Interdepartmental Doctoral Program in Anthropological Sciences, Department of Anthropology, Stony Brook University, Stony Brook, NY 11790, USA.
Human Biology Program, Department of Biology, University of Indianapolis, 1400 E. Hanna Ave., Indianapolis, IN 46227, USA.
Homo. 2023 Apr 14;74(1):1-15. doi: 10.1127/homo/2023/1486.
Discussions of the evolution of sexual dimorphism in torso shape and the pectoral region assume that this dimorphism exists independently of body size. We test this assumption in two human populations and further examine what is needed to understand sexual dimorphism in the pectoral region. Modern human males have broad shoulders and narrow hips relative to females, lending males a more triangular torso. The wider female pelvis is commonly attributed to obstetric pressures while the broader male pectoral girdle has been argued to be an adaptation that improves hunting or intrasexual competition. While sexual dimorphism in the pelvic girdle is known to exist after adjusting for body size across human populations, most studies of sexual dimorphism in the pectoral girdle have not adjusted the data to account for sexual size dimorphism or compared different ancestral groups. The aforementioned hypotheses explaining sexual dimorphism in the clavicle and scapula as products of natural selection are predicated on the untested assumption that sex differences do not scale with body size. This study tests this assumption by comparing various measurements of the pectoral girdle, the pelvic girdle, and six pectoral-pelvic indices of black and white South Africans of known sex and height to test whether the sexes and ancestral groups will differ in these values after adjusting for differences in body size. Comparisons of ancestral groups reveal that white South Africans have larger pectoral and pelvic dimensions than black South Africans, but that blacks have larger index values than whites. Regardless of differences in ancestry and body size, males have significantly broader pectoral regions as indicated by comparisons of both individual pectoral measurements and pectoral-pelvic indices. This pattern of sexual dimorphism is reversed in the pelvic region where females have larger skeletal elements. In addition to finding both absolute and relative differences in mean values for the pectoral and pelvic skeleton, females and males and blacks and whites differ in the scaling relationship of these traits, suggesting different allometric trajectories for these bones that may be explained by their distinct evolutionary functions, their adaptations to specific environments, or by changes in lengths due to age. These results suggest that sexual dimorphism in the pectoral region is not a product of scaling and that differences in this region reflect adaptive forces acting in unique ways on each sex, consistent with the assumptions of earlier evolutionary explanations.
讨论躯干形状和胸部区域的性二态性的进化假设这种二态性独立于体型存在。我们在两个人群中检验了这一假设，并进一步研究了理解胸部区域性二态性所需的条件。现代男性相对于女性而言，肩部较宽，臀部较窄，使男性的躯干呈三角形。女性较宽的骨盆通常归因于分娩压力，而男性较宽的胸带则被认为是一种适应，可提高狩猎或同性竞争的能力。尽管在跨人群调整体型后已知骨盆带存在性二态性，但大多数有关胸带性二态性的研究都没有调整数据以考虑性体型二态性或比较不同的祖先群体。解释锁骨和肩胛骨性二态性为自然选择产物的上述假说，是以未经检验的假设为前提的，即性别差异不会随体型而变化。本研究通过比较黑人和南非白人的胸带、骨盆带以及六个胸-盆指数的各种测量值来检验这一假设，这些南非人的性别和身高已知，以测试在调整体型差异后，性别和祖先群体在这些值上是否会有所不同。对祖先群体的比较表明，南非白人的胸带和骨盆尺寸大于南非黑人，但黑人的指数值大于白人。无论祖先和体型大小的差异如何，男性的胸区都明显更宽，这一点可以通过比较单个胸区测量值和胸-盆指数来证明。这种性二态性在骨盆区域中是相反的，在骨盆区域中，女性的骨骼元素更大。除了发现胸带和骨盆骨骼的平均值存在绝对和相对差异外，女性、男性、黑人、白人在这些特征的缩放关系上也存在差异，这表明这些骨骼的不同的异速生长轨迹可能是由其独特的进化功能、对特定环境的适应或因年龄而导致的长度变化所解释。这些结果表明，胸部区域的性二态性不是缩放的产物，而且该区域的差异反映了对每个性别的适应力以独特的方式发挥作用，这与早期进化解释的假设一致。
