Found in variety of habitats-polar ice caps, mountains, forests,grasslands and dark caves
Some of them are adapted to fly or live in water
Presence of mammary glands for nourishing the young ones - most unique mammalian character
Two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying
Skin of mammals is unique in possessing hairs
External ears or pinnae are present
Different types of teeth are present in the jaw (Heterodont teeth)
Heart is four chambered
Homeothermous
Respiration by lungs
Sexes are separate
Fertilization is internal
Viviparous (with few exceptions)
Direct development
