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By Root 282 0

We too rely on signals as arbitrary as a widowbird’s tail and a bowerbird’s crest. Our signals include faces, smells, hair color, men’s beards, and women’s breasts. What makes those structures less ludicrous than a long tail as grounds for selecting a spouse—the most important person in our adult life, our economic and social partner, and the coparent of our children? If we think that we have a signaling system immune to cheating, why do so many people resort to makeup, hair dyes, and breast augmentation? As for our supposedly wise and careful selection process, all of us know that when we walk into a room full of unfamiliar people, we quickly sense who attracts us physically and who doesn’t. That quick sense is based on “sex appeal,” which just means the sum of the body signals to which we respond, largely unconsciously. Our divorce rate, now around 50 percent in the United States, shows that we ourselves acknowledge the failure of half of our efforts to select mates. Albatrosses and many other pair-bonded animal species have much lower “divorce” rates. So much for our wisdom and their stupidity!

In fact, like other animal species, we have evolved many body traits that signal age, sex, reproductive status, and individual quality, as well as programmed responses to those and other traits. Attainment of reproductive maturity is signaled in both human sexes by the growth of pubic and axillary hair. In human males it is further signaled by the growth of a beard and body hair and by a drop in the pitch of the voice. The episode with which I began this chapter illustrates that our responses to those signals can be as specific and dramatic as a gull chick’s response to the red spot on its parent’s bill. Human females additionally signal reproductive maturity by expansion of the breasts. Later in life, we signal our waning fertility and (in traditional societies) attainment of wise elder status by the whitening of our hair. We tend to respond to the sight of body muscles (in appropriate amounts and places) as a signal of male physical condition, and to the sight of body fat (also in appropriate amounts and places) as a signal of female physical condition. As for the body signals by which we select our mates and sex partners, they include all those same signals of reproductive maturity and physical condition, with variation among human populations in the signals that one sex possesses and that the other sex prefers.

For instance, men vary around the world in the luxuriance of their beard and body hair, while women vary geographically in the size and shape of their breasts and nipples and in their nipple color. All of these structures serve us humans as signals analogous to the red dots and black stripes of birds. In addition, just as women’s breasts simultaneously perform a physiological function and serve as a signal, I shall consider later in this chapter whether the same might be true for men’s penises.

Scientists seeking to understand the corresponding signals of animals can carry out experiments involving mechanical modifications of an animal’s body, such as shortening a widowbird’s tail or painting over a gull’s red spot. Legal obstacles, moral compunctions, and ethical considerations prevent us from performing such controlled experiments on humans. Also preventing us from understanding human signals are our own strong feelings that cloud our objectivity about them, and the great degree of cultural variation and individually learned variation in both our preferences and our bodies’ self-modifications. However, such variation and self-modification can also help us gain understanding by serving as natural experiments, albeit ones lacking experimental controls. At least three sets of human signals seem to me to conform to Kodric-Brown’s and Brown’s truth-in-advertising model: men’s body muscle, facial “beauty” in both sexes, and women’s body fat.

Men’s body muscle tends to impress women as well as other men. While the extreme muscle development of professional bodybuilders strikes many people as grotesque, many (most?)