Do big females have big pelves?

Previous studies have shown that maternal stature is a correlate of both pelvic size and reproductive efficiency. This study addresses the issue of body size and obstetric advantage. The relationship between pelvic size and three nonpelvic measures of body size is determined for females and males. The skeletal sample consists of blacks, whites, and Native Americans. The variables include 28 measures of the pelvis, length and head diameter of the femur, and clavicular length. The coefficient of multiple determination (CMD) is computed for each pelvic measure using multiple regression, with the three nonpelvic measures serving as the independent variables. Partial correlation coefficients are also calculated between each pelvic and nonpelvic variable, while controlling for the other two nonpelvic variables. The results show that all CMDs in females and all but one CMD in males are "low," i.e., below 33%. The sexes are nonsignificantly different in their CMDs for 22 of the 28 pelvic variables; of the six variables that are significantly different, five are of the midplane. The sexes are also broadly comparable in their partial correlations. The results are explained as follows. First, the concordance between the sexes in the relationship between pelvic size and nonpelvic measures of body size is due to their genetic similarity for homologous structures. Second, as pelvic size is at the minimum at the midplane, the sexual differences in CMDs are the result of selection with respect to obstetrics. Third, four explanations for the low CMDs are discussed: 1) lack of populationally or racially specific analysis; 2) nonlinear relationship between pelvic size and nonpelvic measures of body size; 3) combination of negative allometric selection between newborn body weight and maternal stature and weight with positive selection for maternal pelvic size; and 4) hormonally induced increase in pelvic capacity during parturition.