Unexpected gender dimorphism

Male mice do not have nipples.  Neither do stallions.  Pretty much every other male mammal does, because... well, they just do.  No one really seems to have a really solid answer for why, distinct from the simple apparent fact of their existence, and that translates to a lack of good explanation for why not in the case of the rare exceptions.  All babies (in most mammalian animals) are born with them, and in typical males they just develop differently.  Note that it's not fair to say that they don't develop at all-- there's still a bunch of nerves and blood vessels and stuff back there.  It's just not expressed the same way or to the same degree.  As far as an explanation for the mice/rats/horses/possibly platypi goes, the Straight Dope (for example) simply says that nipple development is suppressed in mice by the male hormones.  Which, ok?  I guess?  But a little unsatisfying.