Human cold adaptation: an unfinished agenda.

1975 marked the end of a 20-year period of human biology research on physical environment. The focus then shifted from climatic adaptation to problems of nutrition, disease, and stress. However, many questions about human environmental patterns, especially in reference to their evolution, were abandoned rather than resolved. Assumptions about cold protective functions of low surface area/body mass ratio are entrenched in physical anthropology, despite lack of experimental validation. Since heat loss is controlled by vasoregulation and tissue insulation, a simple physics model of SA:mass may not apply. The issue merits investigation, as do the assumed thermal advantages of foreshortened extremities. Physiological assessment remains our primary research tool. In cold climate natives, elevated basal metabolic rates now appear to be genetically induced. During cold exposure, the body manages heat conservation through well known channels but also by specialized thermogenic functions such as metabolism in brown adipose tissue (BAT). The powerful protective capacity of BAT is largely unexplored either within or between populations of cold exposed human adults. An irony of our profession is that many biological variables seem to have minor effects when compared to behavioral cold protections. This is partly because biological anthropologists may have made incorrect assumptions about what most threatens the well being of cold climate people. Contrasts in environmental behaviors when comparing northern cultures such as Inuit, Athabaskan, and Norse are particularly instructive. Adaptations to life in the cold may ultimately reveal their secrets through biocultural research design modeling of environmental research. With both practical and theoretical gains still wide open, the field needs renewed attention from human biology.

Developmental response to cold stress in cranial morphology of Rattus: implications for the interpretation of climatic adaptation in fossil hominins.

Adaptation to climate occupies a central position in biological anthropology. The demonstrable relationship between temperature and morphology in extant primates (including humans) forms the basis of the interpretation of the Pleistocene hominin Homo neanderthalensis as a cold-adapted species. There are contradictory signals, however, in the pattern of primate craniofacial changes associated with climatic conditions. To determine the direction and extent of craniofacial change associated with temperature, and to understand the proximate mechanisms underlying cold adaptations in vertebrates in general, dry crania from previous experiments on cold- and warm-reared rats were investigated using computed tomography scanning and three-dimensional digitization of cranial landmarks. Aspects of internal and external cranial morphology were compared using standard statistical and geometric morphometric techniques. The results suggest that the developmental response to cold stress produces subtle but significant changes in facial shape, and a relative decrease in the volume of the maxillary sinuses (and nasal cavity), both of which are independent of the size of the skull or postcranium. These changes are consistent with comparative studies of temperate climate primates, but contradict previous interpretations of cranial morphology of Pleistocene Hominini.

Physiological anthropology: past and future.

Environmental studies in adaptive human biology by North American anthropologists have a history of strong investigative research. From both laboratory and field work, we have gained major insights into human response to physical and social challenges. While these results were considered by most professionals to belong within evolutionary biology, in fact the intellectual structure sprang almost entirely from physiological equilibrium models. Consequently, physiological process itself was the focus. Further, most of the physiological patterns were not linked directly to important outcomes such as work output, reproductive success or survival. About 1975, American physiological anthropologists, led by Paul Baker, turned to studies of health, change and stress response. These studies were strong, but were still neither genetic nor evolutionary in intellectual structure. Evolutionary human biology was taken over by a new body of theory now called "behavior ecology", positing that selfish genes control human behavior to promote their own reproduction. This was paralleled by strong use of evolutionary theory in some areas of molecular biology. However, although physiological anthropologists have not focused on evolution, we have been developing powerful causal models that incorporate elements of physiology, morphology, physical environment and cultural behavior. In these "proximate" biocultural models, it is of little importance whether outcomes such as work or energy management are genetically based. Our future offers two major challenges. First, we must confirm causal links between specific physiological patterns and outcomes of practical importance to individuals and societies. Second, if we are to take our place in evolutionary biology, the one overarching theory of life on earth, we must understand the heritability of physiological traits, and determine whether they play a role in survival and reproduction.

Climate, racial category, and body proportions in the U.S.

In 1955, Newman and Munro reported correlations between physical characteristics and climate in a white male U.S. Army sample. For example, the body weight-to-mean annual temperature correlation was -0.460. Because the men descended from relatively recent immigrants to North America, physical clines implicitly derived from differential lifetime growth rather than from natural selection. Consequently, both causation and adaptive function of Bergmann's and Allen's biogeographic rules in humans were called into question. Analysis of male and female data from the 1988 U.S. Army anthropometric survey offers new insights to the 1955 study findings. Using state means of the male subsample identifying themselves as white, as did Newman and Munro, no significant correlations were found between climatic variables and height, weight, BMI, or other body proportions. With individual data rather than state mean values, neither white male nor white female samples showed morphology to climate correlations. Relationships seen in the earlier white sample have disappeared, possibly due to a more uniform growth environment and mobility in the U.S. Black males and females showed some body trait to climate correlations but only at r values of around 0.10. Using state means from the combined sample (racial identification ignored), strong correlations are seen. As examples, mean annual temperature correlates to male relative sitting height at r = -0.634 and to female relative forearm length at r = 0.645. However, these values are evidently spurious, being products of the higher percentages of whites enlisting from colder areas of the U.S.

