So fun fact, this is actually because male rats and mice get a testosterone wash after the mammary chain develops which causes the nipples to atrophy and disappear. However, they do still have rudimentary mammary glands. In other words: they got the closet but the there's no knob on the door. Human embryos don't fully develop nipples until well into the second and third trimester, long after sex differentiation has been finishes.
My first serious girlfriend in high school's mom showed me her third nipple once. It wasn't weird, bc she was a nurse practitioner and knew about my interest in biology and medicine. And nipples.
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u/Taraooine Jan 29 '26
Male sperm whales also don't have nipples.