Latitude and intersocietal variation of human sexual dimorphism of stature

In a recent article Alexander et al.(1979) argue that intersocietal variation in human sexual dimorphism of stature can be explained by differing degrees of male- male competition for mates found in different types of societal marriage systems. In this paper we reexamine their data and suggest that their results may be more simply explained by reference to the latitude of the societies in their sample. Our investigation indicates that latitude has a significant influence on the degree of societal sexual dimorphism of stature. Peoples of the arctic region and the short peoples of the equatorial regions are less dimorphic in stature than midlatitude peoples. Thus, we conclude that the sociobiological hypothesis presented by Alexander et al.cannot alone explain the intersocietal variation in sexual dimorphism of stature and that environmental factors must be taken into account in the explanation of such variation.This research was supported by a Faculty Research Grant from North Texas State University.     

中国现代人群两性身高差异分布及其影响因素

本文对中国现代人群的两性身高差异分布状况及其影响因素进行了分析。选用152处中国现代人群（含69处汉族人群和83处少数民族人群）的男、女性身高数据,计算两性身高差异指数,并对比该指数在南、北方汉族和少数民族人群间的分布差异,同时分析纬度、气候、体格大小与城乡环境因素对两性身高差异程度的影响。结果表明,中国男性的平均身高比女性高出约7.16%(4.72%~9.26%）;南、北方汉族和少数民族之间的两性身高差异程度相似,北方汉族和南方汉族两性身高差异程度相似,但北方少数民族的两性身高差异明显大于南方少数民族。此外,两性身高差异程度与纬度、气温年较差和年均风速呈低度线性正相关,与年均气温、年均降水量和年均相对湿度呈低度线性负相关,而与体格大小和城乡环境并无显著关联。这提示遗传和自然环境因素在中国现代人群两性身高差异的区域化演变中更趋主导性,而社会环境因素的影响程度相对较低。     

QUANTITATIVE GENETICS OF SEXUAL DIMORPHISM IN HUMAN BODY SIZE

A classical data set is used to predict the effect of selection on sexual dimorphism and on the population means of three characters—stature, span, and cubit—in humans. Given selection of equal intensity, the population means of stature and of cubit should respond more than 60 times as fast as dimorphism in these characters. The population mean of span should also respond far more rapidly than dimorphism, but no numerical estimate of the ratio of these rates was possible. These results imply that sexual dimorphism in these characters can evolve only very slowly. Consequently, hypotheses about the causes of sexual dimorphism cannot be tested by comparing the dimorphism of different human societies. It has been suggested that primate sexual dimorphism may be an allometric response to selection for larger body size. We show that such selection can indeed generate sexual dimorphism, but that this effect is too weak to account for the observed relationship between dimorphism and body size in primates.     

Human size evolution: no evolutionary allometric relationship between male and female stature

In many animal groups, sexual size dimorphism tends to be more pronounced in species with large body size. Similarly, in a previous cross-cultural analysis, male and female stature in humans were shown to be positively allometrically related, indicating a similar relationship where populations with larger stature were more dimorphic. In this study, we re-examine the hypothesis of an allometric relationship between the sexes using phylogenetic methodology. First, however, we tested whether there exist phylogenetic signals in male and female stature. Data on mean stature from 124 human populations was gathered from the literature. A phylogenetic test showed that male and female stature were significantly associated with phylogeny. These results indicate that comparative methods that to some degree incorporate genetic relatedness between populations are crucial when analyzing human size evolution in a cross-cultural context. Further, neither non-phylogenetic nor phylogenetic analyses revealed any allometric relationship between male and female stature. Thus, we found no support for the idea that sexual dimorphism increases with increasing stature in humans.     

Human marriage systems and sexual dimorphism in stature.

