Human Adaptation, Somatotyping and Demography: Bergmann's and Allen's Rules, the Human Adaptability Programme, Adaptation to Heat, Cold and High Altitude, Somatotyping from Kretschmer and Sheldon to Parnell and Heath–Carter, and Demography — Its Multidisciplinary Nature, Anthropological Demography, Fertility, Morbidity, Mortality and Migration, and Crow's Index of Selection Intensity Worked

Adaptation is any feature — genetic, developmental, physiological or cultural — that improves an organism's fit to its environment. Anthropologists distinguish genetic adaptation (selected over generations: sickle cell against malaria, the Tibetan EPAS1 allele), developmental adaptation or plasticity (acquired irreversibly during growth: the large chests of Andean children), acclimatisation (reversible physiological adjustment within a lifetime: sweating more after weeks in the heat) and cultural adaptation (clothing, housing, fire, diet), which in humans buffers all the rest. Two ecogeographic rules, framed for warm-blooded animals, apply to human body form. Bergmann's rule (Carl Bergmann, 1847): within a species, populations in colder climates have larger body mass, because volume (heat production) rises faster than surface area (heat loss) as size increases — so a large body conserves heat and a small one sheds it; Roberts (1953) showed mean body weight in humans falls with mean annual temperature. Allen's rule (Joel Allen, 1877): populations in colder climates have shorter extremities — limbs, ears, noses — relative to the trunk, again reducing surface area per unit mass; the stocky, short-limbed Inuit and the linear, long-limbed Nilotes are the textbook pair, and the Neanderthal body is the fossil case. Both rules are read through the surface-to-volume ratio and the relative sitting height or the crural index, and both are statistical tendencies with exceptions produced by diet, activity and gene flow. The Human Adaptability Programme (HAP) was one of the seven sections of the International Biological Programme (IBP, 1964–74), directed on the human side by Joseph Weiner: a coordinated, worldwide study of human populations in extreme environments — high altitude (Andes, Himalaya, Ethiopia), the Arctic (Inuit, Sami), the tropics and deserts (Kalahari San, Australian Aborigines), and isolated islands — using standardised methods of anthropometry, physiology, nutrition, genetics and demography (Weiner and Lourie's IBP Handbook No. 9, 1969). It established the comparative baseline for everything in this chapter, and Indian participation (the Anthropological Survey, the universities) produced data on tribal and high-altitude groups.

Somatotyping is the classification and quantification of body build (physique). Ernst Kretschmer, a German psychiatrist, in Körperbau und Charakter (Physique and Character, 1921), sorted patients into the pyknic (round, stocky, short-necked — associated in his sample with manic-depressive illness), the athletic (muscular, broad-shouldered), the leptosomic or asthenic (thin, narrow, long-limbed — associated with schizophrenia) and the dysplastic (mixed or abnormal); his typology and his psychiatric correlations are historical. William Sheldon, in The Varieties of Human Physique (1940, with Stevens and Tucker), replaced types with three continuous components named for the embryonic germ layers and rated each on a seven-point scale from standardised photographs (photoscopic method) with height/∛weight as the guide: endomorphy (roundness, softness, predominance of the digestive viscera), mesomorphy (muscle and bone, the athletic build) and ectomorphy (linearity, fragility, a large surface relative to mass). A somatotype is written as three digits in that order — 7-1-1 the extreme endomorph, 1-7-1 the extreme mesomorph, 1-1-7 the extreme ectomorph, 4-4-4 balanced — and Sheldon's claim of correlated temperaments (viscerotonia, somatotonia, cerebrotonia) in The Varieties of Temperament (1942) is discarded. R.W. Parnell (Behaviour and Physique, 1958) made the rating anthropometric rather than photographic, with the M.4 deviation chart: fat (three skinfolds), muscularity (bone breadths and limb girths) and linearity (height/∛weight) scored against age-specific tables, keeping Sheldon's 1–7 scale. Barbara Heath and Lindsay Carter (1967; Carter and Heath, Somatotyping: Development and Applications, 1990) produced the method now standard: an anthropometric somatotype from ten measurements — height, weight, four skinfolds (triceps, subscapular, supraspinale, medial calf), two bone breadths (biepicondylar humerus and femur) and two girths (flexed upper arm and calf) — with an open-ended rating scale (not capped at 7), age- and sex-independent, and a somatochart on which the three components are plotted as a single point by the coordinates x = ectomorphy − endomorphy and y = 2 × mesomorphy − (endomorphy + ectomorphy). Applications: kinanthropometry and sports science (the mesomorphic profiles of wrestlers and sprinters, the ecto-mesomorphy of high-jumpers, the endo-mesomorphy of shot-putters), growth and nutrition studies (somatotype changes with age and undernutrition), ergonomics, medicine (the association of endomorphy with cardiovascular risk), and the anthropological comparison of populations — Indian tribal and caste groups have been somatotyped in a large literature from the 1970s.

