Re: Why do human females have breasts while other mammals don't?

The full volume of adipose tissue in the breast is probably not required 
for normal lactation, since women nursing their babies for 6 months or more 
can undergo a reduction of breast volume to the pre-pregnant state and 
still produce an adequate amount of milk (see article by JC Kent, et al. in 
Experimental Physiology vol. 84:435, 1999--you can access scientific 
articles at the "Pubmed" site at www.ncbi.nlm.nih.gov).  The excess adipose 
tissue in the female breast is related to the sex difference in overall 
adipose tissue that normally exists between men and women.  Adipose depots 
respond to estrogen by increasing in volume; an interesting (and 
unsolved!) question is why fat cells in the breast, as compared with those 
quite close by in the skin, are so much more responsive to estrogen.  This 
individual sex difference in anatomy is in turn related to the more general 
question of why humans exhibit sex differences in overall appearance, eg., 
women are shorter than men.  Some primates show an even more dramatic 
sexual dimorphism in stature (females are half the size of males).  This 
extreme sexual dimorphism tends to correlate with intense male competition 
for the possession of many females at once; larger males are more 
successful at polygamy than small males (see "Intrasexual competition and 
body weight dimorphism in primates" Am. J. Physical Anthropology vol. 103, 
p. 37, 1997 by JM Plavcan).  Presumably sex differences in anatomy that do 
not directly relate to reproductive function may serve as cues for 
reproductive behavior, but this question is difficult to examine 
experimentally.  You might be interested in reading a book of mine, 
"Hormones: molecular messengers" by JK Young, Franklin-Watts Publ., 1994 
that discusses some of these topics (it likely is in your library).