Birth sex ratios and maternal social rank in a captive colony of rhesus monkeys (Macaca mulatta).

Data from a 35-year study of rhesus macaques (Macaca mulatta) at Madingley, Cambridge, were used to investigate sex ratio biases associated with maternal rank. Data were available from two colonies, the Old colony (1960-81) and New colony (1982-93). Overall, top-ranking mothers gave birth to 30.9% sons, while non-top mothers gave birth to 58.4% sons. Among non-top mothers, middle- and bottom-ranking ones had 59.0 and 55.0% sons, respectively. Top mothers' daughter biases were strongest in matrilines with two adult females in the year the infants were conceived (15.4 sons and 14.3% sons in Old and New colonies). Non-top mothers' son biases (88.9 and 71.0% in Old and New colonies) were strongest in matrilines with 3 females. The findings are discussed in relation to the colonies' small matriline sizes and data on breeding performance and infant survival, which indicate the costs to mothers of different rank of having different sex infants. Overall, top-ranking mothers were more likely to breed in two successive years (78.6%) than non-top mothers (56.7%). Infant survival to 7 days was significantly higher in the New colony (89.0%) than the Old colony (75.3%), with daughters born to Old colony mothers doing especially poorly. We point out that between-group and between-species comparisons of sex ratio effects depend critically on how females are assigned to rank categories, and require information about divergences of sex ratios from 50:50 in each category. (C) 1996 Wiley-Liss, Inc.