Sexual dimorphism body weight and mating systems

Sexual dimorphism in adult body weight is most pronounced in those primates, such as geladas, hamad-ryas baboons and gorillas, which have polygynous mating systems (Figure 7.1). Competition between adult males for access to females is intense in such species, as is the case in other polygynous mammals (as, for example, elephant seals or red deer). Selection for increased male body size has also occurred in anthropoids such as macaques, chacma baboons and mangabeys, which have multi-male/ multi-female mating systems. In many cases sex differences are not as pronounced in multi-male/ multi-female forms as in polygynous primates, but there are exceptions, as will be discussed below. In monogamous primates, such as the owl monkeys, marmosets, and gibbons, the two sexes are usually very similar or equal in size.

Figure 7.1 also shows the degrees of sexual dimorphism in canine length in the various mating systems, and these broadly correlate with the data on body size. The largest canines occur in males of polygynous primate species, whereas there is little or no sexual dimorphism in the monogamous species. However, as discussed in Chapter 1, reduction in sexual dimorphism in canine size is an ancient hominid trait, occurring throughout the australop-ithecine lineage as well as in the genus Homo. Comparative studies of sexual dimorphism in canine size among extant primates are of limited value in attempts to reconstruct the origins of human mating systems.

Human beings are sexually dimorphic in body size, with the ratio of adult male-to-female body weight being approximately 1.1-1.2. Considerable individual variation is apparent, both within and between human populations, and some examples are provided in Figure 7.2. In the fourteen populations represented here, men average 63.5 kg and women 52.3 kg in body weight. Men are also taller than women on average, and the ratio of adult male/ adult female height is 1.06 for the thirteen examples included in Figure 7.2. These sex differences are quite modest; slightly larger than those which typify monogamous primates, but much less than in many polygynous forms. Consider, as examples,

Body size dimorphism

Relative canine size

Figure 7.1 Body size sexual dimorphism (adult male body weight divided by adult female body weight) and relative canine sizes in primate genera which have monogamous (M), polygynous (P), or multi-male/multi-female (M/M) mating systems. Data are means ± SEM.

Source: From Dixson (1998a); after Clutton-Brock (1991).

180 H

100 80 7060

T3 O CO

co CQ

o tc

co 2

Figure 7.2 Sex differences in body weight and height in various human populations. Open bars = men; Black bars = women. Source: Based on data in Collard (2002) and derived mainly from Houghton (1996).

male/female body weight ratios in the gorilla (2.4), hamadryas baboon (1.7), and gelada (1.6).

Martin, Willner, and Dettling (1994) showed that sexual dimorphism in adult body weight is more pronounced in the Old World anthropoids than among the New World forms. The prosimians should be considered separately, because many of them lack sexual dimorphism, and in some lemurs, females exceed males in body size. The Old World monkeys and apes also include the largest species of primates, and there is a tendency for sexual dimorphism to scale with increases in body size (Rensch's rule). More importantly, the fact that many of the larger Old World primates are terrestrial, or semi-terrestrial, may have favoured the evolution of larger males to combat risks of predation. In gorillas, for example, the dominant silverback male plays a crucial role in defending the females and offspring when the group is threatened in any way. Without the silverback, the group is leaderless and relatively defenceless (Schaller 1963). Thus, the evolution of larger male body size may be due to the entwined effects of natural selection (via predation risk) and sexual selection (inter-male competition for access to females). This should not surprise us. Moreover, the comparatively modest but consistent tendency for men to be heavier and taller than women may reflect ancient effects of natural and sexual selection during human evolution in Africa. One way of examining in more detail the degree of sexual selection in males of extant species is to compare ratios of male/female body weight with the socionomic sex ratio in groups of these species. The socionomic sex ratio of a primate group is the number of adult females divided by the number of adult males. The ratio tends to be highest in polygynous species (where only a single adult male may be present) and lowest in monogamous species (where there is often just one adult male and female). Previous analyses of this type have been criticized (Martin et al. 1994) on the basis of the limited sample sizes used and an over-representation of monogamous primate species (Clutton-Brock, Harvey, and Rudder 1977; Alexander et al. 1979). Figure 7.3 shows the results of an expanded analysis, made possible by virtue of the fact that field data on the socionomic sex-ratios of primate groups are now much more complete. One hundred and eleven species representing a total of thirty-nine genera of monkeys and apes are included in the analysis represented in Figure 7.3. There is, in general, a positive relationship between increasing sexual dimorphism in body weight and increasing numbers of adult females per adult male in social groups of anthropoids. It is important to note, however, that there is a wide scatter of data points on the graph. Some species with multi-male/multi-female mating systems exhibit greater body size dimorphism than more typically polygy-nous species. Some unusual socionomic sex ratios occur, as in the mandrill where adult females may outnumber adult and subadult males by a ratio of 6:1, despite the occurrence of a multi-male/multi-female mating system (Abernethy et al. 2002).

