The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Is female morphology selected for economy and male morphology selected for efficiency? Evidence from studies on humans

CARA M. WALL-SCHEFFLER1 and MARCIE J. MYERS2.

1Biology, Seattle Pacific University, 2Biology, University of St. Catherine

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Female humans are typically smaller than males with relatively broader pelvises. While these sexually dimorphic characters are touted as adaptations for successful child-carrying and birthing, their interaction with other survival necessities must be considered. Particularly, locomotor and thermoregulatory costs are influenced by sexually dimorphic traits such as body size. We compare data on males (N=10) and females (N=9) walking at four self-selected speeds, while oxygen consumption and core temperature are monitored, in order to assess morphological components that differentiate economy (cost/time) and efficiency (cost/distance) in the context of movement-related thermoregulatory challenges. Males had significantly larger masses and lower limb lengths (p<0.03), and relatively narrower pelvises (p=0.13). Optimal walking curves were calculated for each individual. Mean core temperatures at rest were subtracted from the mean at each walking speed to assess changes in core temperature during activity. Males had faster optimal walking speeds (10.9%,p=0.02), higher costs at the optimum (23.4%,p=0.002), and increased curvature (22.1%,p=0.28) of their optimal walking curves. While there were no significant differences in core resting temperatures (p=0.52), males showed a larger increase in body temperature (36.9%,p=0.14) during activity. Females’ slower walking speeds, reduced metabolic costs and smaller masses indicate females are generally more economical walkers than males. Keeping heat loads down is essential for successful reproduction. These data suggest that females’ small size promotes heat loss while slow speeds prevent excessive heat gain. Increasing heat load by adding subsistence or child burdens may cause females to face a conflict between economical and efficient walking speeds.

This study was funded by the Murdock Charitable Trust, reference number 2006194:JVA:11/16/2006.

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