All mammals male or female have nipples. But for the most part, the males of the different mammal species, while they can produce a sort of gluey substance lack the anatomy to actually produce milk for a baby. Just having the show of nipples alone is not enough. In fact the female and male anatomy in that area are completely different. Men simply do not have the glands or the proper constitution to produce milk. I believe that nipples are a mammal feature, like hair, warm blood and other characteristics that define mammals. I mean, fish certainly don't have them, neither do chickens nor insects. nipples are just another definitive feature of mammals. Also, I believe that nipples on men are more for protection for the male, they are sensitive in an area where things need to be sensitive considering the important organs that are located there. Men's nipples are not for producing milk but they are there as sensitive parts of the skin that slightly protrude from the rest of the skin in that area.