Masculine secondary sexual adornments a comparative perspective

Many of the secondary sexual adornments that are visually distinctive and sexually dimorphic in primates are difficult to quantify. It is probably for this reason that most authors have concentrated on sex differences in more readily measurable traits, such as body weight or canine tooth size. Exact measures of masculine adornments, such as the lengths of capes of hair, beards, or crests, or quantitative data on the distribution and brightness of sexual skin colours, are difficult to obtain for comparative studies. When 'fleshy' secondary sexual characters are of interest, such as the large nose of the male proboscis monkey or the bulbous flaps at the mouth corners of the male golden snub-nosed monkey, comparative measurements can be especially challenging (Figure 7.18).

Nasalis Larvatus Skica

Figure 7.18 Secondary sexual adornments in adult male anthropoids. Upper row: left: Red uakari (Cacajao calvus), centre: Proboscis monkey (Nasalis larvatus), right: Golden snub-nosed monkey (Rhinopithecus roxellana). Lower row: left: Mandrill (Mandrillus sphinx), centre: Bornean orangutan (Pongo pygmaeus), right: Man (Homo sapiens).

Figure 7.18 Secondary sexual adornments in adult male anthropoids. Upper row: left: Red uakari (Cacajao calvus), centre: Proboscis monkey (Nasalis larvatus), right: Golden snub-nosed monkey (Rhinopithecus roxellana). Lower row: left: Mandrill (Mandrillus sphinx), centre: Bornean orangutan (Pongo pygmaeus), right: Man (Homo sapiens).

Source: From Dixson (1998a).

In order to overcome these problems and to quantify sex differences in visual traits across a wide range of primate species, rating scales have been devised (Dixson, Dixson, and Anderson 2005). In our studies, a 6-point scale was used, ranging from zero (no difference occurs between adult males and females, e.g. both sexes have a crest of hair of the same size and colour) up to five (maximum dimorphism, with males possessing a prominent trait that is absent, or virtually so, in females). For example, in Figure 7.18, the massive pendulous nose of the male proboscis monkey receives a score of 5, by comparison with the female's much smaller snub nose. The male orangutan (the Bornean species shown in Figure 7.18) has large cheek flanges, absent in females (score = 5), a beard (score = 4) and a fibrous or fatty hump on the crown of the head (score = 4). Using this approach, it was possible to rate the degrees of sexual dimorphism in visual traits involving the trunk, limbs, and head for a total of 124 species (representing thirty-eight genera) of monkeys and apes, as well as H. sapiens.

Figure 17.19 shows scores for sexual dimorphism in visual trait scores for adult male primates analysed according to their principal mating systems (monogamy, polygyny, or multi-male/multi-female). Polygyny is associated with significantly higher ratings for expression of masculine secondary sexual visual traits across the genera and species included in the sample. This finding applies whether the human mating system is classified as either principally polygynous or monogamous. In general, the order of visual trait development in male anthropoids according to their mating systems is (from highest to lowest): polygyny > monogamy > multi-male/multi-female. Multi-male/multi-female species, such as macaques, mangabeys, and chimpanzees, have very low ratings for male-biased sexual dimorphism in visual traits. Sexually dimorphic visual traits are poorly developed in males of most monogamous primate species; however, there are some notable exceptions, as in certain gibbon species and in the white-faced saki (Pithecia pithe-cia). Adult males of this species exhibit a distinctive white facial disc and black pelage on the body and limbs (total score = 10, Figure 7.20). It is important to note, however, that these colour changes develop in male sakis before they reach puberty and are not equivalent to androgen-stimulated secondary sexual traits, such as the human beard.

The relatively conspicuous visual traits of adult human males include facial hair, male pattern baldness, and body hair; all these traits vary considerably in their expression both within and between human populations. A score of 10 was allocated for these sexually dimorphic characters in men (Dix-son et al. 2005), which is high, given the relatively modest sex differences in human body weights. Among the monkeys and apes which have polygy-nous mating systems, scores for male-biased sexual dimorphism in visual adornments are significantly correlated with the degree of sexual dimorphism in body size; the largest males have the most striking secondary sexual adornments (Figure 17.19). It is probable that the human male would justify even higher scores using this rating scale, if sex differences in facial morphology were to be taken into consideration.

Human sex differences in body composition are also more pronounced than differences in body weight alone would indicate. Thus, as discussed earlier, the ratio of muscle mass between men and women is 1.53, using data obtained by Clarys et al. (1984). Inclusion of this sex difference in adult muscle mass ratio in Figure 7.19 places H. sapiens firmly in line with polygynous non-human primates as regards the degree of expression of masculine visual adornments. These data may be interpreted as strengthening the case for effects of sexual selection operating within polygynous mating systems during evolution of the genus Homo.