If we consider not only sex differences in human body size but also differences in muscle mass and strength between men and women (these will be discussed below), then these equate to a likely socio-nomic sex ratio between two and three. As regards human evolution, such a ratio might have occurred in an ancestor having either a polygynous or a multi-male/multi-female mating system. Given the strong evidence assembled in Chapters 2-4 indicating the absence of sexual selection within a multi-male/ multi-female system during human evolution, it is much more likely that the data in Figure 7.3 are indicative of a polygynous background to human evolution. Two further lines of comparative evidence may be advanced in support of this conclusion.

Firstly, there is the question of reproductive bimaturism and its relationship to mating systems among the anthropoids. In many monkeys and apes, males take longer to reach sexual maturity than females; this is usually correlated with a later onset of puberty and slower progression through the stages of puberty and adolescence in males. These effects are most pronounced in species where sexual dimorphism in body size is apparent, so that males are larger than females and take longer to attain fully adult size. The ages at which males and females reach puberty and attain full body size in various Old World anthropoids including Homo sapiens are shown in Table 7.1. The age at which

Socionomic sex ratio

Figure 7.3 Relationships between socionomic sex ratios (xaxis) and body weight sexual dimorphism (y axis) in New World monkeys, Old World monkeys, and apes. Sexual dimorphism is most pronounced in genera with large socionomic sex ratios (i.e. number of adult females per group/number of adult males). Mating systems are indicated by symbols: • = monogamous; A = polygynous; • = multi-male/ multi-female; □ = mixed: polygynous or multi-male/ multi-female groups both occur. Source: Data are derived principally from Campbell et al. (2007) and also from Smuts et al. (1987); Davies and Oates (1994); Jablonski (1998), and Kappeler (2000).

puberty begins in males may be defined as the time when the androgenic functions of the testes increase markedly, and this is usually associated with overall increases in testicular growth, penile size and progressive secondary sexual development. There is considerable individual and inter-specific variation in the timing of these events. In boys, for example, increases in size of the testes and scrotum occur at about 11.5 years of age on average, and the penis begins to enlarge by 12.5 years. Despite individual differences in the onset of puberty, the general progression of physical changes follows an orderly sequence in members of both sexes (Figure 7.4). In girls, as in females of other Old World anthropoids, the occurrence of the first menstrual flow (menarche) provides a useful marker of puberty. However, this marker is not useful for the New World monkeys, because menstruation occurs in relatively few of these species. In girls, the earliest sign of puberty is usually growth of the breast-bud, and this may occur at anywhere between 8-13 years of age (average 11 years: Figure 7.4). Both sexes show increased growth in height; this is the adolescent growth spurt. However, it will be noted in Figure 7.4 that the growth spurt also begins earlier in girls than boys. This difference is the characteristic of other anthropoids in which the sexes are dimorphic in body size and in which sexual bimaturism occurs, in association with polygynous or multi-male/multi-female mating systems (Table 7.1).

One of the many contributions made to this field by J. M. Tanner was his discovery that since the

Sex differences at puberty Girls

Breast bud

Pubic hair begins

--mrTTTTTITTnTIITrnTmTTTTTm--

Peak height spurt

Menarche

Pubic hair adult

Breast adult

Boys

Genital dev. begins

Pubic hair begins

Peak height spurt

-^ttttttTTTTÏÏÏÏÏÏÏÏTTïïTTTTTT^ Genitalia adult

Pubic hair adult

13 14 15 Age (years)

16 17 18 19 20

Figure 7.4 Timing of physical and associated changes throughout puberty in girls and boys. Source: After Marshall (1970).

