The Interactive Influence of Climate and Energetic Factors on Human Nasal Morphology

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Cold‐dry environments require considerable transfers of heat and moisture from the respiratory tract to inspired air in order to prevent thermal damage and desiccation of the lungs. Accordingly, humans indigenous to cold‐dry environments typically exhibit narrow nasal passages, as these enhance intranasal air‐conditioning by increasing contact between respired air and the heat/moisture‐rich nasal mucosa. Yet, cold‐dry environments are also metabolically expensive due to thermoregulatory demands, requiring greater oxygen intake than in tropical environments. Thus, humans from colder environments are also predicted to possess larger nasal passages (i.e., cross‐sectional areas) in order to transmit larger volumes of air during each breath. Still, despite substantial evidence that both climate and metabolism likely influence nasal size and shape, virtually no research has investigated the interplay of these two factors on nasal anatomy. Here it is hypothesized that climate‐mediated nasal narrowing may necessitate a compensatory increase in nasal height to ensure the airways remain large enough to transmit a metabolically adequate volume of oxygen. To test this, we collected 17 linear measurements from the nasal skeleton of 119 modern humans from 10 climatically diverse geographic areas (Arctic Circle, Europe, Iran, Australia, North Africa, Khoisan, South African Bantu, East Africa, West Africa, Papua New Guinea). Measurements of associated postcranial elements were then used to estimate body mass and basal metabolic rate (BMR) for each individual. Climatic data (i.e., monthly air temperature and absolute humidity) were similarly collected for the geographic provenance of each individual. These morphological, energetic, and climatic data were subsequently employed in multivariate analyses to assess potential interactive influences of climate and metabolism on nasal morphology. Our results indicate that most measurements of nasal complex breadth are significantly correlated with climate (all r‐values >0.45, all p‐values <0.009), but not BMR. Conversely, nasal height is more strongly correlated with BMR (r=0.47, p=0.02) than climate. Additionally, nasal passage cross‐sectional area demonstrates a positive association with BMR (r=0.74, p=0.0007), while passage cross‐sectional shape exhibits a significant relationship with climate (r=0.52, p=0.0017) with taller/narrower airways found in colder‐drier environments. Our results support assertions that nasal narrowing in colder climates necessitates a concomitant increase in nasal height to maintain an overall airway size capable of meeting energetic demands for oxygen intake. Moreover, these patterns of nasal morphology appear consistent with well‐established ecogeographic distributions in human body size/shape attributed to Bergmann’s and Allen’s Rules. Thus, future research employing larger and more diverse samples appears poised to provide far‐reaching insights regarding climatic adaptation during human evolution.