I read an article a while back as to why, when there is no functional need for them, men have nipples, and this was because all you guys started out as females.
Males have nipples because they form during embryonic development following a default developmental pathway that is not linked to sex hormones. A specific body segment is genetically programmed to produce that tissue as the skin forms. When we go into puberty, under the influence of some hormones, the tissue there responds locally to turn the nipple area into a breast.
For this reason, males given the right stimulus (physical and or chemical) can augment their glandular tissue there too and produce milk. Males of some species do this anyway to participate in breast feeding - bats are an example.
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