Anthropology, The University of Texas at San Antonio
Friday All day, Plaza Level
Traditionally gibbons (family Hylobatidae) have been classified as living in monogamous social groups with limited variation. This view has eroded as observations of multi-male and multi-female groups have accumulated. Concomitantly, some researchers now describe gibbon social organization as remarkably flexible, a characteristic that more closely allies them with the great apes. To provide context for this assertion we conducted a literature review to determine mean group size and range in gibbons relative to other primates that have been reported to live predominantly in two-adult groups. We predict that if gibbons in fact display greater social flexibility, group size in gibbons will be larger than in other putatively monogamous primates.
Mean groups size in all genera considered was below 5: Callicebus [3.2 ± 0.49; (1.7-4.8], Pithecia [4.3 ± 0.99; (2.3-9.0)], Aotus [3.3 ± 0.24; (2.3-4.1)], Hoolock [3.0 ± 0.20; (2.9-3.2)], Hylobates [3.8 ± 0.26; (2.6-5.0)], Nomascus [4.3 ± 1.19; (2.0-6.3)], and Symphalangus [3.9 ± 0.11; (3.8-4.0)]. Data for a number of other species (e.g., Presbytis potenziani) was too limited to allow statistical comparison. Group size among gibbon genera did not differ, but as predicted, group size in hylobatids was significantly larger than in Callicebus [t(28.41)=2.75, p=0.011] and Aotus [t(57.18)=3.4, p=0.001], though not in Pithecia [t(17.8)=-0.76, p=0.480]. Significantly, Pithecia is frequently identified as diverging from a strict one-male/one-female social organization. Our results provide broad support for the view that gibbons are more socially flexible than other monogamous primates. Future analysis will examine group composition and adult sex ratios.