The class Mammalia, encompassing over 6,600 species, is defined by the presence of hair, three middle ear bones, and mammary glands. These specialized glands produce milk to nourish the young, a characteristic from which the class name “mammal” originates. The structures that deliver this milk—nipples or teats—are a common feature across the vast majority of the group. However, the universality of this milk-delivery structure depends on the evolutionary history and diversity within the mammalian lineage.
The Universal Trait and Biological Purpose
For the two largest groups of mammals, the marsupials and the placentals, the nipple is a standardized biological structure. Its primary purpose is to deliver milk from the underlying mammary gland to the nursing offspring. Milk is produced within secretory cells and travels through a network of ducts that converge at the nipple. Suckling by the infant stimulates nerve endings, triggering the release of the hormone oxytocin. This hormonal signal causes muscle-like cells to contract, forcing milk out through the ducts in a process known as the milk-ejection or “let-down” reflex.
Mammals That Don’t Have Nipples
While the vast majority of mammals rely on nipples for nursing, a small and ancient group proves the exception: the monotremes. This unique order includes the platypus and four species of echidnas, which are the only mammals that reproduce by laying eggs. Female monotremes possess true mammary glands and produce milk, confirming their place within the class Mammalia. However, they completely lack the raised, centralized nipples or teats found in other mammals. Instead, milk is secreted through specialized pores or ducts that open directly onto a patch of skin on the mother’s abdomen. The young, called puggles, do not suckle but instead lap up the milk that pools in grooves or clings to the surrounding fur. This less direct method of feeding is considered a more primitive form of mammalian lactation.
Diversity in Nipple Number and Arrangement
For the mammals that do possess nipples, the number and physical arrangement on the body vary dramatically, generally correlating with the species’ typical litter size. This variation is a result of embryonic development along the “milk line,” or mammary ridge, which extends from the armpit area down to the groin. Nipples develop symmetrically and in pairs along this ridge. Mammals that typically give birth to a single or small litter, such as primates, horses, and goats, tend to have a limited number of nipples, often only one or two pairs. Conversely, animals known for large litters, like pigs, mice, and dogs, can have 10 to 18 nipples arranged in a double row along the abdomen. This arrangement maximizes the mother’s ability to nurse all offspring simultaneously.
Why Male Mammals Have Nipples
The presence of nipples on male mammals, which do not lactate, is explained by the timeline of embryonic development. All developing mammalian embryos, regardless of their eventual sex, follow the same initial genetic blueprint. This early developmental path includes the formation of the mammary glands and nipples, which occurs very early in gestation. In humans, nipples begin to form before the sixth or seventh week of pregnancy. Sexual differentiation, triggered by the presence of the SRY gene in males, only begins to influence the reproductive system later. By the time the male fetus begins producing testosterone, the nipples are already established structures and their development is largely complete. Because having nipples is not detrimental or costly, there has been no evolutionary pressure to eliminate them, leaving them as a harmless remnant of the shared developmental history.