There is a certain tribe in Africa (the Aka) whose males are known to provide their babies with a nipple for when the women are out and about hunting for food. This is mostly to soothe the newborns and remind them that "mommy isn't too far away", but it is possible that it has eventually led to some of their men developing the ability to lactate.
You must have received quite a few stares when you set out to prove that men can also produce prolactin