Nipples exist primarily to deliver milk from a mother’s body to her infant. They are the exit point for a network of ducts inside the breast that collect and release milk during breastfeeding. But nipples do more than just dispense nutrition. They play roles in hormonal signaling, infant bonding, sensory response, and even helping newborns find food through scent. And the reason everyone has them, regardless of sex, comes down to how human embryos develop.
How Nipples Deliver Milk
Each nipple is pierced by 15 to 20 tiny openings called lactiferous ducts, arranged in a radial pattern like spokes on a wheel. Each duct drains an entire lobe of the breast. Just before reaching the nipple surface, these ducts widen into small reservoirs where milk collects between feedings. When a baby suckles, the milk stored in these reservoirs is released.
The nipple also contains smooth muscle fibers running in both circular and longitudinal directions. These muscles cause the nipple to become erect when stimulated, which helps an infant latch on. The tissue is densely packed with nerve endings, making it one of the most sensitive areas on the body. That sensitivity isn’t incidental. It’s the trigger for the entire milk production cycle.
The Hormonal Chain Reaction
When a baby suckles, the pressure and stimulation activate a nerve that sends signals to the brain’s hypothalamus. In response, the pituitary gland releases oxytocin, a hormone that causes tiny muscles around the milk-producing glands to contract and squeeze milk into the ducts. This process is called the let-down reflex, and it can kick in within seconds of a baby latching on, or sometimes just from hearing a baby cry.
Oxytocin released during breastfeeding also causes the uterus to contract after childbirth, helping it return to its pre-pregnancy size. So nipple stimulation serves a dual recovery function: feeding the infant while simultaneously aiding the mother’s physical healing. The same hormone promotes feelings of calm and emotional bonding, which is one reason breastfeeding is closely linked to early attachment between parent and child.
How the Areola Helps Newborns Find Food
The darker skin surrounding the nipple, the areola, contains small raised bumps called Montgomery glands. These glands are a hybrid of oil-producing and milk-producing tissue, and they secrete a substance that serves multiple purposes at once.
The secretion lubricates the nipple and areola, protecting the skin from cracking during repeated breastfeeding. More remarkably, it produces a scent that newborns respond to instinctively. In a 2009 study published through the National Institutes of Health, researchers found that the odor of areolar secretions intensified newborns’ breathing patterns and triggered appetitive mouth movements more strongly than any other stimulus tested. Babies who had never breastfed still responded, suggesting the reaction is hardwired rather than learned. These volatile compounds help guide a newborn to the breast and may play a role in kick-starting the bonding process within the first hours of life.
Why Everyone Has Nipples
One of the most common questions about nipples is why males have them at all. The answer lies in how embryos develop. For the first several weeks of development, all human embryos follow the same body plan regardless of genetic sex. Nipples form early in this shared blueprint, before sex hormones begin differentiating male and female anatomy.
In most male mammals, including humans, the nipples simply remain after this early stage. They don’t disappear because there’s no active biological mechanism to remove them. Having nipples costs males nothing in terms of survival, so evolution has never selected against them. In fact, researchers describe male nipples as the evolutionary norm across mammals.
A handful of species are exceptions. Male mice, for instance, lose their nipples during fetal development. Testosterone produced by the fetal testes activates a specific receptor that triggers cell death in the nipple bud, causing it to shrink away before birth. Male guinea pigs produce the same testosterone, but their developing breast tissue lacks the receptor needed to respond to it, so their nipples survive. The difference between these two species illustrates that nipple loss requires a specific, active process. Without it, nipples stay put.
Nipples Across the Animal Kingdom
Not all mammals even have nipples in the traditional sense. The platypus and echidna, the only egg-laying mammals, lack nipples entirely. Instead, milk seeps through patches of skin on the mother’s abdomen, and the young lick it up from the surface. This is likely the more ancient system, predating the evolution of nipples altogether.
Marsupials took the nipple in a different direction. Newborn kangaroos and opossums, born at an extremely early stage of development, crawl to a teat and latch on immediately. In some pouchless marsupial species, the infant’s mouth essentially fuses around the nipple, forming a seal that keeps the tiny newborn attached while its jaw joint develops. The teat swells inside the mouth, creating a lock-and-key arrangement that allows passive “pump-sucking” rather than the active suckling that human infants do.
Placental mammals, the group humans belong to, evolved nipples that project outward and are designed for a baby that can actively create suction. The number of nipples across species generally tracks with litter size: dogs have eight to ten, cats have six to eight, and humans typically have two.
Common Nipple Variations
Nipples vary more than most people realize. Roughly 10% to 20% of the population has one or more inverted nipples, where the nipple retracts inward instead of pointing outward. This is usually a congenital trait, present from birth, and often causes no problems until breastfeeding, when latching can be more difficult. Various techniques and devices can help draw the nipple out for feeding.
Extra nipples, sometimes called supernumerary nipples, are also surprisingly common. Prevalence varies by population, ranging from about 0.2% in some groups to over 5% in others, and they occur at similar rates in males and females. These extras appear along the “milk line,” an embryonic ridge that runs from the armpit area down the torso toward the inner thigh. The armpit region accounts for 60% to 70% of cases. Most extra nipples are small and easily mistaken for moles.
What Nipple Changes Can Signal
Because nipples sit at the endpoint of a complex duct system, changes in their appearance or behavior can sometimes reflect what’s happening deeper in the breast. Nipple discharge is one example. Milky discharge outside of pregnancy or breastfeeding can point to hormonal imbalances involving prolactin. A sticky, multicolored discharge is often related to duct widening, a common and usually benign condition in people approaching menopause.
Watery, blood-tinged, or bloody discharge deserves closer attention. In a review of 370 patients with these types of discharge, about half had benign growths inside the ducts, 31% had fibrocystic changes, and 13.5% had breast cancer. The likelihood of a serious cause increases after age 50, particularly when the discharge comes from one side only or accompanies a lump. A newly inverted nipple that wasn’t always that way is also worth having evaluated, as it can indicate tissue pulling inward from a growth.
For people undergoing mastectomy for breast cancer, modern surgical techniques can often preserve the nipple and areola. In a recent meta-analysis of robotic nipple-sparing mastectomy, cancer recurrence rates were under 1%, and complication rates were low, with nipple tissue loss occurring in fewer than 0.5% of cases. Preserving the nipple doesn’t just improve cosmetic outcomes. For many patients, it plays a meaningful role in body image and emotional recovery.