Why do straight men devote so much headspace to those big, bulbous bags of fat drooping from women's chests? Scientists have never satisfactorily explained men's curious breast fixation, but theorists are gonna theorize. Mammary glands are a defining feature of mammals, but humans seem unique in granting mammaries a large sexual role. That's not to say interest in nipples is entirely unheard of elsewhere in the animal kingdom: In the book "Biological Exuberance: Animal Homosexuality and Natural Diversity" Stonewall Inn Editions, , Canadian biologist Bruce Bagemihl notes that a couple of primate species, including humanity's close relative the bonobo, have been seen stimulating their own nipples while masturbating.
The breast is one of two prominences located on the upper ventral region of the torso of primates. In females, it serves as the mammary gland , which produces and secretes milk to feed infants. At puberty , estrogens , in conjunction with growth hormone , cause breast development in female humans and to a much lesser extent in other primates. Breast development in other primate females generally only occurs with pregnancy. Subcutaneous fat covers and envelops a network of ducts that converge on the nipple , and these tissues give the breast its size and shape.
In the primate world, plump breasts last only as long as breastfeeding doesexcept in humans. Women are busty all the time, even after menopause. According to some scientists, the trait is an evolutionary trick for snagging men and signals a woman's ability to feed her children. But no one can confirm an answer as to why women are the only apes with sizable chests. Even though they appear full, a woman's breasts are only filled with milk after a she gives birth.