Do all or most male mammals have nipples? Is male lactating common or rare? Do most mammals help to nurse their child?
In most mammalian cases, nipple-like structures seem to be the norm. As always, it is never 100%, the exception to the rule here are the horse and the platypus. In humans, we were female before we were male, the X chromosome being fertilized by the Y chromosome, and the existence of nipple-like structures are the norm. On the subject of lactation, again, the majority sides with the female. There is an example listed of Old World fruit bats, but even that is sketchy at best in terms of male lactation. For a mammal to lactate, an abundance of the enzyme lactase needs to be present, which is what occurs upon mammalian live-birth. Since this does not occur with males, the likelihood of male lactation is slim to none with humans. To answer the last question, again, there is a low participation factor of both parents nursing, based on the dearth of male ability to lactate. Nipples on the male side of mammalia appear to be for superficial ornamentation only, not much in terms of actual functionality.