Partially shared consensus decision making and distributed leadership in vervet monkeys: older females lead the group to forage

Objectives: Group living can be advantageous, but for motile organisms, collective movements become necessary. We are just beginning to understand the many ways that animal groups make movement decisions and maintain cohesion. We examined start attempts and success in leading collective group movements in vervet monkeys (Chlorocebus pygerythrus) characterized by matrilineal groups and territoriality. Materials and methods: We recorded 179 start attempts in a single group of vervets at Lake Nabugabo, Uganda and examined individual success in three situations (departing from sleeping site, moving to forage, returning to sleeping site) relative to dominance rank, age, and sex. Results: Sex and age were associated with both the number of start attempts and success in leading group movements, but there was no effect of dominance rank. Older females were most successful at leading group movements, especially toward foraging sites, while adult and subadult males almost always led the group out of the sleeping site. Discussion: Collective group movements in vervet monkeys appear to be based on distributed leadership and partially shared consensus decision making. Older females may be repositories of ecological knowledge, resulting in their success at leading the group to forage. Male motivation to lead the way out of the sleeping site appeared related to accessing human food sources before other group members. Young natal males achieved some success leading group progressions because they were motivated to make many initiations, which may be related to their life-stage. These results give us a better understanding of the processes underlying collective movements in cohesive animal groups.