Estimating body Mass and Nutritional Status from Subadult Hominin Skeletons

Abstract

Hominin body size and shape are key components to reconstructing phylogeny, life history, adaptation, and behavior in ancestral populations^1^. For adults, geometric properties of the femoral midshaft cross-section are used to infer locomotor behavior^2-5^, subsistence practices^6-7^, and functional adaptations^8-10^. Recently research has focused on patterns of compact bone ontogeny, particularly in regard to changes in bone strength with the acquisition of bipedal locomotion^11-13^ and examining population differences in ontogenetic trajectories for subadult humans^14-16^ and Neandertals^16^. Because femoral midshaft geometry is primarily shaped by biomechanical strains-weight bearing, locomotion, and muscle action-ontogenetic research requires estimates of body mass from the skeleton. Currently we use the width of the distal end of the femur to estimate subadult body mass but articular ends are constrained by pressures for joint congruence. Here I demonstrate the strong and consistent scaling relationship between body mass and femoral torsional strength (J) derived from measurement of midshaft cross-section geometry. Bone ends are most significantly affected by locomotor strain during the second year of life, during the initial acquisition of obligate bipedalism, after that time they are increasingly constrained by joint congruence. Results of a comparison of body mass for body size in prehistoric populations support the hypothesis that the midshaft is also a more sensitive indicator of population differences in body mass and activity levels during ontogeny. The femoral midshaft can be used to detect starvation and growth disruption in past populations because there is a mechano-biological interaction effect whereby disruptions in nutritional, metabolic and hormonal status lead to low body mass and activity levels^1, 11-13, 17-22^.