It is estimated that breast-feeding began more than 200 million years ago, the time of the first mammals (Which laid eggs and hatched them). Subsequently no less than 18,000 different species of mammals evolved.

The kangaroo has the unique ability to produce two types of milk from two teats for young of different ages. It is curious that the number of teats or nipples varies throughout the mammalian world. The horse and elephant, for instance, have only two nipples; and though the cow has only one udder the single massive gland empties into four teats. The dog has five paired glands. The hog can have as many as eighteen mammae. Supernumerary nipples are common in many species, e.g. primates, rodents, ruminants. The largest number of nipples is present in Centetes, a primitive insectivore with twenty-two to twenty-four nipples and as many as thirty six young at birth.