Pair Bonds in Monogamous Apes: a Comparison of the Siamang Hylobates Syndactylus and the White-Handed Gibbon Hylobates Lar

Abstract

AbstractDescriptions of the social systems of gibbons (Hylobates, Hylobatidae) have typically emphasized generically uniform attributes such as 'monogamy' and 'territoriality'. This has prevented testing of the hypothesis that pair bonds differ in the siamang (Hylobates syndactylus) and the white-handed gibbon (Hylobates lar) (Chivers, 1972). I replace a description of sociality based on mating system and group size/composition with quantitative measurement of social interactions and spatial relations between wild adult males and females in three heterosexual pairs of siamang and two pairs of white-handed gibbons studied for 2.5 years at the Ketambe Research Station (Sumatra, Indonesia). Siamang pair bonds show greater heterosexual cohesion than those of white-handed gibbon as reflected in higher rates of affinitive interactions such as close proximity, relaxed physical contact, embraces, and communal use of sleep trees. Although males are more responsible than females for the maintenance of close proximity in both species, sex differences in intra-pair allogrooming suggest divergent mechanisms maintaining pair bonds in the two species. In white-handed gibbons, the female rarely initiates grooming and grooms her mate significantly less than he grooms her, partly because she solicits grooming from him at higher rates while simultaneously ignoring more of his 'presents' for grooming. In siamang, the contributions of the sexes to grooming are more equivalent and reciprocal. Taken together, these results suggest that investment of the sexes in maintaining the pair bond is more asymmetrical in white-handed gibbons (i.e. males contribute relatively more than females) and more mutual in siamang. Although mate guarding may have been the selective force behind the origin of pair bonds in both species, greater intra-group feeding competition in the gibbon and substantial paternal care in the siamang may account for the evolution of more reciprocal and stronger pair bonds in the latter. Future research on more groups is necessary to clarify the proposed species differences in light of existing intraspecific variation in social behavior.