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.