HUMAN SEXUAL DIMORPHISM: OPPOSITES ATTRACT Table 7.1 Sexual bimaturism in Old World monkeys, apes, and humans

Species

Females (age in years)

Males (age in years)

At puberty

Full grown

At puberty

Full grown

Chlorocebus aethiops

2.8

-

5

-

Erythrocebus patas

-

3

4

5

Miopithecus talapoin

4

5

-

6

Lophocebus albigena

3.6

5

4.5

7

Cercocebus atys

3

5

5

6

Mandrillus

2.75-4.5

5

4-8

8-10

sphinx

(3.6)

(6)

Papio cynocephalus

4.8

7

5.7

9

P. hamadryas

4.3

5.6

5.8

10

Macaca mulatta

2.5

6

3.5

8

M. sylvanus

3.5

5.5

4

7.5

Hylobates spp.

<6

8

<6

8

Pongo pygmaeus

7?

10-11

6-7

14+

Pan troglodytes

7.5-8.5

10-11

6-9

16.5

Gorilla gorilla

7.75

10

6-8

12-15

Homo sapiens: P

15-16

-

17

-

H. sapiens: M

12.8-13.2

16-18

13-14

20+

Source: Data from McCormack (1971); Tanner (1978); Harcourt et al. (1980); Dixson (1981); Dixson et al. (1982); Goodall (1986); Kingsley (1988); Wickings and Dixson (1992b); Bercovitch (2000); and Campbell et al. (2007). P = pre-industrial revolution; M = modern.

Source: Data from McCormack (1971); Tanner (1978); Harcourt et al. (1980); Dixson (1981); Dixson et al. (1982); Goodall (1986); Kingsley (1988); Wickings and Dixson (1992b); Bercovitch (2000); and Campbell et al. (2007). P = pre-industrial revolution; M = modern.

industrial revolution the age of puberty has declined steadily in Europe and in the USA. The average age at which girls reach menarche declined by about 4 months per decade, from 15-16 years in 1860, to 12.813.2 years during the 1960s (Tanner 1978). Menarche typically occurs in girls after the peak of the adolescent growth spurt in height. Girls increased in average height during the nineteenth and twentieth centuries, as well as entering puberty at a younger age. The same was true for boys, and the growth of the larynx, which takes place during puberty due to increased secretion of testosterone, now occurs at a significantly younger age than would have been the case historically. An ingenious demonstration of this was provided by Daw (1970) who examined the records of the ages at which 'breaking' and deepening of the voice occurred among boys in the J. S. Bach Choir in Leipzig. For the years 1729-49, such changes typically occurred at around 17 years of age. Nowadays, boys who are only 13 or 14 years old exhibit comparable development of the larynx resulting in deepening of the voice.

From the earliest phase of human evolution, the sexes would have exhibited bimaturism of reproductive development, with males passing through puberty, adolescence, and attaining sexual maturity later than females. Although the timing of these events occurs at younger ages in various modern human societies, the sequence of changes in growth, development of the reproductive systems and secondary sexual traits remains stable, as does the tendency for girls to mature earlier (Figure 7.4). As can be seen in Table 7.1, such bimaturism is typical of monkeys and apes which are sexually dimorphic in body weight. Indeed, Martin et al. (1994) were able to show that the age of attainment of sexual maturity is usually the same for males and females in those primates where the sexes are of equal size. However, for sexually dimorphic species (for which the ratio of adult male/adult female body weight exceeds 1.15) males reach maturity later than females. Human beings fall within the latter group, and the larger size and slower development of the human male is consistent with an evolutionary past involving a significant degree of inter-male competition for access to mating partners.

