Sexual dimorphism and biomechanical loading in occipital bone morphological variation

We explore the contribution of biological sex and biomechanical activity from subsistence to occipital bone variation. Previous studies have used occipital bone traits to determine biological sex and identify ancestry to differing degrees of success. Biomechanical stress from variation in subsistence and gender-based divisions of labor could perhaps explain some of the noise in the signal for these grouping variables. To explore this possibility, we used metric (foramen magnum length and breadth, external occipital protuberance depth, lambda-inion length, bicondylar breadth) and nonmetric traits (general occipital form, presence of a nuchal crest, and nuchal line count). We collected original data and mined published data for our analysis using skeletal collections of Native American hunter-gatherers and horticulturalists, a historic military site, and contemporary study collections. We find that the foramen magnum area exhibits sexual dimorphism and is not influenced by subsistence but the accuracy of sex estimation is only 71%, suggesting the chance of being correct at slightly more than two-thirds. All traits exhibited sex-based variation but only bicondylar breadth and lambda-inion exhibited subsistence-based variation. Given the limited amount of variance explained by either sex or sex and subsistence, biomechanics may still play a role but not from the influence of subsistence practices. Additional data from a wider array of skeletal samples, perhaps with known occupation, is warranted if we are to understand how occipital variation is shaped. PRACTITIONER POINTS The study examines the role of biological sex and biomechanical stress in shaping occipital morphology. We introduce an area variable to describe the foramen magnum and apply ectocranial metric and nonmetric states for the first time to sex estimation. We share methods and R code for cleaning data, testing assumptions, and analysis of research questions.