from another answer: "small non-striated myocytes (muscle cells) arranged cylindrically within the nipple are responsible for the nipple becoming erect when they are stimulated" physiologically, the muscles contract from interplay of sympathetic and parasympathetic stimulation, that is stimulation of the autonomic nervous system commonly referred to as the "fight or flight" or "rest and digest" response but which is essentially employed by the nervous system for anything requiring coordinated physiological response to stimulus physiological effects of sexual stimulation are generally coordinated by the parasympathetic nerves (e.g. lubrication, erection), while resolution is coordinated by the sympathetic system (e.g. climax, orgasm) this is why worry, anxiety and fear can interfere with sexual activity, as the sympathetic effects of these may interfere with the parasympathetic stimulation required in various phases of sexual activity the point here is, see predator, run, live, have sex another day :) ______________________________________________________ so, that's the physiology of it. teleologically, why do female (and male) nipples often become erect during arousal, i.e. what is the evolutionary advantage of this happening? well, there is an almost freudian link psychosexual link between breastfeeding and sexual arousal, but that is probably just an accident of evolution and the way our autonomic system is wired, although i'm sure some sociologists and anthropologists could come up with better answers for this ...