The influence of climate on body proportions and form has been an area of study for over a century, starting with animal observations by Bergmann (1847) and Allen (1877) and moving to patterns in extant and extinct human populations around the world (Newman and Munro, 1955; Mayr, 1956; Schreider, 1964; Crognier, 1981; Trinkaus, 1981; Ruff, 1993, 1994; Katzmarzyk and Leonard, 1998; Tilkens et al., 2007). Considerable focus has been placed on populations in Africa, Europe, and the Americas (Schreider, 1964; Crognier, 1981; Holliday, 1997; Auerbach, 2007; Savell et al., 2016), but Asia remains largely underrepresented despite its long history of habitation (Bae et al., 2017), myriad of environmental conditions (Weightman, 2002), high genetic diversity (Cann et al., 1987; Karafet et al., 2001), and immense population size. When Asian remains are incorporated into regional or global adaptation studies, they typically have smaller samples (Katzmarzyk and Leonard, 1998; Betti et al., 2014; Maddux et al., 2017) and originate from the same countries, with the most frequent sampling among the modern and historic Japanese (Temple et al., 2008; Temple and Matsumura, 2011; Fukase et al., 2012, 2016; Kudaka et al., 2013) and Negrito populations from the Philippines (Betti et al., 2012; Kurki, 2013).

Clear clines in accordance with Bergmann's and Allen's rules of increased body breadth, reduced intralimb indices, and rounder crania with greater nasal projection, increased nasal height, and reduced nasal breadth are prevalent when examining populations of Africa and Europe from the equator to the arctic as the climate changes from hot and humid to cold and dry (Carey and Steegmann, 1981; Crognier, 1981; Franciscus and Long, 1991; Holliday, 1997; Savell et al., 2016). In contrast, Indigenous populations of the Americas exhibit broader bi-iliac breadths at all latitudes than those seen in Old World populations with muted gradation of limb proportions and unexpected variation in cranial and nasal shape for particular temperatures and humidity levels (Newman, 1953; Ruff, 1994; Auerbach, 2007). The disparity in morphological patterns could be due in part to the shorter habitation time to adapt to new environments in the Americas and the speed at which populations first crossed the region (Meltzer, 2002), but may also be due to natural selection acting upon regions of the body independently which would allow for different evolutionary tempos of the pelvis and limbs (Auerbach, 2012). As Asia is the ancestral place of origin of New World populations and a part of the Old World, a more comprehensive examination of how climate influences East Asian skeletal structure would enhance our understanding of global diversity and how environment influences human body form.

Despite considerable literature dedicated to ecogeographic research examining the head (Thomson and Buxton, 1923; Carey and Steegmann, 1981; Franciscus and Long, 1991; Roseman and Weaver, 2004; Harvati and Weaver, 2006; Hubbe et al., 2009; Yokley, 2009; Evteev et al., 2014) and postcrania (Newman and Munro, 1955; Schreider, 1964; Holliday, 1997; Katzmarzyk and Leonard, 1998; Stock, 2006; Savell et al., 2016) in isolation, there remains a paucity of studies that use a holistic approach to examine the cranium and postcranium simultaneously. Limited research investigating the impact of climate on the body as a whole has led to a lack of consensus on the strength of climatic influence on different parts of the human form within the individual. Greater correlations between cranial morphology and climatic variables using global data are supported by Beals et al. (1984), while conversely, examinations of European (von Cramon-Taubadel et al., 2013) and Native American populations (Auerbach, 2007) have found postcranial measurements to exhibit stronger correlations with climate. The discrepancy between these findings could be attributed in part to various factors, including which cranial and postcranial traits were compared, how differences in environment were quantified, and the particular statistical methods used in analyses. Additional holistic investigations on the evolutionary relationship between cranial and postcranial form within the body of the same individuals will assist in the establishment of a general agreement.

Although postcranial studies exploring the influence of climate ordinarily center upon limb dimensions and torso breadth, inclusion of the distal-most extremities is merited. Animal studies examining the applicability of Allen's rule have found temperature to impact not only limb length, but ear and foot size as well as tail length (Lee et al., 1969; Serrat et al., 2008; Serrat, 2014). Limited exploration of the hands in humans suggests that a reduction in metacarpal (Betti et al., 2015) and manual digit proportions (Payne et al., 2018a) provide better thermoregulation at colder temperatures. Expansion of ecogeographic studies to encompass parts of the body with greater exposure to the elements, and therefore, greater thermoregulatory challenges to overcome, will further our understanding of the relationship between climatic factors and the human skeleton.

Traditional evaluation of climatic influence has drawn associations between skeletal variation and various climatic factors (temperature, precipitation, humidity, vapor pressure, etc.) and their interactions (Thomson and Buxton, 1923; Newman and Munro, 1955; Carey and Steegmann, 1981; Crognier, 1981; Beals et al., 1984). The existence of a correlation between climate variables is useful in that it implies a potential adaptive response to environmental conditions. However, the relationship between body form and climate has proven to be more complex than previously assumed as traits may not vary between populations solely due to climatic pressure. As such, recent ecogeographic research has begun accounting for geographic proximity and genetic relatedness (Roseman, 2004; Harvati and Weaver, 2006; Evteev et al., 2014; Roseman and Auerbach, 2015; Katz et al., 2016; Betti, 2017), which, when combined, form the genetic structure of a population through the neutral evolutionary forces of drift, gene flow, and migration. Other researchers have also started examining particular regions of the body, such as the nose, from a functional point of view and have tested each part of the respiratory tract (Fukase et al., 2016; Evteev et al., 2017; Maddux et al., 2017).

