Local Resource Competition and Facultative Adjustment of Sex Ratios in Relation to Competitive Abilities

Abstract

I have suggested that local resource competition and male dispersal may generally lead to (1) intense competition among females for resources and reproductive opportunities, (2) limiting the recruitment of females, (3) extensive maternal investment in vulnerable female offspring, and (4) facultative adjustment of the sex ratio of progeny in relation to maternal competitive abilities. This argument appears to be consistent with evidence drawn from several species of macaques and baboons. The size of groups appears to be limited by the availability of resources; females actively compete over access to resources, and limit the number of females born and raised in their groups through the effective harassment of females carrying female fetuses and immature females. In response to such harassment, females appear to invest more in their daughters than sons, and to adjust the sex ratio of their progeny in relation to the expected reproductive success of their sons and daughters. High ranking females whose female progeny are likely to survive to become high ranking, reproductively successful adults produce more daughters than sons; while low ranking females whose female offspring are unlikely to survive to maturity or to reproduce successfully, produce more sons than daughters.