Mammals need to drink milk to increase their chances of survival during the young age or after it is born. The milk, the mammals get from the mammary gland of their mother are useful in increasing the immunity of the child. Without this milk the mammal will most probably not survive until the child develops its own immune system. Through the milk the mammal child gets the immune system of the mother. The protein of the milk helps the child to gain strength and grow. This is the main reason the mammals drink milk. The adults also drink milk of cows and other animals to get the required protein.