Derived from the Latin word 'mamma', Mammals are a group of warm-blooded animals. The female ones carry mammary glands (produce milk) to nourish their young ones, hence, they belong to the class Mammalia. The presence of neocortex, fur, and three middle ear bones distinguishes them from other reptiles and birds. Except for platypus and echidna, all other mammals (including humans) give birth to the next generation. They are blessed with well-developed brains. Studies say that there are more than 5400 mammal species on earth. Out of which one-fifth are known to be threatened or extinct.