Preliminary study on hand preference in captive northern white-cheeked gibbons (Nomascus leucogenys).

The postural origin hypothesis and the task complexity hypothesis propose that hand preference in non-human primates evolved in association with body posture and task complexity, respectively. The results of previous studies testing these two hypotheses, however, vary greatly with the different primate species and methods used. To investigate the effect of body posture and task complexity on hand preference, we recorded bouts of hand usage in nine captive northern white-cheeked gibbons (Nomascus leucogenys) housed at Beijing Zoo as they reached for food items in a ground-reaching task, a box task, and a tube task. The results showed that four to seven of the nine gibbons displayed a hand preference at the individual level in different tasks, and that hand preference in individuals was task-specific; there was no group-level hand preference in any task. The box task seemed to elicit a greater strength of hand preference than the ground-reaching task at the individual level. Although the small sample size rules out drawing any strong conclusions concerning hand preference at the group level, our results suggest that the suspensory reaching posture might increase the expression of hand preference at the individual level. This study provides preliminary information on hand preference in captive northern white-cheeked gibbons, and will be helpful for future studies addressing the origin and evolution of hand preference in small apes.