Abstract

Both extinct and extant hominin populations display morphological features consistent with Bergmann's and Allen's Rules. However, the functional implications of the morphologies described by these ecological laws are poorly understood. We examined this through the lens of endurance running. Previous research concerning endurance running has focused on locomotor energetic economy. We considered a less-studied dimension of functionality, thermoregulation. The performance of male ultra-marathon runners (n = 88) competing in hot and cold environments was analysed with reference to expected thermoregulatory energy costs and the optimal morphologies predicted by Bergmann's and Allen's Rules. Ecogeographical patterning supporting both principles was observed in thermally challenging environments. Finishers of hot-condition events had significantly longer legs than finishers of cold-condition events. Furthermore, hot-condition finishers had significantly longer legs than those failing to complete hot-condition events. A degree of niche-picking was evident; athletes may have tailored their event entry choices in accordance with their previous race experiences. We propose that the interaction between prolonged physical exertion and hot or cold climates may induce powerful selective pressures driving morphological adaptation. The resulting phenotypes reduce thermoregulatory energetic expenditure, allowing diversion of energy to other functional outcomes such as faster running.

Highlights

  • Ecogeographical rules Effective thermoregulation is a key challenge facing individuals in different environments

  • It was hypothesised that ecogeographical patterning in human morphology would be observed in ultra-marathon runners successfully completing races in hot and cold environments

  • The present study further develops the idea of an energetic relationship between limb morphology, thermoregulation and locomotion

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Summary

Introduction

Ecogeographical rules Effective thermoregulation is a key challenge facing individuals in different environments. The two ecogeographic rules contend that endotherms will tend to be larger (Bergmann 1847) and display shorter limbs and body appendages (Allen 1887) in colder environments. Both rules are underpinned by the fundamental thermodynamic principles by which the body produces heat through cellular activity, such that heat generation is proportional to body mass, while heat loss through radiation, convection and evaporation is proportional to body surface area. Large body size is thereby associated with a large heat-producing mass relative to the heat-losing surface area, and is favoured in colder conditions; in warmer conditions the inverse holds

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