspring, food collection etc.
5. Present in all the climatic conditions: They are present on the land, in oceans and even at places with extreme climates. You can find camels in the hot and dry deserts, polar bears in the arctic circle with extreme cold and ice conditions. Many mammals are also found on the hill and mountain tops, forests, deep caves, grasslands etc.
5. Mammals have well-developed organs. Most organs like the heart, brain, kidney which are not highly evolved in other animals are developed in mammals. For examples, animals like fish, reptiles, amphibians have two or three chambered hearts. But mammals have four-chambered hearts. Two chambers are meant for handling of pure blood and two for pumping impure blood.
6. They are the largest of all the animals on the planet. Of all the animals, mammals are the largest animals. An elephant is the largest one on the land while the blue whale is the largest in the oceans.
They weight in few tones and measure up to few meters.
7. They have the same set of the organ system. All the mammals have the same set of organs and organ systems. In other animals, there is variation in organs and organ systems in between species and genus. This is not so among mammals.
8. Obligate vegetarians: Most animals are either vegetarians or non-vegetarians. They are clearly defined in most cases.
Mammals are the only group which have obligate vegetarians. These include the cow, goat, rabbit, buffalo, monkeys, horse, deers etc. These are pure vegetarians in class.
Some of them are pure non vegetarians like the lions, tiger, hyena, etc.
Few of the mammals prefer both type of diet. They can eat meat or vegetarian diet based on availability. These include mammals like the dogs, cats, rats etc.
Further, they tend to be part of food chain. They are available for other animals as a part of the diet. For examples, birds eat rats, goats, monkeys, deer etc. Reptiles like the snakes, crocodiles eat rats, squirrels, buffaloes, humans etc.
9. Concentrate their urine. Most animals cannot concentrate their urine. This is because of lack of the Henley’s loop and vasa recta. In animals these are present and the urine flows in opposite directions creating counter current.
10. Presence of hair and glands in the skin: Most animals have scales on the skin like in fishes, birds, reptiles etc. Orelse the surface could be slimy like in the amphibians, mollusks etc.
But in animals, skin gives out prominent hair. Besides, the skins may also have glands beneath it.