From John, a student

I'm an animal science student and during our mammogenesis lecture a professor brought up the obvious, but seemingly overlooked notion of the size of the female human breast. Other species do have obvious teats and great apes have breasts, but humans are at the extreme end – obviously not all – but it does make one wonder on a selection and genetic level, why?

Ask Carole: Chimpanzee wearing spectacles Photograph: Public Domain

Carole replies:

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.

1) Miller, G (2000) The Mating Mind: How sexual choice shaped the evolution of human nature. Vintage

2) Moller, AP, Soler, M and Thornhill R (1995) Breast asymmetry, sexual selection, and human reproductive success. Ethology and Sociobiology. 16 (3): 207-219

3) Singh, D (1995) Female health, attractiveness, and desirability for relationships: Role of breast asymmetry and waist-to-hip ratio. Ethology and Sociobiology. 16 (6): 465-481