Abstract
This study provides the first statistically significant evidence that the mechanisms of how pair bonds are created or maintained differ between gibbon taxa. We examine the pair bond in captive pairs of three genera of gibbons (Hylobatidae): siamangs (Symphalangus, N = 17 pairs), crested gibbons (Nomascus, N = 7 pairs), and pileated gibbons (Hylobates pileatus, N = 9 pairs). In the first part of this study, we determine three generally-accepted indicators of pair-bond strength (mutual grooming, behavioral synchronization and partner distance). A pairwise comparison of our samples reveals a difference in relative partner distances between siamangs and pileated gibbons, suggesting that siamangs may have a stronger pair bond than pileated gibbons. No difference among the three taxa was found in other variables believed to indicate pair bond strength. In the second part we examine the amount of partner-directed grooming in each sex. In siamangs, males invest significantly more into pair bonds than females, whereas the opposite is true in crested and pileated gibbons. Our results for siamangs correspond to predictions derived from the ‘mate-defense hypothesis’ for the evolution of pair bonds, whereas our results for crested gibbons and pileated gibbons correspond to predictions derived from the ‘male-services hypothesis’.
Highlights
Whereas the genera of great apes are known to differ strongly among each other in their social structure, the small apes or gibbons clearly are a more uniform group [1–3]
It has repeatedly been suggested, that gibbon taxa may differ in subtle details of their social organization [16]: Wild family groups of Malayan siamangs (Symphalangus syndactylus) appear to be more tightly knit than those of white-handed gibbons (Hylobates lar), with siamang intra-group distances being shorter and intra-group communicatory signals being fewer or less conspicuous to observers, and paternal infant-carrying only occurring in siamangs [17–21]
Paternal infant-carrying appears to be absent in some wild and many captive siamang groups and varies dramatically among males of the same population [10, 23–25], while it may occasionally occur in other gibbon taxa, at least in captive groups [23, 26]
Summary
Whereas the genera of great apes are known to differ strongly among each other in their social structure, the small apes or gibbons clearly are a more uniform group [1–3]. Non-monogamous (extra-pair) matings and groups with multiple adult males and multiple adult females occasionally occur in gibbons [7, 9–15]. It has repeatedly been suggested, that gibbon taxa may differ in subtle details of their social organization [16]: Wild family groups of Malayan siamangs (Symphalangus syndactylus) appear to be more tightly knit than those of white-handed gibbons (Hylobates lar), with siamang intra-group distances being shorter and intra-group communicatory signals being fewer or less conspicuous to observers, and paternal infant-carrying only occurring in siamangs [17–21]. Paternal infant-carrying appears to be absent in some wild and many captive siamang groups and varies dramatically among males of the same population [10, 23–25], while it may occasionally occur in other gibbon taxa, at least in captive groups [23, 26]
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