Neandertal cold adaptation: physiological and energetic factors.

European Neandertals employed a complex set of physiological cold defenses, homologous to those seen in contemporary humans and nonhuman primates. While Neandertal morphological patterns, such as foreshortened extremities and low relative surface-area, may have explained some of the variance in cold resistance, it is suggested the adaptive package was strongly dependent on a rich array of physiological defenses. A summary of the environmental cold conditions in which the Neandertals lived is presented, and a comparative ethnographic model from Tierra del Fuego is used. Muscle and subcutaneous fat are excellent "passive" insulators. Neandertals were quite muscular, but it is unlikely that they could maintain enough superficial body fat to offer much cold protection. A major, high-energy metabolic adaptation facilitated by modest amounts of highly thermogenic brown adipose tissue (BAT) is proposed. In addition, Neandertals would have been protected by general mammalian cold defenses based on systemic vasoconstriction and intensified by acclimatization, aerobic fitness, and localized cold--induced vasodilation. However, these defenses are energetically expensive. Based on contemporary data from circumpolar peoples, it is estimated that Neandertals required 3,360 to 4,480 kcal per day to support strenuous winter foraging and cold resistance costs. Several specific genetic cold adaptations are also proposed--heat shock protein (actually, stress shock protein), an ACP*1 locus somatic growth factor, and a specialized calcium metabolism not as yet understood.

Field test of self-paced work capacity: Ifugao rice farmers.

This article reports results of a field test of work capacity on 30 male farmers ranging in age from 15-54. It involved a self-paced walk from the valley floor, up the mountain wall, and return, with heart rate monitoring. The route was 2.21 km long with a vertical rise of about 200 m. At its steepest, the grade was about 34 degrees, requiring long runs of steps cut into the mountain face. The purpose of this research was to determine whether biological and behavioral traits of individual men help to explain household economic productivity to which they contribute. The traits included anthropometry and a new measure of self-paced, voluntary work capacity (heart rate x time). Income per productive adult increased as did the relative fitness index (heart rate increase above resting x min taken to finish the course). However, the number of rice bundles earned per family, expected to increase with more fit workers, increased with the number of household adults and sitting height (adjusted R(2) = 0.392), but not with self-paced fitness markers. Am. J. Hum. Biol. 12:192-200, 2000. Copyright 2000 Wiley-Liss, Inc.

China productivity project: General description and analysis of productivity.

The amount of work that people do is a focal point of human life, an outcome with extraordinarily complex roots. The physical task itself, the natural setting, biological work capacity, and behavioral patterns presumably condition productivity. This paper presents a model by which work output of Chinese cycle haulers was investigated, and outlines investigative techniques including work physiology, health assessment, cold response, and ethnography of the workplace and home. The objective is to explain variation in work done on a daily, monthly, and seasonal basis. This paper also quantifies work output, or productivity, using long-term pay records as measures of productivity. While pay records, which show statistically normal distributions, serve as the primary dependent variable in the analysis, field observations and experiments offer supplementary data on the behaviors that produce work output. In a sample of 48 men, various measures of biological capacity and behaviors, such as motivation, predict overall productivity regardless of season. Since mean daily pay and monthly pay have different predictors, there is much individual choice in how many days per month one works. © 1995 Wiley-Liss, Inc.

Physical size, school performance, and visual-motor maturity in the Philippines.

The broad theoretical issue approached in this work is whether supposedly adaptive or maladaptive biological variation has any real impact on people's lives and well-being. From a poor barrio of a rural lowland Philippine fishing community, a sample of 25 boys and 25 girls who had completed first grade at 8 years was measured. This population may be considered moderately undernourished by NCHS but not Philippine standards. Correlations between anthropometry, school grades, and visual-motor skills were calculated. Grades, taken here to have both cognitive and social-behavioral components, correlated best to percentage of median height (NCHS and Philippine), less strongly but still significantly to visual motor skill, and not to weight for height. Visual-motor skills (estimated by the Beery VMI) did not correlate to anthropometry. Visual-motor performance, however, is low by U.S. standards, a finding discussed in a behavioral context.