These new assessments have been beneficial as clinal phenotypic variation in some traits are better explained by population structure than climate (von Cramon-Taubadel, 2014; Roseman, 2016). Cranial morphological variation has been strongly influenced by neutral evolutionary forces, making it a good representation of how population and evolutionary history shapes morphology in tangent (Relethford and Harpending, 1994; Roseman, 2004). However, when examined in sections, facial shape and facial features along with vault size have been found to better represent adaptation to climate than the temporal or neurocranium (Harvati and Weaver, 2006; Hubbe et al., 2009; Katz et al., 2016). The influence of drift, migration, and gene flow has also been identified in explorations of postcranial morphology. Comparison of within-population variance of the humerus, radius, femur, tibia, and pelvis with climate and geographic distance supports a relationship between the lower limb elements and minimum temperature while finding pelvic shape better preserves population history (Betti et al., 2012). In addition, mixed model comparisons of postcranial ecogeographic measurements and intralimb indices have highlighted that population structure alone or in combination with latitude better fits the data than models using only latitude (Roseman and Auerbach, 2015). Taken together, these studies emphasize that morphological variation in body form and ecogeographic trends can no longer be assumed to result solely from climate adaptation.

Although our understanding of how climatic pressures and neutral evolutionary forces affect skeletal morphology has improved with new approaches to the assessment of ecogeographic trends, patterns of selection between the sexes remain largely unexplored. Most studies have used only male samples (Newman and Munro, 1955; Crognier, 1981; Hubbe et al., 2009; Betti et al., 2012; Fukase et al., 2012; Roseman and Auerbach, 2015; Savell et al., 2016), which can result in the presumption that females will exhibit the same variation tendencies. Although this approach tends to occur because of the limited availability of female samples or to mitigate the need to account for sexual dimorphism, the sexes adaptive thermoregulatory responses might not mirror each other as closely as surmised. Male and female allometric patterns and levels of variance differ for maximum limb element lengths (Holliday and Ruff, 2001), which can impact intralimb indices and could support different thresholds of climatic sensitivity. Evaluation of Indigenous populations of the Americas by Auerbach (2007) found females and males did not always share morphological correlations with climate, but when they did, the association was more pronounced in males.

Regional morphological proportions and their relation to the body as a whole fluctuate from infancy to adulthood due to different development patterns (Bogin, 1997), but correlations between ecogeographic body proportions and latitude have been identified in children as early as one year of age, which are then maintained into skeletal maturity (Cowgill et al., 2012). As males exhibit longer periods of skeletal growth (Dunsworth, 2020) and show greater environmental sensitivity in various avenues of prenatal research (Stinson, 1985), they could be more susceptible to climatic stress postnatally as it can elongate their early critical phase of growth, which has been identified in animal studies as the period temperature exerts the greatest impact upon morphology as well as their maintenance phase where growth continues but temperature's effect is no longer significant (Serrat, 2013). Furthermore, populations living in cold environments have greater total energy expenditure than those living in other climates (Ocobock, 2016), and it has been proposed that a reduction in allocable energy for bone and muscle growth during development will have a greater impact on male form (Tague, 1989; Waxenbaum and Feiler, 2020). This would result in sex-specific patterns of association with climate as climatic factors would exert weaker adaptive pressure on females and more on males.

By including both sexes in analyses, patterns of climate influence in the body can also be explored on traits traditionally limited to studies of sexual dimorphism. Males and females within a population share the population's genetic history and as such trait change in one sex will often result in a response in both sexes, but weak natural selection and constant male and female fitness levels can allow for dimorphism to occur (Lande, 1980). Although sexual dimorphism tends to be constrained due to similar sex-specific heritability and cross-sex genetic correlations (Poissant et al., 2010), it is still impacted by the environment. The pelvis is widely accepted as the most dimorphic part of the body, but most studies evaluating the magnitude of ecogeographic trends only look at the maximum breadth of the ilia while little attention has been placed upon the size and shape of the bony birth canal, which may be influenced by climate in addition to obstetric constraints. Pelvic morphology varies geographically, and the lower constraints and covariance between sections of the human pelvis compared to other primates would allow for greater adaptive response to varying environments (Betti, 2017). Differences in latitude have been associated with variation in the shape of the bony birth canal with higher latitudes exhibiting larger transverse diameters while lower latitudes have increased anteroposterior diameters with reduced transverse widths (Kurki, 2013). This points to pelvic canal morphology being influenced by body size, which in turn is impacted by climatic variables.

This study takes a multidisciplinary approach to the evaluation of patterns throughout the body in relation to climate using a large East Asian skeletal sample. Analysis is focused upon the influence of minimum temperature of the coldest month as past studies have indicated that winter temperature extremes may place greater environmental stress on the body than any other season of the year or climate variable (Beals et al., 1983; Katz et al., 2016). The main aim is to determine if trends in cranial and postcranial morphology in the populations examined exhibit directional patterns when compared to changes in temperature after accounting for the effects of population structure. In doing so, subsequent determinations can be made as to whether ecogeographic patterns exhibited in East Asia fit those identified in other parts of the world and if traits throughout the body share similar correlation strengths with climate. In addition, the inclusion of data from both sexes allows for the comparison of climatic association in males and females to determine if potentially adaptive variation is occurring on the same traits and with matched intensity.