Contemporary populations of Homo sapiens are sexually dimorphic on a variety of traits. In terms of stature, men are reliably between 4% and 10% taller than women in well-sampled human populations. Are cross-cultural differences in the magnitude of sexual dimorphism consistent with expectations from sexual selection theory? Prior studies have provided conflicting answers to this question in part because they failed to agree on how the force of sexual selection should or could be operationalized. Here we offer a simple and unbiased method for operationalizing sexual selection and retest two separate predictions from earlier work (Alexander et al., 1979) about its expected impact on stature dimorphism in a sample of 155 societies. Neither prediction matches the observed cross-cultural distribution of dimorphism. However, this is not the consequence of a random distribution of dimorphism across societies. Instead, the data exhibit a robust and unexpected pattern.     

Sexual dimorphism in stature and women's work: a phylogenetic cross-cultural analysis.

The following cultural variables were tested for their association with sexual dimorphism: sexual division of labor, type of subsistence (hunting and agriculture), and polygyny. The transmission of these traits among populations was investigated. All the traits were found to be associated with phylogeny, indicating that they are inherited from mother to daughter populations. A cross-cultural comparative method was used which controls for the statistical effects of similarity due to common ancestry (Galton's problem). Cross-cultural variation in sexual dimorphism in stature is negatively associated with women's contribution to subsistence. Women are taller, relative to men, in societies where women contribute more to food production. This may be because female nutritional status is better in these societies. No relationship was found between sexual dimorphism and other aspects of subsistence or polygyny. These results are discussed in relation to other studies of sexual dimorphism in modern and archaeological populations, and in relation to cross-cultural variation in sex-biased parental investment.     

Is sexual dimorphism in the femur a "population specific phenomenon"?

The patterns of sexual dimorphism as well as the differences in amount between the populations were studied on a sample of 162 male and 159 female left femora, which were classified as Zulu, Sotho, Xosa and South Africans of European extraction. Multivariate analyses revealed that even adjacent African tribes exhibit a different pattern of sexual dimorphism, but there were similarities between Zulu and European femora. Furthermore, relative size differences, i.e. shape, discriminated more clearly between the sexes than did absolute size. Bicondylar width yielded a statistically significant higher degree of sexual dimorphism in Europeans when compared to African populations. This finding was interpreted in terms of the biomechanical demands on the femur under different living conditions. On the other hand, sexual dimorphism of femoral length did not differ among the populations. This was unexpected since femoral length correlates highly with stature, which was reported to show a lesser degree of sexual dimorphism in Africans than in Europeans. Detailed analyses of the results of the present study led to suggest that different living conditions may affect bones in complex ways of which linear growth is only one aspect.     

Spouse selection by health status and physical traits. Sardinia, 1856–1925

Military medical information and data from civil registers of death and marriage have been used to study the role of physical characteristics and health conditions in explaining access to marriage for the male population of Alghero, a small city located in Sardinia Island (Italy), at the turn of 19th century. Literature data about contemporary populations have already demonstrated the influence of somatic traits in the mate choice. The results presented here show that men with low height and poor health status at the age of 20 were negatively selected for marriage. This holds true also in a society where families often arranged marriages for their children. This pattern of male selection on marriage was found to be particularly marked among the richest and wealthiest SES groups. Our hypothesis is that this social group carefully selected for marriage those individuals who were apparently healthier and therefore more likely to guarantee good health status and better life conditions to offspring. In evolutionary terms, the mate choice component of sexual selection suggests that the height of prospective partners could be claimed as one of the determinants, along with other environmental causes, of the observed higher stature of men belonging to the wealthiest social strata of the Alghero population. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Human fertility variation, size-related obstetrical performance and the evolution of sexual stature dimorphism

In several animal species, change in sexual size dimorphism is a correlated response to selection on fecundity. In humans, different hypotheses have been proposed to explain the variation of sexual dimorphism in stature, but no consensus has yet emerged. In this paper, we evaluate from a theoretical and an empirical point of view the hypothesis that the extent of sexual dimorphism in human populations results from the interaction between fertility and size-related obstetric complications. We first developed an optimal evolutionary model based on extensive simulations and then we performed a comparative analysis for a total set of 38 countries worldwide. Our optimization modelling shows that size-related mortality factors do indeed have the potential to affect the extent of sexual stature dimorphism. Comparative analysis using generalized linear modelling supports the idea that maternal death caused by deliveries and complications of pregnancy (a variable known to be size related) could be a key determinant explaining variation in sexual stature dimorphism across populations. We discuss our results in relation to other hypotheses on the evolution of sexual stature dimorphism in humans.     

