Maternal Condition and Offspring Sex Ratio in a Group of Free-Ranging Rhesus Monkeys: An Eleven-Year Study

Abstract

The extent to which variations in maternal condition could be used to explain lineage-related variations in offspring sex ratios was examined in one social group of free-ranging rhesus monkeys over a 11-yr period. Sex ratios were examined as a function of several factors thought to be related to maternal condition: individual and lineage rank, age, fecundity, survival, neonatality, and rates of stillbirths and neonatal deaths. Maternal condition was also assessed more directly with ordinal-level measures of body fat. Significant differences among the lineages resulted from a strong tendency for middle-lineage females to produce biased proportions of daughters. It could not be argued using criteria of body fat, maternal age, survival, or fecundity that these females were in better or worse condition than those in other lineages. Maternal age or dominance was not related to body fat or offspring sex. Middle-lineage females had significantly higher neonatality rates, primarily because they had significantly shorter intervals between viable infants and smaller proportions of stillbirths and neonatal deaths. Over the whole group, females who had high neonatality also had high proportions of daugheters. However, there was no evidence that this relationship depended on variations in maternal condition or on differential mortality of offspring of a particular sex. Hence, maternal condition does not appear to be a likely explanation for variations in sex ratios observed in this group. Random variation or a genetic predisposition in the middle lineage to produce daughters, along with a general tendency for shorter interbirth intervals following the birth of viable daughters than sons, may account for the differences in neonatality. There is a need to clarify the concept and measurement of maternal condition and to assess its relationship to various factors (e.g., rank, age, productivity) at each research site.