NEONATAL SIZE AND INFANT-MORTALITY AT HIGH-ALTITUDE IN THE WESTERN HIMALAYA

A prospective study was undertaken in Ladakh, India, a high-altitude r egion of the Himalaya, to investigate the effects of small average bir th size on neonatal mortality. While such studies exist from high-alti tude regions of the New World and shed light on the adaptive status of high-altitude-dwelling populations there, this is the first to examin e this relationship in the Himalaya. In a sample of 168 newborns, birt hweight and other anthropometric measurements were reduced relative to Andean and Tibetan newborns. Logistic regression and hazard analysis showed that neonatal biological characteristics such as weight, fatnes s, and circumferences were important predictors of survival probabilit ies of infants, especially in the neonatal period. Low Rohrer's Ponder al Index (PI) was particularly strongly related to poor survival outco me. Males and females showed no significant differences in mortality r isk. Data derived from reproductive histories revealed that neonatal m ortality accounted for 70-80% of total infant mortality in Ladakh. Com pared to other high-altitude studies, small newborn size in Ladakh was associated with much higher mortality risks; mortality risk rose dram atically with birthweights below the mean (2,764 grams), which charact erized 50% of all newborns. It is argued that newborns in Ladakh are s ubject to strong directional selective forces that favor higher birthw eights that incur lower risks of neonatal mortality, while Andean infa nts are subject to relatively mild selection pressure at both ends of the birthweight distribution. Given the overall small size at birth of Ladakhi newborns and the poor survival outcomes of newborns below the mean, it is suggested that this population is less well adapted in a biological sense to the stresses inherent in this high-altitude enviro nment than are Andean populations, perhaps due to the relatively recen t colonization of the area and the substantial genetic admixture that has occurred in the past. (C) 1994 Wiley-Liss, Inc.