Career moves: Transfer and rank challenge decisions by male long-tailed macaques

Transfers to other social groups and active acquisition of high dominance rank carry a substantial risk for a primate male. Therefore, we expect that such actions will affect his reproductive success. Demographic changes were studied in three groups of long-tailed macaques (Macaca fascicularis) over a 12-year period at Ketambe, Gunung Leuser National Park, Indonesia. Natal dispersal by males is expected to result in reduced inbreeding; secondary transfers may improve access to mates. Previous study revealed that, in this population, siring success was strongly correlated with dominance rank, with top dominance concentrated in young adult males. Thus, few decisions have such profound consequences for a male's fitness as those concerning transfer to a group where he may challenge for top dominance and how long he can occupy high rank. This paper examines in detail patterns in male transfer and rank acquisition in relation to reproductive success. We also attempt to identify the factors affecting a male's decisions to transfer or challenge to attain top dominance. Males transferred on average every 45.5 +/- 3.5 months (N = 90), with longer residences for the younger males. In general, subadult males seemed to follow a low risk strategy: all males left their natal group as subadults, at a median age of 5 yr (N = 32) and most settled in an adjacent group with at least some of their peers. Most subadults did not transfer again until after reaching adult features. Average tenure of top dominance was 25 mo. Attempts to take over top dominance were most often successful if the tenure of the former top dominant had lasted at least one reproductive season and if the challenger was already a long-term resident of the group. Males who attempted to take over upon immigration into a group ('bluff immigrants') were less successful. Peers of a newly established top dominant tended to transfer to another group instead of challenging him. Thus, the timing of a take-over attempt relative to the social circumstances seems important. Age of maturation into fully adult features and the first attempt to challenge for top dominance varied between males. Sons of high-ranking mothers were more successful in acquiring top dominance in a nonnatal group than low-born males. Groups with an unstable male dominance hierarchy attracted more, mostly young, immigrants and lost more, mostly older, emigrants. During such periods, infant mortality was higher than during stable periods. The data support the hypothesis that transfer by males is a self-chosen option with a high probability of improving a male's reproductive opportunities. We conclude that these males show sophisticated assessment and decision-making.