At birth, the young no longer can rely on the direct exchange of nutrients through the placenta (or in monotremes, through nutrient stored in the egg). Thus, nutrition of young requires an additional process, and milk is the nutrient that serves that purpose. Milk is unique to mammals, and all species of mammals are capable of producing milk.
Milk production occurs in the mammary glands, which resemble sweat glands in form but become mature only following parturition or birth of young. The milk is delivered through nipples or teats (except in monotremes), and typically the number of teats is roughly twice the average litter size. Although males have fewer and smaller teats that are vestigial, only females produce milk and consequently they have larger teats. All rules have their exception and in mammals, there is one species in which males can also produce milk and nurse young, the Dyak fruit bat (Dyacopterus spadiceus).