Problems with primate sex ratios

Birth sex ratios of baboons in Gombe National Park, Tanzania, show an overa ll male bias of cn. 20%, but there is no obvious explanation for this trend . Individual females did not alter their sex ratios according to persistent levels of local resource competition. Sex ratios showed an unexpected rela tionship between age and rank: subordinate females had more sons when they were young; dominant females had more sons when they were old. The sex rati o of low-ranking females also varied with the severity of environmental con ditions during pregnancy Our findings suggest that mammalian sex ratios mig ht be the product of more complex processes than is generally recognized or that sex-determining mechanisms impose sufficient constraints to prevent a daptive variation in all contexts.