Does Covariation Between Cranial and Pelvic Shapes Alleviate the Obstetric Dilemma? Insights From a Brazilian Sample

Cranial and pelvic bones could have evolved in response to each other during human evolutionary history due to the increasingly tight fit between the baby's head and the mother's pelvis during delivery. A recently identified covariation pattern between these sets of bones and stature has shown important evidence of such an evolutionary trade-off, alleviating the chances of obstructed labor. Here, we tested the validity of this covariation pattern in a different sample, from a population with known high rates of C-section. 98 computed tomographies were used to perform statistical covariation tests (two-block partial least squares and ANOVA Procrustes) between pelvic and cranial shape, as well as other anthropometric variables, like stature, body mass, and BMI. Additionally, measurements were taken from cranial and pelvic bones for classic morphometric analyses. The results have shown an important sexual dimorphism in pelvic bones' shape but no correlation between them and stature or cranial size or shape. In terms of size, the sexual dimorphism on the true pelvis was also noticeable. The fact that the results obtained from this sample do not corroborate previous findings suggests the absence of this pattern in some populations. It also suggests that the current ideal rates of C-sections proposed by the World Health Organization might not be considering the existing diversity among human populations that may account for variable levels of difficulties in birth.