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 do not socialize much. But mammals socialize well for reasons like safety, rearing of offspring etc.
5. Present in all the climatic conditions: They are present on the land, in oceans and even at places with extreme climates. Camels in the desert, polar ice regions, mountains, forests, caves, grasslands etc.
5. Mammals have well-developed organs. Most organs like the heart, brain, kidney which are not highly evolved are developed in animals. For examples, animals have a four-chambered heart while reptiles have a three-chambered heart.
6. They are the largest of 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 non-vegetarians. They tend to other animals as a part of the diet. For examples, birds eat reptiles, fish, amphibians, insects etc. Reptiles like snakes eat amphibians, fish, birds, mammals etc. Mammals are the only group which has obligate vegetarians. Mammals like the cow, goat, buffalo, monkey are pure vegetarians.
9. Concentrate their urine. Most animals cannot concentrate their urine. This is because of Henle’s loop and vasa recta where the urine flow in opposite directions creating countercurrent.
10. Presence of hair and glands in the skin: Most animals have scales on the skin like in fishes, birds, reptiles etc. Or the surface is slimy like in amphibians, mollusks etc. But in animals, skin gives out prominent hair. Besides, the skins also have glands beneath it.