How to put this delicately, which being obscure?
In each species of mammal (that is, animals possessing mammary glands), with only two exceptions that I can think of, the male and the female do not significantly differ in the appearance of the mammary gland area. One of the two exceptions is "cattle", whose sexual dimorphism is, I presume, the non-natural result of selective breeding.

But in the other exceptional case, humans, this particular dimorphism is clearly not caused by selective breeding. What then was the evolutionary causation? Ladies, explain yourselves! It would seem to be, from a purely mechanical-engineering point of view, a severely inefficient construction.

or can anyone offer a third species of mammal exhibiting this dimorphism?