Birth Sex Ratios in Toque Macaques and Other Mammals: Integrating the Effects of Maternal Condition and Competition

Mammalian life histories suggest that maternal body condition and social dominance (a measure of resource-holding potential) influence the physical and social development of offspring, and thereby their reproductive success. Predictably, a mother should produce that sex of offspring which contributes most to her fitness (as measured by the number of her grandchildren) and that she is best able to raise within the constraints imposed by her condition, social rank, and environment. Such combined effects were investigated by monitoring variations in body condition (weight) and behavior of female toque macaques, Macaca sinica of Sri Lanka, in a changing forest environment over 18 years. Maternal rank, by itself, had no influence on offspring sex, but did affect maternal body condition. The combined effects of rank and condition indicated the following: mothers in robust condition bore more sons, whereas those in moderate condition bore more daughters, but both effects were expressed most strongly among mothers of high rank. Where the consequences of low rank were felt most acutely, as shown by poor condition, mothers underproduced daughters. Environmental quality directly influenced rank and condition interactions, and thus sex ratios. These relationships, and data from other mammals suggest an empirically and theoretically consistent pattern of sex allocation in mammals. New predictions integrate effects, proposed by Trivers and Willard, that are rooted in male mate competition, which is universal among polygynous mammals, with those of local resource competition (and/or female reproductive competition), which are not universal and differ in intensity between the socioecologies and local environments of different species.