On the road again: competitive effects and condition-dependent dispersal in male baboons

Historically, intertroop movement by males in female-philopatric species has been investigated without any consideration of the potential for variance in the competitive ability of males. This is despite the fact that in many species, particularly among the primates, males tend to move multiple times between groups and often vary substantially in their competitive ability. We investigated this issue using 7 years' data from a long-term study of chacma baboons, Papio hamadryas ursinus. We predicted that variance in competitive ability would promote differentiation in dispersal strategies, with competitively effective males favouring long-term measures of a troop's potential reproductive output and low-quality males favouring short-term measures. Using time series analysis, we found that overall patterns of movement were significantly associated with two independent measures of a troop's demography: absolute female number and excess male number. Furthermore, when the analysis was broken down by competitive ability, we found that only high-quality males responded to the absolute number of females and only low-quality males were sensitive to excess male number. Therefore, we conclude that variance in competitive ability has promoted the evolution of alternative dispersal strategies in male chacma baboons.