Mammals have a large number of features that are not present in other species. Two of these distinguishing characteristics are the presence of hair and the possession by the female of structures called mammary glands which secrete milk for the young.
These two features are fairly obvious in most mammals, especially in the familiar domestic species such as cows, horses, dogs, and cats. The hair is clearly apparent; the mammary glands, well represented by the udder and teats of the cow, are also easily discovered in the females of most species. Even the sleek-skinned whales and other supposedly hairless mammals either have the hair represented by bristles, about the mouth or possess hair during their younger stages.
Mammals are warm-blooded: their body temperature remains relatively constant despite the variation of the outside temperature. For this reason, they are not so strictly limited in their distribution as are many cold-blooded species; one type of mammal or another may be found in areas with very adverse climatic conditions.
Representative mammals include horses, cows, lions, elephants, deers, rabbits, whales, kangaroos and a host of similar animals.