Leading limb preference during brachiation in the gibbon family member, Hylobates syndactylus (siamangs): a study of the effects of singing on lateralisation.

Individual-level lateralisations are common among vertebrates, however population-level preferences are usually reserved for specific tasks. In humans, handedness is thought to be related to the hemispheric processing of specific aspects of language, including speech. Although nonhuman primates do not possess speech, gibbons are known to produce elaborate vocal displays often referred to as song. To investigate the evolutionary effects of singing on hand preference, this study examined leading limb preference during brachiation in the large-bodied, melodious gibbon, Hylobates syndactylus (siamangs). A total of 13 male and 12 female siamangs were observed in captive and semi-captive settings for leading limb preference during vocal and nonvocal behaviour. No significant results were found for the effect of vocalisation at the group level, although individual-level data indicated a trend towards a right shift in the vocal condition, especially for females, where 8 out of 12 demonstrated a greater reliance on a right leading limb during the vocal versus nonvocal conditions. Males were not found to possess significant preferences in either the vocal or nonvocal conditions alone, however when condition data were combined, the number of individual males lateralised was significantly greater than predicted by chance. These findings support individual, but not population-level, effects for vocalisation on leading limb preference in siamangs, and emphasise the importance of testing for sex difference in handedness and lateralisation research.