Rep. Barney Frank was "upstaged by his own chest" during a speech he gave Monday with erect nipples. According to the Huffington Post, his appearance “raised questions about how cold it was in the House chamber.” Why do nipples harden in the cold?
No one knows, but it's probably for the same reason that cold gives you goose bumps. Low temperatures stimulate the tiny muscles attached to your hair follicles, which cause the hairs to stand on end. In other, hairier mammals, this process, known as the pilomotor reflex, can produce a layer of insulation. In humans, it seems to be vestigial. The areolae (the colored areas surrounding the nipples) also have smooth muscle cells that contract when stimulated, and cold weather can make the skin pucker inward while the nipples stick out. The squeezing of these muscles may also lead to increased protrusion of the Montgomery glands—the little bumps around the nipples that secrete fluid to facilitate breast-feeding. (Both men and women have Montgomery glands; under the right circumstances, men can even lactate.) Few studies have been done on why the areolae, in particular, might be reactive to the cold.
The areola is one of the most sensitive areas of the body, and it can be stimulated in both men and women. In addition to their being sensitive to cold, nipples can also become made erect as a result of breast-feeding or sexual arousal. Both stimulate the release of the hormone oxytocin, which triggers the pilomotor reflex. In breast-feeding mothers, this can be prompted by tactile stimulation, or simply by the sight of a hungry baby. Men and women are equally susceptible to the pilomotor reflex during sexual activity, though the effect is less noticeable on smaller, male breasts.
Some people get erect nipples more frequently and easily than others, and this can be a source of pain or discomfort. In extreme cases, doctors suggest wearing breast petals (pasties), or nipple protectors (which are also available for men). Nipple protrusion also varies across mammalian species. Gorillas have been observed to develop erect nipples when physically aroused and beating their chests. Pinnipeds (including seals, sea lions, and walruses) have retractable nipples that remain tucked when pups aren’t feeding on them. Adult male rats don’t have nipples, and male horses don’t have teats. The nipples of a female dog shrivel after they get spayed. The platypus and the echidna don’t have nipples at all, but rather secrete milk through the pores in their skin.
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Explainer thanks Todd Disotell of New York University, Dana Holwitt of Montclair Breast Center, and Philip Myers of the University of Michigan.