The influence of humidity and precipitation on skeletal morphological variation in East Asia

Human ecogeographic proportions in Europe, Africa, and the Americas are well studied, but Asia remains underrepresented despite its diverse range of climates, latitudinal expanse, and long history of habitation. The monsoon is a significant environmental force across East Asia with two distinct phases: a summer phase and winter phase. The southwest winds of the summer component cross the equator bringing heavy rain during the warmer, humid months of the year while the winter's northeastern winds bring cold, dry air down from Siberia during the year's coldest months. Previous assessment of East Asian body form and minimum temperature during the monsoon's winter phase has found body size to conform with Bergmann's rule while limb length patterns did not follow Allen's rule. These analyses found that Southeast Asian are small overall, possibly due to the greater selective pressure of living in a humid climate which is exacerbated by the monsoon's heavy rain summer component. Reduction of body size in these populations could allow for better thermoregulation as the climate conditions of Southeast Asia are not conducive to heat dissipation via sweat evaporation. For this study, measurements throughout the body were gathered on female (N=439) and male (N=460) individuals from seven East Asian countries and represent 10 populations. Osteometric data and proportional indices were combined with appropriately matched autosomal SNP data, precipitation, and humidity data using linear mixed model analysis to evaluate the selective pressure of these climatic factors specifically associated with the summer phase of the monsoon. The influence of both climate variables was found to vary depending on the region of the body assessed. This further supports the need for ecogeographic research to assess the body as a whole and highlights the complexity of climatic adaptation.