Very little is known about the origins of the human beard, or of male pattern baldness, and the marked reduction of human body hair. One suggestion is that male pattern baldness might have a function in heat loss and in thermoregulation, especially in bearded men (Cabanac and Brinnel 1988). The argument is not convincing. Body hair is sparse in human beings; its reduction during human evolution, together with increased sweat gland activity, has been linked to specializations for thermoregulation in tropical savannah environments (Wheeler 1984; Jablonski 2006). It has also been suggested that hair reduction may have been adaptive in lessening ectoparasite loads (such as lice) in ancestral humans (Rantala 1999; Pagel and Bodmer 2003). Indeed, of

Mating system Mating system

□ Monogamous Q Polygynous ^ Multi-male / Multi-female

Balding Sexual Selection

Figure 7.19 Upper: Ratings of the degree of male-biased sexual dimorphism in visual traits in monkeys, apes, and humans, in relation to their principal mating systems. The genus Homo is included among the polygynous taxa in this analysis. *P < .05; **P< .01; ***P< .001 for analyses at the genus level and (on the right) at the species level after statistical correction for possible phylogenetic biases in the data set. Lower: Correlation between ratings for male biased sexual dimorphism in visual traits and sexual dimorphism in adult body weight in polygynous anthropoid genera. Slope of the regression line y = 9.738 x — 9.81 1; r2 = .78, P < .001. Source: Redrawn from Dixson, Dixson, and Anderson (2005).

Body weight ratio (M/F)

Figure 7.19 Upper: Ratings of the degree of male-biased sexual dimorphism in visual traits in monkeys, apes, and humans, in relation to their principal mating systems. The genus Homo is included among the polygynous taxa in this analysis. *P < .05; **P< .01; ***P< .001 for analyses at the genus level and (on the right) at the species level after statistical correction for possible phylogenetic biases in the data set. Lower: Correlation between ratings for male biased sexual dimorphism in visual traits and sexual dimorphism in adult body weight in polygynous anthropoid genera. Slope of the regression line y = 9.738 x — 9.81 1; r2 = .78, P < .001. Source: Redrawn from Dixson, Dixson, and Anderson (2005).

the multiple theories advanced to explain the loss of body hair during human evolution, Marcus Rantala (2007) suggests that the parasite hypothesis is the most compelling. He proposes that once early members of the genus Homo began to live in more 'fixed home bases' the risk of infestation by ectoparasites would have increased considerably. Reduction in body hair would have been highly adaptive under these conditions.

The thicker distribution of body hair in men, as compared to women, may represent an epiphe-nomenon in many modern populations, rather than having significant effects upon attractiveness. In one study conducted in the UK, women found male chest and abdominal hair to be attractive (Dixson et al. 2003). Attractiveness ratings for mesomorphic and endomorphic male images increased if hair was added to the chest and abdomen. Images including

Figure 7.20 The male white-faced saki (Pithecia pithecia). This South American monkey lives in small family groups and is thought to be monogamous. It is highly sexually dimorphic in appearance, as only males have the white face and black pelage.

Source: Author's drawing from a photograph by Dr Russ Mittermeier.

trunk hair were also rated as being older by women. Given that there is a strong cross-cultural propensity for women to prefer men somewhat older than themselves as marriage partners, masculine body hair might have represented one of the many signals of masculine maturity in ancestral populations of H. sapiens. Contrary to this view, it must be stressed that women in China, New Zealand and the USA (California), find the absence of male trunk hair to be more attractive; the inclusion of greater amounts of hair in male images renders them correspondingly less attractive to women (Figure 7.21). In the only study conducted in Africa, Cameroonian women did not show any consistent preference for male trunk hair or its absence (Dixson et al. 2007b).

Beard growth in men is stimulated by testosterone, and the size and colour of the beard differs with age and also between individuals. Darwin (1871) noted the considerable variations which

China

Africa

None

China

Africa

None

Male trunk hair

Figure 7.21 Women's ratings for the attractiveness of images of men that vary in amounts of hair on the chest and abdomen. Studies were conducted in China, New Zealand, USA, and Africa. *P < .05; ***P < .001.

Male trunk hair

Figure 7.21 Women's ratings for the attractiveness of images of men that vary in amounts of hair on the chest and abdomen. Studies were conducted in China, New Zealand, USA, and Africa. *P < .05; ***P < .001.

Source: Data are from Dixson et al. (2007a; 2007b; in press).

occur in different human populations as regards the presence or relative absence of facial hair in men. He concluded that 'as far as the extreme intricacy of the subject permits us to judge, it appears that our male ape-like progenitors acquired their beards as an ornament to charm or excite the opposite sex.' However, it is by no means clear that the human beard plays a significant role in female assessments of male attractiveness. It is the case that some studies have produced evidence for positive effects of male facial hair upon judgments of attractiveness (Pelligrini 1973; Hatfield and Sprecher 1986). In Pellegrini's study, both women and men rated photos of men with full beards as being more masculine, dominant, attractive, and older than pictures of the same men with less facial hair or when clean-shaven. As Barber (1995) has suggested, the beard's ability to influence such perceptions of masculine social status 'may be attributable to the fact that it exaggerates another epigamic trait of the face, the male chin'. Sexual dimorphism, with men having a larger face, particularly the lower part, and a larger chin than women, is accentuated by the growth of the beard. Experiments are required, in order to determine how far beards may have originated as status ornaments, via intra-sexual selection between males, and to what extent they are attractive to women. It is interesting to note, however, that many men remove their facial hair by shaving each day; this is more readily understandable in terms of reducing possibly inappropriate signalling of masculine status, than as an attempt to appear less attractive to women.

100 Hair Growth Tips

100 Hair Growth Tips

100 Hair Growth Tips EVERY Balding Person Should Know. This Report

Get My Free Ebook


Post a comment