This study tested some predictions of the local-resource-competition hypothesis and local-resource-enhancement hypothesis of sex-biased maternal investment in a population of pigtail macaques (Macaca nemestrina leonina). Offspring sex ratios in high- and low-ranking matrilines were biased in opposite directions as predicted. Sex ratio in the middle-ranking matriline was female biased similar to the high-ranking matrilines but became increasingly male biased with maternal age, similar to the low-ranking matrilines. Interbirth intervals following surviving daughters were longer than those following sons, irrespective of rank, suggesting that daughters were more costly to rear than sons. Infant mortality was not significantly different for sons and daughters. The findings of this study are consistent with some, but not all, of the predictions of local-resource-competition and local-resource-enhancement hypotheses and suggest that middle-ranking mothers have specific investment strategies that change in relation to their age.