Source: Original cartoon by Alex Martin

Natural selection, whereby environmental features favor particular adaptations, is the keystone of the Darwin/Wallace theory of evolution. But Darwin soon recognized a second kind of selection. Within a species, females and males may exert selection on each other. Darwin called this sexual selection in his 1871 book The Descent of Man, and Selection in Relation to . It affects traits unconnected with the sex organs, notably body size and adornments. Selection can take place within one sex, as with weaponry for contests between males, or between sexes, as in mate choice. Darwin specifically cited the human beard as a response to sexual selection serving mate .

Source: From Wikimedia Commons. Wallace photograph by Sims, 1889; Wellcome Images; Creative Commons Attribution 4.0 International license. Darwin photograph by Julia Margaret Cameron (1815-1879); work in the public domain.

Traits that evolved under sexual selection — secondary sexual characteristics — typically develop when individuals mature sexually ( in humans). In mammals, such traits are commonly located on the head. Because vision predominates in primates, they show many striking types of facial adornment. Examples are the prominent nose of male proboscis monkeys and a beard combined with cheek flanges in male orangutans. The human beard also belongs in this category.

Source: Adapted from a figure in Dixson et al. (2005) and used with kind permission from Alan Dixson.

Basics of beard growth

James Hamilton and colleagues conducted pioneering studies of human beard growth in the 1950s and 1960s, and notable research by Valerie Randall followed. Importantly, different head hair regions show different growth patterns, indicating separate adaptations. Chin hair is replaced most rapidly, in about three months, while scalp hair is slowest, taking four months to over three years.

“Hairless” areas on the human body are actually covered by short, fine, unpigmented vellus hairs. “Hairy” regions, including the scalp, eyebrows, and eyelashes, have longer, thicker, pigmented hairs. Each hair follicle shows a growth cycle in which hair replacement alternates with rest phases.

Source: Adapted from figures in Hamilton et al. (1958)

In one landmark study, Hamilton and colleagues investigated beard growth in 365 Japanese males aged 1-88 years. Beards grew far less than in Caucasians, indicating a genetic difference, confirmed by the fact that Japanese men in Tokyo resembled those living in New York. Moreover, beard growth was significantly more similar in identical male than in non-twin brothers or unrelated males. The overall pattern was a steep increase in beard growth during sexual maturation (ages 15-20), a gradual increase to age 45 and then a slow decline over subsequent decades. Pigment loss from beard hairs first began at age 40 and increased thereafter. Beard appearance hence changes considerably across the lifespan.

control of beards

The development of secondary sexual characteristics, such as beards, during maturation reflects hormonal differences between the sexes. “Male” hormones (androgens), including various forms of , typically control the development of both weapons and adornments. One clear sign that male hormones are involved, noted by Aristotle, is that secondary sexual traits fail to develop if males are castrated before maturity. In human eunuchs, beard growth is suppressed, but administering testosterone stimulates it.

Source: Adapted from Anonymous (1970).

An anonymous report in Nature in 1970 provided intriguing evidence for a possible connection between androgens and beards in adult men. Over two years, the author — a field biologist — was virtually isolated on a remote island for several weeks at a time. He noticed that his beard grew less during spells on the island, but began to increase the day before he left, reaching unusually high levels during his first few days on the mainland. To test his suspicion that resumed sexual activity was the trigger, he measured beard growth by weighing shavings from an electric razor once daily. A marked increase consistently occurred whenever sexual activity resumed, although it soon returned to baseline. Interestingly, the growth increased before sexual activity. The author also conducted experiments on himself and found that small doses of various androgens placed under his tongue stimulated beard growth comparable to that provoked by sexual activity.

Subsequent authors have downplayed this anonymous account, but surely the possibility that beard growth is linked to sexual activity deserves further investigation?

Beards as signals

Various studies have tested Darwin’s original suggestion that human beards evolved under female choice, but with mixed results. Several authors have suggested that beards, rather than serving mate attraction, may amplify aggressive displays and boost perceptions of social dominance. Furthermore, a cross-cultural occurrence of human responses is a prerequisite for any evolutionary hypothesis. For these reasons, wide-ranging studies, including a cross-cultural comparison conducted by Barnaby Dixson, are especially welcome. As an initial test, Dixson designed a questionnaire combining the presence/absence of a beard with facial expressions and assessed responses of Europeans (New Zealand) and Polynesians (Samoa). In these two cultural settings, both men and women attributed higher social status to bearded men, and women in fact rated faces without beards as more attractive.

Source: Adapted from figures in Dixson et al. (2016).

Subsequently, in a 2016 paper, Dixson and colleagues used a more sophisticated approach to test the effects of beards in combination with facial indicators of masculinity (pronounced brow ridge; more robust jawline). Allowance was also made for the differences between short-term and long-term relationships. Computer graphic manipulation was used to generate male faces with five different degrees of masculinity, varying from clean-shaven through light or heavy stubble through to fully bearded. Image-rating tests were conducted online with 8,520 women aged 18-100, randomly assigned to assess three different aspects: (1) general physical ; (2) attractiveness for a short-term liaison; and (3) attractiveness for a long-term relationship. A significant interaction between beardedness and masculinity emerged. With clean-shaven faces, both increased and decreased masculinity reduced attractiveness, but this effect was less marked if stubble or a beard was present. The presence of facial hair enhanced attractiveness for long-term relationships, but not short-term liaisons.

Male pattern baldness

But the story has an additional twist, because another conspicuous feature of male heads may also send a signal: male pattern baldness. James Hamilton documented balding in a 1951 paper discussing findings from over 300 men and 200 women aged 20-94 years. In adult men, but usually not in women, hair loss occurred progressively with age, affecting 58 percent of men aged 50-92 years.

Source: Adapted from figures in Hamilton (1951).

As with beardedness, balding runs in families, with differences between human populations presumably reflecting genetic distances. Hamilton found that in Chinese men, baldness was less common and tended to occur later than in Caucasians. As with beards, androgens are involved in male pattern baldness, which can be induced by treatment with testosterone. But with balding the effect is opposite: Androgens suppress the growth of hair follicles. This “androgen paradox,” studied in detail by Valerie Randall, dramatically illustrates the fact that different hair traits can behave in very different ways.

But comparatively little research has been done on the possible signaling functions of male pattern baldness. A 1996 paper by Frank Muscarella and Michael Cunningham considered this in tandem with beards. They noted that baldness typically develops later in life and may signal social maturity: non-threatening dominance allied with and nurturance. Tests used manipulated male facial images, with three levels of cranial hair (full, receding, bald) and two levels of facial hair (beard with moustache, clean-shaven). Reduced head hair was associated with increased perceptions of social maturity and appeasement, but with decreased perceptions of attractiveness and aggressiveness. By contrast, facial hair was perceived as more aggressive, less appeasing, less attractive, and lower on social maturity.

Overall, the studies to date indicate that any signal function of beards has more to do with dominance relationships than with female mate choice. In the future, it will be important to conduct studies that assess multiple kinds of sexually selected traits together, rather than examining just one or two. In particular, it will be necessary to study the combined effects of beards and balding in relation to male age.

Important note: I am very to Barnaby and Alan Dixson for providing very helpful feedback and permission to use figures from their papers.

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