Academic journal article Journal of Anthropology

Size and Shape: Morphology's Impact on Human Speed and Mobility

Academic journal article Journal of Anthropology

Size and Shape: Morphology's Impact on Human Speed and Mobility

Article excerpt

Cara M. Wall-Scheffler 1, 2

Recommended by Benjamin Campbell

1, Department of Biology, Seattle Pacific University, Suite 205, 3307 3rd Avenue West, Seattle, WA 98119-1997, USA 2, Department of Anthropology, University of Washington Seattle, WA 98195, USA

Received 27 March 2012; Revised 23 June 2012; Accepted 11 July 2012

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

In recent years a number of key paleoanthropological finds and reconstructions have pushed into the forefront ideas of mobility strategies within and between hominin groups or species. Understanding mobility strategies is important in order to place groups within a particular adaptive environment or niche [1]. Distances traveled, time spent traveling, and group composition of a traveling party all determine the behavioral tradeoffs that provide the basis for selection pressures. For example, increasing the speed of one task, can increase the availability of time for another task (e.g., tool development [2]). Conversely, if essential foraging behavior takes up most of the day-light hours, social behaviors might suffer and group fragmentation may occur [3-5]. Studies that integrate mobility and reproductive success clearly show that the energetics of daily walking influence interbirth-intervals and offspring survivorship [6-9]. Studies that applied meta-analyses of ongoing selection in human populations further show a close relationship between locomotor morphology and reproductive success [10] and locomotor stress and drops in ovarian hormones [11]. Furthermore, changes in mobility have consistently been shown to influence weight loss/weight gain, and ovarian function and fecundity have consistently been shown to be particularly sensitive to changes in the metabolic balance/weight, such that an increased imbalance between energy in and energy out decreases fertility [12-17]. Such sensitivities have been shown both in energy-rich, healthy populations and also in lower body mass, less healthy populations. Thus, the speed of moving across a landscape and the energetic efficiency of mobility must be assessed together in order to understand the selection pressures involved in maintaining reproduction and accessing resources across any series of niche adaptations.

Humans and other mammals have been shown to have a curvilinear cost of transport (CoT: the metabolic cost to travel a given distance) relationship with speed during both walking and running [18-22]. This means that there is a speed at which the CoT is minimized. In reconstructions of mobility patterns and daily energy budgets [23, 24], assumptions are generally made that all people are traveling at their optimal speed--the speed at which the metabolic cost of walking is at the lowest [19, 25-30]. Evidence supports this general assumption that people (and other animals) adjust their speed by numerous amounts of physiological input, including energetic [27, 29, 31, 32], muscular [33, 34], and thermoregulatory [35-37], so that the speed of travel is generally near this minimum.

Given the evidence that people walk near their minimum CoT speed [19, 27-29], it remains possible that size and proportions significantly impact the speed at which walking occurs [38-40], and, as such, human populations with dimorphism may be suspected of either having different mobility strategies within the group or that part of the population accepts an energetic burden for walking at sub-optimal speeds. The extent to which the dimorphism impacts actual optimal speed and the penalties for each morph to walk at sub-optimal speeds have not been measured in extant populations. Here, data on the metabolic cost of human walking are presented in order to determine the effect of within-species size variation on the CoT curves. …

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