Demography (Achille Guillard coined the word in 1855) is the statistical study of human populations — their size, composition and distribution and the three processes that change them: fertility, mortality and migration. It is multidisciplinary by nature: its data come from censuses, vital registration and surveys (statistics); its explanations from economics (Malthus's Essay of 1798; the demographic transition of Thompson 1929 and Notestein 1945, from high to low birth and death rates), sociology (family, marriage, education), geography (distribution, urbanisation), public health and medicine (morbidity, mortality), biology and genetics (fecundity, selection) and history (reconstitution from parish registers). Anthropological demography is the anthropologist's contribution and complement: it studies small populations — a tribe, a caste, a village, a band — often without registration data, by genealogies, censuses of its own and long residence; it treats fertility, marriage and death as embedded in kinship, ritual and economy rather than as individual choices; it reads the demography of hunter-gatherers (Howell's Dobe !Kung, 1979) and of pastoralists and peasants as adaptations; and it feeds population genetics with the mating structure and reproductive variance that selection works on. Its measures are the demographer's. Fertility (actual reproduction, as against fecundity, the biological capacity) is measured by the crude birth rate (live births per 1,000 mid-year population), the general fertility rate (births per 1,000 women aged 15–49), age-specific fertility rates and the total fertility rate (the average number of children a woman would bear over her life at current age-specific rates — replacement level about 2.1), with the gross and net reproduction rates counting daughters; its determinants are the proximate ones of Davis and Blake (1956) and Bongaarts (1978) — marriage, contraception, abortion, breastfeeding and postpartum abstinence, sterility — behind which stand education, income, religion, infant mortality and the value of children. Morbidity, the burden of illness, is measured by incidence (new cases per 1,000 per year) and prevalence (existing cases at a point of time), and its determinants are nutrition, sanitation, crowding, occupation and health services. Mortality is measured by the crude death rate (deaths per 1,000 mid-year population), age- and cause-specific rates, the infant mortality rate (deaths under one year per 1,000 live births, the most sensitive index of living conditions), the under-five and maternal mortality rates, and life expectancy at birth from the life table; its determinants are nutrition, infection, sanitation, medical care, income, sex (women outlive men) and age. Migration is a change of usual residence across a boundary, internal or international, in-migration and out-migration giving net migration; Ravenstein's laws (1885), Lee's push and pull factors (1966), Zelinsky's mobility transition, and the anthropological types — seasonal, circular, chain and forced migration, rural–urban and tribal displacement — are its frame, and its determinants are employment, land, marriage, education, conflict and disaster. Worked: a district has a mid-year population of 50,000; in the year there were 1,500 live births, 400 deaths and 60 infant deaths. CBR = 1,500 ÷ 50,000 × 1,000 = 30 per 1,000; CDR = 400 ÷ 50,000 × 1,000 = 8 per 1,000; the rate of natural increase = 30 − 8 = 22 per 1,000 = 2.2% a year; IMR = 60 ÷ 1,500 × 1,000 = 40 per 1,000 live births. If the seven five-year age-specific fertility rates (15–19 to 45–49) sum to 700 per 1,000 women, TFR = 5 × 700 ÷ 1,000 = 3.5 children per woman.

Natural selection can act only where individuals differ in survival to reproductive age and in the number of offspring they leave, and James Crow (1958) proposed an index of the total opportunity for selection that a population's demography allows, computed from those two components. The mortality component Im = Pd ÷ Ps, where Pd is the proportion of live-born who die before reproductive age and Ps = 1 − Pd the proportion who survive. The fertility component If = Vf ÷ x̄², where x̄ is the mean number of live births per surviving woman (or per person) who reaches reproductive age and Vf the variance of that number. The total index I = Im + If ÷ Ps — the fertility component is divided by Ps because only survivors reproduce. Worked: in a tribal population 20% of children die before maturity, so Pd = 0.2, Ps = 0.8 and Im = 0.2 ÷ 0.8 = 0.25. Among surviving women the mean number of live births is 4 with a variance of 4, so If = 4 ÷ 16 = 0.25. I = 0.25 + 0.25 ÷ 0.8 = 0.25 + 0.3125 = 0.5625. If mortality is later halved (Pd = 0.1: Im = 0.111) and the variance of fertility stays, I = 0.111 + 0.25 ÷ 0.9 = 0.111 + 0.278 = 0.389: the index falls, and the fertility component now supplies most of the opportunity for selection — the pattern Crow and Spuhler found as populations pass through the demographic transition. The index measures opportunity, not the direction of selection or its genetic effect; it has been computed for many Indian tribal and caste populations (Basu, Reddy, Bhasin and others), with typical values between 0.3 and 1.5, higher where child mortality and variance in family size are high. Selection intensity so measured is the link between the demographer's rates and the evolutionist's forces, which is why the syllabus places it here.