Patterns of Variation in Human Sexual Dimorphism.

Anthropometric and densitometric data for 64 females and 31 males were collected in order to examine three aspects of sexual dimorphism, including the degree of sex differences in anatomical regions, the influence of total body size and composition on regional sex differences, and the extent to which patterns of soft-tissue distribution are shared by both sexes. All subjects were White volunteers from the Ann Arbor, Michigan area. Data were collected between December, 1978 and March, 1979. No significant differences were found between the two genders in either socioeconomic status or health history. Linked-segment models were used to define anatomical regions, with circumferences, diameters and heights measured for each segment, and skinfolds taken where appropriate. Segmental volumes and limb muscle circumferences were calculated from these measurements, and were validated hydrostatically. In addition, total body density was determined by underwater weighing. Females were found to have generally thicker skinfolds. Percent sexual dimorphism, calculated as 100 (M/F-1), ranged from four percent greater in the males (suprailiac skinfold) to 61 percent greater in the females (lateral mid-thigh skinfold). On the other h and , males had larger segmental volumes. Average values ranged from six percent larger gluteal segments in the females to 68 percent larger lower forearm segments in the males. Similarly, limb muscle circumferences were between 13 percent (calf) and 28 percent (forearm) greater in the males. Characteristic sex differences in total arm and leg length were contributed primarily by the extreme proximal and distal segments of each limb. Using stepwise multiple regressions, bi-iliac diameter and biacromial diameter were found to be more predictive of segmental volume and composition than was height. Analysis of covariance was used to statistically equalize mean sex differences in total size and in body composition. After equalization of size and composition, skinfold thicknesses and segmental volumes were altered from 17 to 22 percent. Muscularity was less affected, with an average change of seven percent. Generally, males and females of the same adjusted size were found to be more dissimilar in fatness but less dissimilar in segmental volume and limb muscularity than were typical males and females. Those regions which were most dimorphic following covariate adjustment of both size and fatness were the gluteals, mid-thigh and upper calf for the females and the forearm and dorsal trunk for the males. Patterns of soft-tissue distribution were isolated using cluster analysis, pattern profiles and principal components analysis. Characteristic patterns shared by each sex included a general inverse contrast of limb and trunk fatness, and a so-called "athletic" pattern characterized by large chest, arm and thigh volumes, elevated muscularity and stature, and reduced fatness. The results of this study offer support to an adaptive model of mosaic reduction in sexual dimorphism based on considerations of energy efficiency and muscular strength. Moreover, they suggest that previous suppositions about the degree and nature of sex differences in morphology may require re-examination.PhDPhysical anthropologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/157695/1/8017217.pd