Socio-behavioral convergence in female affiliative behaviors in Pan and Ateles

Pan and Ateles converge in many aspects of their social organization. Both are characterized by a high degree of fission-fusion dynamics, in which species exhibit high variation in party size, composition, and spatial cohesion. Within this framework, chimpanzees (Pan troglodytes) and spider monkeys (Ateles sp.) exhibit the most similarity in subgrouping patterns and social relationships. Specifically, significant overlap is found in the context of female social relationships and patterns of male-female aggression. Here, we examine how affiliative behaviors mediate female social relationships in captive spider monkeys, chimpanzees, and bonobos. Focal data were collected from five female Ateles geoffroyi at Brookfield Zoo (mean age=13.2, range: 7-21), five female bonobos at Columbus Zoo (mean age=22.0, range: 7-31), and five chimpanzees at North Carolina Zoo (mean age=20.4, range: 15-43). Female dyads did not differ in their rates of total or directional grooming, but spider monkeys engaged in significantly less mutual grooming (Kruskal-Wallis H= 8.917, N=30, p=0.012). All three species exhibited grooming reciprocity. There were no significant differences in the overall use of tension-reduction behaviors. However, there were significant differences in the types of tension-reduction behaviors, with spider monkeys using embraces (H=14.306, p=0.001), bonobos using socio-sexual behaviors (H=14.269, p=0.001), and chimpanzees using kisses (H=11.50, p=0.003). Furthermore, bonobos used tension-reduction behaviors significantly more often in feeding contexts (ANOVA F=14.357, N=15, p=0.001). We suggest that each species use tension-reduction behaviors that are species-typical, but serve as functional equivalents. However, bonobos may experience increased tension in feeding contexts, which suggests differing social and ecological pressures may necessitate an increase in tension-reduction behaviors.