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 Madin gley, Cambridge, were used to investigate sex ratio biases associated with maternal rank. Data were available from two colonies, the Old col ony (1960-81) and New colony (1982-93). Overall, top-ranking mothers g ave birth to 30.9% sons, while non-top mothers gave birth to 58.4% son s. 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 con ceived (15.4 sons and 14.3% sons in Old and New colonies). Non-top mot hers' son biases (88.9 and 71.0% in Old and New colonies) were stronge st in matrilines with 3 females. The findings are discussed in relatio n to the colonies' small matriline sizes and data on breeding performa nce and infant survival, which indicate the costs to mothers of differ ent rank of having different sex infants. Overall, top-ranking mothers were more likely to breed in two successive years (78.6%) than non-to p 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 dep end critically on how females are assigned to rank categories, and req uire information about divergences of sex ratios from 50:50 in each ca tegory. (C) 1996 Wiley-Liss, Inc.