Function
The function of the mammary glands in female breasts is to nurture the young by producing milk, which is secreted by the nipples during lactation. However, zoologists point out that no female mammal other than the human has breasts of comparable size when not lactating and that humans are the only primate that have permanently swollen breasts. This suggests that the external form of the breasts is connected to factors other than lactation alone.
The mammary glands that secrete the milk from the breasts actually make up a relatively small fraction of the overall breast tissue. It is commonly assumed by biologists that the real evolutionary purpose of women having breasts is to attract the male of the species; that, in other words, breasts are sexually dimorphic, or secondary sex characteristics. One theory is based around the fact that, unlike nearly all other primates, human females do not display clear, physical signs of ovulation. This could have plausibly resulted in human males evolving to respond to more subtle signs of ovulation. During ovulation, the increased estrogen present in the female body results in a slight swelling of the breasts, which then males could have evolved to find attractive. In response, there would be evolutionary pressures that would favor females with more swollen breasts who would, in a manner of speaking, appear to males to be the most likely to be ovulating.
Some zoologists (notably Desmond Morris) believe that the shape of female breasts evolved as a frontal counterpart to that of the buttocks, the reason being that whilst other primates mate in the typical piggy-back position, humans are more likely to successfully copulate mating face on. A secondary sexual characteristic on a woman's chest would have encouraged this in more primitive incarnations of the human race, and a face on encounter would have helped found a relationship between partners beyond merely a sexual one.
Others believe that the human breast evolved in order to prevent infants from suffocating while feeding. Since human infants do not have a protruding jaw like our ancestors and the other primates, the infant's nose might be blocked by a flat female chest while feeding. According to this theory, as the human jaw became recessed, so the breasts became larger to compensate.
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