Breast Development without Pregnancy/Lactation

In great ape females, the breasts are not  fully developed in the virgin female and they only expand during pregnancy to then begin producing milk after birth.  Following completion of lactation, the breast involutes again, until the next pregnancy. In striking contrast, virgin human females have fully formed breasts, which do not undergo much greater expansion during pregnancy and lactation, and they then do not involute after lactation is over. Neither the reason nor the biological nor the developmental basis of this very striking difference are understood.  Based on the sexual attractiveness of the breasts in many human cultures, one hypothesis put forward is that the human breast is a form of sexual advertisement, perhaps replacing the genital swellings found in some great apes. It is also interesting that there have so far been no reports in great apes of breast cancer, a very common disease in humans.