Joanna, the answer to your question  -- why do male mammals have nipples too
-- is quite simple.  The basic mammalian body plan is that of a female.  In
humans, from conception to about 12 weeks fetal age, you cannot tell the sex
of the child -- they all look female.  If no testosterone is added to the
fetus, then it develops into a female.  If testosterone is added to the
fetus, either by its own testicles starting to produce testosterone, or by
testosterone from the mother's environment, then the testosterone induces
changes which result in a male.  That is why you can get a person who is
genetically a female (two X chromomes) but externally seems to be a male
(testosterone contamination of the mother during pregnancy, like from her
job).  You can also get people who are genetically male (one X and one Y
chromosome) but who either don't produce testosterone for some reason, or
whose cells have no testosterone receptors.  They externally seems to be female.

It is much easier, evolutionarily, to have one basic plan.  There are not
separate genes that code for males to be bigger, have more body hair, have a
deeper voice, have facial hair, have a penis and testicles, have broader
shoulders and slimmer hips, put on more weight around the middle, become
bald in middle age, etc.  There is just the one gene that says "Hey
testicales, start making testosterone now.  And lets make a bunch later
(puberty)."  All the differences follow from the presence of testosterone.
So, nipples are the basic plan.  In females, they continue to develop at
puberty and during pregnancy and lactation, in response to female hormones,
including estrogen.  In males, they are just quiescent.
Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University