Relationship of sexual dimorphism in canine size and body size to social,behavioral, and ecological correlates in anthropoid primates

Among anthropoid primates there are interspecific differences in the degree of sexual dimorphism in both body size and canine size. Within the suborder body size dimorphism and canine size dimorphism are positively correlated,r=0.76. This correlation suggests that the two dimorphisms are equally developed in some species, while in other species there is a differential degree of sexual dimorphism. An analysis of these results and their relation to social organization and other ecological variables reveals: (1) the degree of canine size dimorphism is closely related to the amount of male intrasexual selection in a given mating system; and (2) the degree of body size dimorphism is also related to male intrasexual selection, but may be modified (either enhanced or diminished) by selection pressure from factors such as habitat, diet, foraging behavior, antipredator behavior, locomotory behavior, and female preference.     

Multivariate and geometric morphometrics in the analysis of sexual dimorphism variation in Podarcis lizards

Podarcis bocagei and P. carbonelli are two closely related lacertid species, very similar morphologically and ecologically. We investigated sexual dimorphism patterns presented by both species in allopatry and in sympatry. Sexual size and shape dimorphism patterns were analyzed using both multivariate and geometric morphometric techniques. Multivariate morphometrics revealed a marked sexual dimorphism in both species--males being larger with more robust habitus and females presenting a longer trunk. General patterns of sexual size dimorphism are not modified in sympatry, although there is evidence for some morphological change in male head size. The application of geometric morphometrics offered a more detailed image of head shape and revealed that males present a more developed tympanic area than do females, while females have a more rounded head. Differences in the degree of sexual shape dimorphism were detected in sympatry, but no consistent patterns were observed. From the results of the study, and based on previous knowledge on the populations studied, we conclude that the morphological differences observed are probably not caused by exploitative competition between the species, but rather appear attributable to the modification of the relative influence of sexual and natural selection on both sexes.     

Sex and Weapons: Contrasting Sexual Dimorphisms in Weaponry and Aggression in Snapping Shrimp

Sexual dimorphisms in weaponry and aggression are common in species in which one sex (usually males) competes for access to mates or resources necessary for reproduction – sexually dimorphic weaponry and aggression, in other words, are frequently the result of intrasexual selection. In snapping shrimp, the major chela (snapping claw) can be a deadly weapon, and males of many species have larger chelae than females, a pattern readily interpreted as resulting from intrasexual selection. Thus, males might be expected to show more sex‐specific aggression than females, and be more aggressive overall. We tested these predictions in two species of snapping shrimp in a territorial defense context. Neither of these predictions was supported: in both species, females, but not males, engaged in sex‐specific aggression and females were more aggressive than males overall. These contrasting sexual dimorphisms – larger weaponry in males but higher aggression in females – highlight the importance of considering the function of weaponry and aggression in contexts other than direct competitions over mates. In addition, species differences in the degree of sexual dimorphism in chela size were due to differences in female, not male, chela size, and the species with greater sexual dimorphism in weaponry was significantly less aggressive overall; also, while paired and solitary males did not differ in residual chela size, for the species with greater sexual dimorphism, females carrying embryos had smaller residual chela sizes. These results suggest that understanding the sexual dimorphisms in weaponry and aggression in snapping shrimp requires understanding the relative costs and benefits of both in females as well as males.     

Using visual modelling to study the evolution of lizard coloration: sexual selection drives the evolution of sexual dichromatism in lacertids

