Mammals are a class of animals that give birth to live young and feed them with mother’s milk. There are about 4000 – 5000 different species of mammals. The largest known mammal is the blue whale and the smallest known is the bumblebee bat. All mammals are warm-blooded meaning their body temperature stays the same irrespective of the atmospheric temperature. They do this by generating their own internal heat. All mammals have hair and backbone. They need air to breathe and the females produce milk. Body hair helps retain internally produced heat. Their brains are normally bigger and more complex than any other animal’s. They communicate to each other by sounds, touch, visual clues and odor.
The mammals may be grouped as placental, marsupials and monotremes. Lions, mice, elephants, horses and hippos are placental mammals. Placental females have a structure called placenta which connects the embryo with the mother. Marsupials like kangaroos, Tasmanian wolves, marsupial mice and moles give birth to young ones that are at a very early stage of development. For example, young kangaroos crawl into their mother’s pouch to continue growing.
Monotremes are an exception in a sense that they lay eggs instead of giving birth to live young. Spiny anteaters and the duck billed platypus are the only egg laying mammals called as monotremes. They hatch from eggs warmed outside the mother’s body. Like other mammals, they are also fed milk by the mothers. For instance, young platypus licks the milk droplets that falls off the mother’s hairs attached to the mother’s stomach.