Humans are the only mammals with permanently swollen breasts. We have boobs all the time, other animals only have them when nursing
The full, plump bosom seen in the human ape is an anomaly. No other primate has a permanent breast. During lactation all the ape species develop a full breast to store milk. In non-human primates (and other mammal species) a full breast is a clear indication the female is suckling young. Not so in humans. In addition, females in early adolescence can start developing a breast before menarche and females maintain breasts post menopause, so the full breast is not a reliable indicator of fertility. Neither is size an indicator of milk production – bigger breasts don’t necessarily produce more milk. It is the symmetry of the breasts that indicates the phenotypic quality and fitness of the individual female, not the size.
The sex appeal of rounded female buttocks and plump breasts is both universal and unique to the human primate1. Fertile women tend not to store fat around the abdomen, so the waist of a fertile female is usually slimmer than her hips. Other female primates do not have fat deposited on the rump. For example, the female gorilla has a skinny posterior and stores fat on her abdomen, as do human males. So it has been widely theorised that the plump buttock and bosom of modern women are sexual ornaments, selected for by ancestral males2. Seen from a distance the adult female form, either from behind or from the front, can be recognised as distinct from the male of the species. An hourglass figure, plus youthfulness, would have attracted male hominids looking for mating opportunity3. The hourglass figure remains attractive to modern males. Over the centuries females attempting to increase their mate choice have dressed to exploit this shape (corsets, bustles and wonder bras). If ancestral males had not shown a preference for the mutation producing symmetrical, plump bosoms, modern women’s chests would resemble the flat thoraxes of the other apes.
Today, plastic surgery is used by some women, particularly those working in the sex industry, to enhance bust size and exploit ancient male programming. But the ultimate function of the human breast becomes clear only when an infant is born and lactation begins.