SEASONAL-VARIATION IN HUMAN-REPRODUCTION - ENVIRONMENTAL-FACTORS

Almost all human populations exhibit seasonal variation in births, owi ng mostly to seasonal variation in the frequency of conception. This r eview focuses on the degree to which environmental factors like nutrit ion, temperature and photoperiod contribute to these seasonal patterns by acting directly on the reproductive axis. The reproductive strateg y of humans is basically that of the apes: Humans have the capacity to reproduce continuously, albeit slowly, unless inhibited by environmen tal influences. Two, and perhaps three, environmental factors probably act routinely as seasonal inhibitors in some human populations. First , it seems likely that ovulation is regulated seasonally in population s experiencing seasonal variation in food availability. More specifica lly, it seems likely that inadequate food intake or the increased ener gy expenditure required to obtain food, or both, can delay menarche, s uppress the frequency of ovulation in the nonlactating adult, and prol ong lactational amenorrhea in these populations on a seasonal basis. T his action is mast easily seen in tropical subsistence societies where food availability often varies greatly owing to seasonal variation in rainfall; hence births in these populations often correlate with rain fall. Second, it seems likely that seasonally high temperatures suppre ss spermatogenesis enough to influence the incidence of fertilization in hotter latitudes, but possibly only in males wearing clothing that diminishes scrotal cooling. Since most of our knowledge about this phe nomenon comes from temperate latitudes, the sensitivity of spermatogen esis in both human and nonhuman primates to heat in the tropics needs further study. It is quite possible that high temperatures suppress ov ulation and early embryo survival seasonally in some of these same pop ulations. Since we know less than desired about the effect of heat str ess on ovulation and early pregnancy in nonhuman mammals, and nothing at all about it in humans or any of the other primates, this is an imp ortant area for future research. Third, correlational data suggest tha t there may be some degree of regulation of reproduction by photoperio d in humans at middle to higher latitudes. Populations at these latitu des often show a peak in presumed conceptions associated with the vern al equinox. On the other hand, evidence gathered by neuroendocrinologi sts tends to argue against reproductive photoresponsiveness in humans.