Object manipulation to obtain a food reward in hoolock gibbons, Bunopithecus hoolock

Gibbons (family Hylobatidae) have rarely been studied in terms of object manipulation and tool use. We assessed whether hoolock gibbons can learn to pull on a rake-like object to gain a food reward, in a zero-order manipulation task, without specific training. Their learning style was assessed along with their understanding of the tertiary relationships between the rake, a goal object and an environmental feature. Hoolocks used a rake to pull in an out-of-reach food item in less than 90 s on first presentation. The gibbons' behaviour suggests that perceptually restructuring the environment may be within this species' abilities. Their causal understanding of three factors, the rake, the reward and a trap into which the reward could fall and be lost, was moderately better than that of chimpanzees, Pan troglodytes, and capuchins, Cebus spp., on a similar task. Learning of simple associative rules, rather than understanding the physical properties of the trap, might explain the gibbons' performance.