Animals that gives birth to living babies, not through eggs, and feeds them on milk are called ‘mammals’. Apart from human beings, animals like whales, cows, buffalos, goats, sheep, etc. are recognized as mammals.
Mammals are worm blooded animals and they have well-developed brains. The basic body type is a four-legged land-borne animal. However, some mammals are adapted for life at sea, air, trees or on two legs. They breathe air through their lungs and their bodies are protected with hair. Mammals use their limbs to walk, climb, swim or fly. Some land mammals have toes that produce claws for climbing or hooves for running. Aquatic mammals like whales and dolphins have flippers. which are evolved from legs.
Mammals require a lot of energy to maintain high and constant body temperature. Hence, they need nutritious and abundant diet. While the earliest mammals were perhaps predators, different species have since adapted to meet their food requirements in a variety of ways. Some mammals eat other animals to feed themselves. They are called carnivorous mammals. On the other hand, as some mammals eat plants, they are called herbivores mammals.
All female mammals nurse their young with milk. Mammary glands in female mammals help in producing milk. These glands are located in the breasts of mammals. They have a hag like shape and are big in size. Hormonal changes take place in the mother shortly before giving birth to her young. This results in increase in development of mammary glands.
Mammary glands produce large drops of fat that mixes with the fluids present in the breasts. This is converted into milk. This milk is sucked by the young ones or the new-born as their primary source of nutrition. They suck it from the teats or the nipples.
According to Wilson and Reeder’s classic reference book ‘Mammal Species of the World’, there are 5,416 known species. They are grouped in 1,229 kinds, 153 families and 29 orders. They differ from one another in their builds, shapes, sizes and habits. Most of them live on land but some live in water and air. For example the whale is a mammal that can survive in water, whereas the bat is also a mammal but it flies in the air. Most of the mammals give birth to less than ten young ones a year. However, humans usually have only one baby at a time.