Male residency and dispersal triggers in a seasonal breeder with influential females

Abstract

Males in female-philopatric social groupings leave their natal groups to pursue successive reproductive opportunities in one or more other groups. In vervet monkeys, Chlorocebus pygerythrus, adult males coexist and physical eviction is not a driver of male movement. Migratory decisions are expected to turn on an evaluation of future reproductive opportunity, as indexed principally by local operational sex ratio and relative competitive ability. Although vervet males' reproductive success is correlated with dominance, they are distinctive in that the attainment of rank is contingent on integration into female sociospatial networks and we expect decisions about continued residency to reflect this. We used 8 years' data from three groups to confirm that male dispersal between groups is seasonal in our population, with a peak that is coterminous with androgen levels and precedes peak mating and conception by 4 weeks. The average length of completed residency was 459 days, with an increase in the logged rate of departure after 1428 days, which is 150 days longer than the estimated modal age at first conception by putative daughters. There were positive correlations between a male's initial and highest rank, and between his highest rank and the length of time to reach it. We found that a male's residency was positively and independently associated with his highest achieved rank and both his grooming centrality and proximity degree. Additionally, increasing rank and proximity degree also had positive effects on residency length subsequent to the attainment of his highest rank. The probability of emigration was associated negatively with both female number and grooming centrality scores. We conclude that emigration from a group is linked to male rank attainment and mediated by a male's integration into female sociospatial networks. We found no evidence that emigration preceded the sexual maturity of putative daughters.