On Fat Deposits around the Mammary Glands in the Females of Homo Sapiens

In an article ¹ in this journal I cited an exchange ² between Zoe and Bloom in James Joyce's Ulysses:

Zoe: . . . Come and I will peel off.

Bloom: (Feeling his occiput dubiously with the unparalleled embarrassment of a harassed pedlar gauging the symmetry of her peeled pears.) Somebody will be dreadfully jealous if she knew.

Perhaps Joyce knew something ³ way ahead of his time. Gauging symmetries would turn out to be one of the favorite activities of theoretical physicists starting in the late 1960s. I described in considerable detail how fundamental physics is now known to be dictated by gauge symmetry in my book Fearful Symmetry: the Search for Beauty in Modern Physics. ⁴ The notion of gauge symmetry turned out to be the magic "open sesame" that has allowed physicists a glimpse of Mother Nature's secrets at the most fundamental level.

For years after quoting this Joycean passage in Fearful Symmetry, I have pondered over the deep meaning behind it. Surely the ability of a human being to "gauge the symmetry of her peeled pears" is of far more import than his or her ability to gauge the symmetry of a nonabelian field theory. Indeed, from the point of view of evolutionary biology, the former led to the latter by a long and torturous chain of events. It is our finely tuned ability to gauge the reproductive value of each other that produced over the evolutionary time scale a species capable of gauging the symmetry of Mother Nature herself.

The five thousand or so living species of the class Mammalia are distinguished from myriad other species by mammary glands that the female of the species uses to nourish her young. There is, however, a striking phenomenon rarely remarked upon, ⁵ namely that Homo Sapiens is the only species I know of whose females, at the age of onset of potential fertility (or technically fecundability), begin to deposit fat around their mammary glands, giving their breasts a roughly globular shape presumably well known to most readers of this journal. In contrast, the mammary glands of the females of most nonhuman mammals in general and of nonhuman primates in particular are not padded, so to speak, and in general are no different from the mammary glands of the male in external appearance. (A friend to whom I made this remark retorted that I must have not been to a farm lately, but biologists would regard farm animals as having been so extensively bred for human purposes that they can no longer be regarded as counterexamples to this general rule.)

This striking sexual dimorphism in breast size in humans, which we tend to take for granted, did not escape the notice of Charles Darwin, that particularly astute observer and presumably the first human to find this fact remarkable. He concluded from this everyday observation that the increase in breast size in women must represent a relatively recent evolutionary event.

First, some physiological facts. The mammary glands develop from sweat glands. In cattle and whales they are located in the groin region, but in primates they are on the chest. Smaller mammals typically have many pairs of mammary glands, according to the size of the litters. (Polymastia is rare but not unknown in humans.) In human females, the breasts are made of glandular tissues encased in fat, with the entire complex suspended by ligaments. It is the fat that gives human breasts their hemispherical shape. The nipple on each breast is surrounded by muscles (forming a pigmented region called the areola) that pull the nipple erect upon tactile stimulation to facilitate suckling. The areola also contains glands which secrete a lubricant during breast feeding. When humans became bipedal, gravity tended to stretch the suspensory ligaments, and in primitive conditions the sagging of the breasts provides one of the clearest indicators of female age. In modern times, women counteract the effect...