
Kris M. answered 07/07/19
Master of Science in Animal Science with 6+ years of Experiene
Hello!
So men have nipples because in the early stages in development, the embryo has the same "developmental map" whether it is a male or female. This "map" happens to include nipples. After 4 to 6 weeks, the Y gene in males will kick in and gear the embryo towards male development, such as testosterone.
Fun fact: In some animal species, such as dayak fruit bats, the males can generate milk!