It is also relevant to mention here that males tend on average to have shorter lifespans than females in many vertebrates, including humans (Mealey 2000; Finch 2007). There is evidence that mating systems may have some impact upon this sex difference. Clutton-Brock and Isvaran (2007) reported that higher levels of inter-male competition for access to mates (such as occur in multi-male/multi-female groups and in polygy-nous one-male breeding units) are associated with reductions in male longevity (relative to females) in natural populations of mammals and birds. Table 7.2 summarizes the data from sixty-nine countries worldwide showing percentages of males and females in human populations at various ages. Initially, males tend to slightly outnumber females, probably due to a slight sex difference in birth sex ratios (105 male:100 female births). However, by 20-39 years of age percentages of males and females are almost equal and from 40 years of age onwards women increasingly outnumber men in the vast majority of countries that conduct population censuses. This sex difference in human longevity is intriguing, as it may be greater than expected for a monogamous mammal. Thus it could be relevant to judgments about the role of polygyny in human evolution. Unfortunately, no firm conclusion can be reached, as very few comparative data are available on longevity in monogamous mammals. Thus, Clutton-Brock and Isvaran's (2007) study relied mostly on information from socially monogamous birds and polygynous mammals.

Returning to the subject of changes in growth at adolescence in human beings, considerable differences occur in the development of muscle mass, fat depots and strength between the two sexes. Boys develop larger muscles than girls as well as having larger hearts and lungs, and they have greater numbers of erythrocytes (and a higher concentration of haemoglobin) in the blood. Harrison et al. (1988) summarize the evolutionary significance of these sex differences by stating that 'the male becomes at adolescence more adapted for the tasks of hunting, fighting and manipulating all sorts of heavy objects, as is necessary in some forms of food-gathering.' By contrast, females invest more physiological resources than males in the production and storage of fat, which is deposited particularly around the buttocks, hips, and thighs, as well as in the developing breast. These changes occur as a result of oes-trogenic stimulation, as adolescent girls transition to womanhood and become reproductively mature (Pond 1998). The physiological demands of pregnancy and lactation are such that these fat reserves play a crucial role in determining the success of female reproduction and the viability of offspring.

Thus, a second point relating to the question of sexual dimorphism and the likely polygynous origin of human mating systems concerns sex differences in adult body composition. Sexual dimorphism in human body composition is much more pronounced than can be conveyed by the measurements of male and female body weight alone. Clarys, Martin, and Drinkwater (1984) dissected male and female cadavers to measure exactly how much various tissues contribute to body composition in adulthood. As can be seen in Figure 7.5, muscle constituted 38.8

Table 7.2 Sex differences in human longevity: Age-related changes in percentages of males and females in the total population, in sixty-nine countries worldwide

Percentage of total population Age group (years)

Note: Data are mean ± SEM percentages of males and females of each age group in the total populations of 69 countries, as listed in The Financial Times World Desk Reference, 2004. * P < .05; ** P< .01.

Q Men

Women

Q Men

Women

Figure 7.5 Human body composition. Men show higher percentages of muscle, and women show higher percentages of fat, in relation to their overall body composition.

Source: Based upon data from Clarys et al. (1984).

Muscle Bone Residue

Skin

Muscle Bone Residue

Figure 7.5 Human body composition. Men show higher percentages of muscle, and women show higher percentages of fat, in relation to their overall body composition.

Source: Based upon data from Clarys et al. (1984).

per cent on average of body mass in men, but only 27.7 per cent of overall mass in women. Conversely, women included an average of 43.6 per cent fat (adipose tissue) in their body make-up, as compared to 28.4 per cent in men. Unfortunately, I have been unable to locate body composition data of the kind summarized here for any of the non-human primates. Nor are the cross-cultural data on sex differences in human body composition adequate for comparative purposes. Nonetheless, it is interesting that the ratio of male muscle mass/female muscle mass in Clarys et al.'s (1984) sample is 27.4:17.89 kg, which yields a sexual dimorphism ratio of 1.53. This is considerably higher than the male/female body weight ratio (1.1) for these same subjects and strengthens the conclusion that selection has favoured increased development of muscular strength in men. Indeed, from puberty onwards, males begin to out-perform females in tests which assess motor performance (e.g. measures of hand-grip, arm-pull, and arm thrust strength: Harrison et al. 1988). Sex differences in human body composition thus also support the conclusion that polygyny, rather than monogamy alone, has played some role in the origin of human sexuality. However, to examine this question further, it will be helpful to enquire how far human body shape and composition might influence mate choice, and whether any such effects are cross-culturally consistent and likely to represent ancient traits for H. sapiens.

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