Sexual selection has been invoked as a major force in the evolution of secondary sexual traits, including sexually dimorphic colourations. For example, previous studies have shown that display complexity and elaborate ornamentation in lizards are associated with variables that reflect the intensity of intrasexual selection. However, these studies have relied on techniques of colour analysis based on human – rather than lizard – visual perception. Here, we use reflectance spectrophotometry and visual modelling to quantify sexual dichromatism considering the overall colour patterns of lacertids, a lizard clade in which visual signalling has traditionally been underrated. These objective methods of colour analysis reveal a large, previously unreported, degree of sexual dichromatism in lacertids. Using a comparative phylogenetic approach, we further demonstrate that sexual dichromatism is positively associated with body size dimorphism (an index of intrasexual selection), suggesting that conspicuous coloration in male lacertids has evolved to improve opponent assessment under conditions of intense male–male competition. Our findings provide the first evidence for the covariation of sexual dichromatism and sexual size dimorphism in lacertids and suggest that the prevalent role of intrasexual selection in the evolution of ornamental coloration is not restricted to the iguanian lineage, but rather may be a general trend common to many diurnal lizards.     

Ancient-to-modern secular changes in Korean stature

Statural growth in human populations is a sensitive indicator of socio-economic well-being, and improvements in socio-economic status are reflected in secular increases in adult height. In the present study, we investigated the statures of historical Korean societies to show how stature changed over time. Applying Fujii's equation, derived from modern Japanese, to the measurement of femora removed from 15th- to 19th-century Joseon tombs, the average heights of Korean adults during the Joseon dynasty were estimated to be 161.1 ± 5.6 cm and 148.9 ± 4.6 cm for males and females, respectively. Plotting statures for successive historical societies against time revealed that Korean heights remained relatively unchanged through to the end of the 19th century, a pattern that differs from that seen in many Western countries in which stature transiently decreases after the Middle Ages. In contrast, a sharp increase in Korean stature was observed at the beginning of the 20th century, similar to trends seen in other nations (although exact timing varies in different countries). There were no accompanying changes of stature sexual dimorphism. The data reported in this study reflect the unique historical experience of Korea; the relative isolation of Joseon society, the late onset of modernization (at the end of the 19th century), and the later occurrence of industrialization (during the 1960s).     

Variable preferences for sexual dimorphism in height as a strategy for increasing the pool of potential partners in humans

Human mate preferences are known to be related to a number of morphological traits. Those relating to female waist-to-hip ratio or body mass index and to male height appear to be distinctive mate-choice criteria and are known to be related to reproductive success. In addition to absolute height, a possibly important mate-choice criterion may be relative height, i.e. the extent of sexual dimorphism in stature (SDS = male height/female height) between oneself and a potential partner. Here, I demonstrate that people adjust their preferences for SDS in relation to their own height in order to increase the potential pool of partners. This causes nonlinearity in assortative mating in relation to height and shows that in relation to intrapopulational SDS both men and women are responsible for stabilizing selection.     

Sex-Biased Sound Symbolism in English-Language First Names

Sexual selection has resulted in sex-based size dimorphism in many mammals, including humans. In Western societies, average to taller stature men and comparatively shorter, slimmer women have higher reproductive success and are typically considered more attractive. This size dimorphism also extends to vocalisations in many species, again including humans, with larger individuals exhibiting lower formant frequencies than smaller individuals. Further, across many languages there are associations between phonemes and the expression of size (e.g. large /a, o/, small /i, e/), consistent with the frequency-size relationship in vocalisations. We suggest that naming preferences are a product of this frequency-size relationship, driving male names to sound larger and female names smaller, through sound symbolism. In a 10-year dataset of the most popular British, Australian and American names we show that male names are significantly more likely to contain larger sounding phonemes (e.g. “Thomas”), while female names are significantly more likely to contain smaller phonemes (e.g. “Emily”). The desire of parents to have comparatively larger, more masculine sons, and smaller, more feminine daughters, and the increased social success that accompanies more sex-stereotyped names, is likely to be driving English-language first names to exploit sound symbolism of size in line with sexual body size dimorphism.     

