Interactive Influence of Climate and Energetics on Human Nasal Morphology

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2023-05

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For more than a century, studies have demonstrated strong statistical associations between climate and modern human nasal morphology, showing that populations indigenous to colder-drier environments exhibit relatively narrower nasal apertures, more projecting nasal bones, and taller/longer/narrower nasal cavities than those from warmer-humid environments. This ecogeographic patterning of nasal morphology has historically been attributed to pressures for intranasal air-conditioning (i.e., heat and moisture transfer) in different climatic conditions. However, recent studies suggest that energetic requirements also represent an important selection pressure on nasal morphology, with greater metabolic demands for oxygen intake necessitating larger nasal passages in order to respire more air with each breath. This theoretical precept has consequently been deemed the "respiratory-energetics hypothesis." Yet, despite substantial evidence that both climatic pressures and metabolic demands likely influence nasal anatomy, virtually no research has investigated the interaction and possible trade-offs of both factors on nasal size and shape. Thus, this dissertation has three objectives, 1) test the respiratory-energetics hypothesis in a mixed-sex sample from a circumscribed geographic locale (i.e., holding climate constant), 2) test the applicability of the respiratory-energetics hypothesis across ecogeographically diverse populations, and 3) investigate the influence of metabolic demands on ecogeographic variation in the nasal index. A total of 232 cranial computed tomography (CT) scans of modern human skeletons from a wide range of climatic regimes were employed in this study. Linear measurements and three-dimensional coordinate landmarks were collected from the nasal skeletons of each individual along with basal metabolic rate (BMR, kcal/day) estimates derived from associated postcranial elements. Multivariate analyses were then performed to complete each of the three objectives. This study found the basic tenets of the respiratory-energetics hypothesis to be supported both across and within modern human populations, with higher BMRs associated with larger nasal passages. Further, this study found that metabolically-mediated variation in passage size is predominantly driven by nasal passage heights and lengths, while breadths appear differentially constrained by climatic pressures.

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