Nipplebearing

Nipplebearing is the anatomical condition of possessing nipples, or teats, connected to mammary glands. In mammals, nipples serve as outlets for milk and their primary biological function is lactation in females, though nipples commonly occur in both sexes. The trait is widespread among mammalian orders, but its presence and form vary across species.

Nipplebearing is not universal. Monotremes, such as platypus and echidnas, lack true nipples and secrete milk onto the fur of their young. Marsupials often have multiple nipples arranged along the belly or chest, supporting the growth of several offspring. In placental mammals, the number and arrangement of nipples vary by species, with humans typically having two, though supernumerary nipples can occur as a developmental variant.

Developmentally, nipples form along the mammary ridge, or milk line, during embryogenesis. In humans, nipples appear in both sexes, and breast tissue development proceeds in females during puberty under hormonal influence, though the nipples themselves are present regardless of sex.

Evolutionarily, lactation and mammary glands are defining features of the mammalian lineage, and nipples provide the anatomical interface for delivering milk to offspring. Variation in nipplebearing reflects differences in reproductive strategies, offspring development, and species-specific lactation demands.

See also: Mammary gland, Lactation.