THE EFFECTS OF CLIMATE ON GROWTH *

Animal studies (Harrison 1958, 1964) and a few human studies (Shapiro, 1939; Ito, 1942; Lasker, 1952; and Hiernaux, 1963) show that environment modifies the genotype so that like genotypes developing in different environments will result in different phenotypes. Environmental factors that have been considered are nutrition, disease, work load and climate. Climate is not the most important of these, but it should not and has not been ignored. Three outstanding climatic extremes are considered to be: ( I ) hot, wet tropics, (2) hot, dry desert, (3) cold, dry Arctic. There are naturally modifications and variations of these. For the present I shall consider only (I) and (2) since I have been concerned with heat. In a hot climate the direct factors of sunlight, temperature, rainfall, humidity and winds affect the individual physiology by the individual’s need to adjust body heat production and eliminate heat in order to maintain normal body temperature (Wulsin, 1948; Roberts, 1953). Physiological changes in the heat acclimatized individual as reported by Bass and his cohorts (1955) are the following: lower pulse rate, lower rectal and skin temperature, higher sweat rate, reduced basal metabolism rate (BMR), more stable blood pressure, reduced urinary 17-ketosteroids. They conclude that the most important adaptations are those concerned with improved cardiovascular function. They did not find that the pituitary-adrenal system was involved as some investigators believe. They did not determine the role of the thyroid in acclimatization except they noted that the BMR was reduced. In addition to the above, there have been studies on the changed vitamin and mineral requirements in the tropics. Lee (1941), Mills (1941) and de Castro (1948) agree that the tropic dweller needs more vitamin B,. De Castro claims that the reason for this is the primarily carbohydrate diet followed by the natives in the tropics since B, is concerned in regulating carbohydrate metabolism. It follows then that a temperate dweller on his usual temperate zone diet should not require more vitamin B,. Concerning other vitamins, de Castro says that the strong sunlight more readily aids the transformation of absorbed carotene to vitamin A and ergosterol to vitamin D. As for the minerals, Lee (1941) believes there is probably greater need for phosphates and chlorides because of greater sweat secretion. On the other hand, de Castro (1948, 1952) sees a reduced calcium requirement because of the increased amount of ultraviolet radiation. It has been shown that there are some physical and physiological types that get along better in extremes of heat than others. Wulsin (1948) discusses the classic study made by Dreosti in South Africa. Using elevated body temperature as the measurement of heat tolerance, he found that 15 per cent of his sample of 3,252 natives had superior heat regulating mechanisms and could endure heat stress without previous acclimatization. These natives would be less likely to suffer from heat stroke while working deep in the mines. Most of the remaining 85 per cent were able to be acclimatized, but a small group appeared to be genuinely heat intolerant. He found no correlation of the heat tolerant 15 per cent with body