Social and environmental factors influencing natal dispersal in blue monkeys, Cercopithecus mitis stuhlmanni

Abstract

We evaluated four factors that may influence the process of male natal dispersal in wild female-philopatric blue monkeys: aggressive eviction by adults, attraction to oestrous females, social integration in the natal group and survivorship risk. Observations of nine males and five females of dispersal age allowed us to compare the social behaviour of males and females and that of dispersing and nondispersing males. We also studied the circumstances of 26 natal dispersals (1997–2006). Aggressive eviction did not appear to be important in most dispersals. We found no significant differences in rates or types of aggression received by juvenile females and males or by dispersing and nondispersing males. Sexual attraction also did not appear to influence dispersal, because males did not disperse more often than expected by chance during conception seasons or during breeding-season influxes of males. Our results confirm low social integration as a correlate of sex-biased dispersal. Juvenile males groomed and sat in contact with groupmates significantly less than females did, although we found few differences between dispersing and nondispersing males. Our results also suggest that males timed dispersals to minimize risks, because dispersal was most likely to occur during months of low rainfall and least likely to occur when fruit was least available. Endogenous factors are probably important in stimulating dispersal, because all males disperse but females never do. Although aggressive eviction may occasionally influence the dispersal process in blue monkeys, given their unimale group structure, our results implicate social integration much more uniformly and strongly.