Significance
We report a study designed to explore the extent to which variation in oxygen delivery physiology of ethnic Tibetan women aged 46 to 86 living ≥3,500 m altitude in Upper Mustang District, Nepal, related to the number of livebirths. Among women with long marriages and early first births, combinations of traits enhancing oxygen delivery to tissues characterized those with the highest lifetime reproductive success. Considering the collective contributions of sociocultural factors and the multiple biological traits contributing to the internal environment provided a fresh way to test hypotheses about ongoing natural selection under the stress of high-altitude hypoxia.