Linearity in the real world: An experimental assessment of nonlinearity in terrestrial locomotion

Abstract

Amongst early human ancestors, cross-sectional geometric properties of lower limb bones are particularly useful for reconstructing mobility patterns. Experimental studies of diaphyseal loads characterizing locomotor activities, however, demonstrate disconnect with theoretical loads predicted from bone morphology alone. This complicates population-level comparisons, and makes specific behavioral inferences tenuous. Lack of a consistent definition for mobility further complicates comparisons. To contribute towards a consensus definition of mobility, here I address one specific relevant factor-what are the effects of nonlinear locomotion or turning. Mice in custom-designed cages accentuating turning (condition 1) and linear movement (condition 2) were compared with mice (control) permitted to move freely in cages. Locomotor behavior of individuals was documented multiple times per day. At the end of the experiment, limb bones were harvested, scanned with high resolution CT, and subjected to structural analyses. Comparing growing BALB/cByJ female mice from a previous experiment and growing C57BL/6J female mice (n = 30, 10 per group) in the present experiment, permits comparisons of structural effects of movement regimes on femoral cortical areas, second moments of area, polar moment of area, and shape ratios, as well as activity profiles. C57BL/6J groups differed in activity level, while BALB/cByJ groups did not. Mice in turning groups tended to have more elliptical diaphyses, while linear and control mice differed comparatively less often. Distinctive diaphyseal shapes in turning mice support the idea that nonlinear movements (e.g., turning) have recognizable effects on long bone diaphyseal structure. This suggests limb loading, likely in side-to-side orientations, is relatively high during turning compared to linear movement.