**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'.

**Keywords:** social structure, pair bond, *Symphalangus*, *Nomascus*, *Hylobates*, sex-specific investment, Hylobatidae
