There seems to be a logical explanation about why women have nipples - for babies. But why do men's bodies retain what appears to be redundant body parts? The Darwinian natural selection process would seem to dictate that male nipples really should not be there. So what's the deal? Why do men have nipples?
The answer is that as embryos men and women have similar tissues and body parts. If anything the embryo follows a 'female template'. That is why nipples are present in both sexes. It is the effect of the genes, the Y chromosome and the hormone testosterone that brings about the changes and masculinises the embryo. Testosterone promotes the growth of the penis and testicles. Because nipples are there before this process begins the nipples stay!
Nipples and breast tissue have no function as such except for perhaps protecting the heart and lungs from injury.
Male Breasts Can Produce Milk and Get Breast Cancer
A certain level of the female hormone estrogen is present in all men. If, as a result of disease or a condition affecting hormones, breast tissue in men can grow (gynecomastia- abnormal enlargement of breasts,) and men can produce milk.
Gynacomastia is more commonly seen in adolescence, a period in development when hormones fluctuate a lot. It is also seen in some men with liver disease and sometimes in alcoholics.
Men can get breast cancer as well as women.