Short stature in African pygmies is not explained by sexual selection

African pygmies' short stature has been studied for more than a century, but the evolution of this extreme phenotype remains unknown. The present study tests the hypothesis that sexual selection, through preference for short partners, may have contributed to the evolution of pygmies' stature. We gathered anthropometric and familial data from 72 Baka pygmy couples and 27 neighboring Nzimé nonpygmy couples from Cameroon. We found evidence for positive assortative mating and partial evidence for the male-taller norm in both groups. This is surprisingly close to results reported for many modern occidental populations, in which sexual selection is thought to exert a positive selective pressure on men height. Semistructured interviews of Baka pygmies concerning height and mate choice suggested that the male-taller norm matches mating preferences. Stature was also positively correlated with the number of serial marriages contracted by men of both populations, while the stature of women was not related to their mating success. Finally, we did not detect any linear or quadratic effect of height on reproductive success for either men or women. Altogether, our results demonstrate that stature influences mate choice in pygmies, and we argue that, if of any influence for sexual selection, mate choice should have favored tallness rather than shortness in our pygmy population. Consequently, this study establishes that sexual selection is a very unlikely candidate to account for the evolution of pygmies' short stature.     

Sexual selection, natural selection and the evolution of dimorphic coloration and ornamentation in agamid lizards

Both sexual selection and natural selection can influence the form of dimorphism in secondary sexual traits. Here, we used a comparative approach to examine the relative roles of sexual selection and natural selection in the evolution of sexually dimorphic coloration (dichromatism) and ornamentation in agamid lizards. Sexual dimorphism in head and body size were used as indirect indicators of sexual selection, and habitat type (openness) as an index of natural selection. We examined separately the dichromatism of body regions "exposed to" and "concealed from" visual predators, because these body regions are likely to be subject to different selection pressures. Dichromatism of "exposed" body regions was significantly associated with habitat type: males were typically more conspicuously coloured than females in closed habitats. By contrast, dichromatism of "concealed" body regions and ornament dimorphism were positively associated with sexual size dimorphism (SSD). When we examined male and female ornamentation separately, however, both were positively associated with habitat openness in addition to snout-vent length and head SSD. These results suggest that natural selection constrains the evolution of elaborate ornamentation in both sexes as well as sexual dichromatism of body regions exposed to visual predators. By contrast, dichromatism of "concealed" body regions and degree of ornament dimorphism appear to be driven to a greater degree by sexual selection.     

Human sexual dimorphism in size may be triggered by environmental cues

Evolutionary biologists mostly assume that polygyny increases sexual dimorphism in size because, under polygyny, larger males monopolize mating opportunities and pass on their 'large male' genes to their sons. Available data on parent-child correlations in height among humans (Homo sapiens) do not support the crucial assumption that height is transmitted along sex lines. This paper instead suggests that human sexual dimorphism in size emerged, not because men got taller, but because women got shorter by undergoing early menarche in response to polygyny. It further speculates that, rather than genetically transmitted, the sexual dimorphism may emerge anew in each generation in response to the degree of polygyny in society. The analysis of comparative data supports the prediction that polygyny reduces women's height, but has no effect on men's, and is consistent with the speculation that the origin of human sexual dimorphism in size may be cultural, not genetic.     

Why are American mink sexually dimorphic? A role for niche separation

American mink are highly sexually dimorphic, with males being up to twice the size of females. Sexual dimorphism may arise for several reasons, including intra- or inter-sexual selection, inter-sexual competition, or divergent reproductive roles. Whether or not dimorphism arises from competition, a degree of niche separation is expected in dimorphic species. Sexual divergence in feeding niche has been reported for many species, including mink. This is likely to be manifested in a greater degree of dimorphism in those structures, such as teeth, that are used for the acquisition of prey. We tested the hypothesis that teeth and other trophic structures of male mink would be significantly larger than those of females, after controlling for underlying skeletal size differences. Canine and carnassial teeth, and several skull dimensions, were larger than predicted in males. There is good evidence that sexual dimorphism in mink trophic apparati is greater than predicted from allometry. We examined the development of dimorphism in various features with age and found that it was not consistent. Several trophic features were dimorphic amongst juveniles, and the degree of dimorphism remained relatively constant with age. Dimorphism in canines, and in relative body mass, was less apparent amongst juveniles and increased with increasing age. We discuss our results in the light of contemporary theories on the evolution and maintenance of sexual size dimorphism and argue that niche separation as a result of dimorphism in trophic features, while probably not the driving force behind sexual size dimorphism, may play a role in its maintenance.