Why do men have nipples?

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Why do men have nipples?

In: Biology

They’re essentially “leftovers” from foetal development. Both male and female foetuses will develop nipples, but only females will develop functional mammary glands later in life (barring some rare exceptions).

Before a certain hormone kicks in all embryos are genderless and evolve the same. It seems the nipples need to form before to be done in time for females, and this causes no significant disadvantage to males so there is no selection pressure for men with less nipples

All foetuses are female by default, and the male hormone (testosterone) doesn’t kick in until after the nipples have already formed. The penis is an extended clitoris, and the labia stitch together to form the scrotum, all caused by the testosterone.

There is an interesting condition where the foetus is “immune” to testosterone, and so even though it’s chromosomes are XY (boy), it does not develop male characteristics.