Spontaneous use of sticks as tools by captive gorillas (Gorilla gorilla gorilla)

The present report describes the spontaneous use of sticks as tools by young adult gorillas (Gorilla gorilla gorilla) in a social group at the San Diego Wild Animal Park, CA, USA. Three 8-year-old gorillas tone female and two males) threw sticks into the foliage of trees, which the gorillas could not climb due to electric wire, to knock down leaves and seeds. Two of the three gorillas selected sticks that were more suitable (i.e, longer or thicker sticks) for throwing. Moreover, they looked up at the target (i.e. the foliage of the tree) before throwing and grasped the stick at a position appropriate for throwing (i.e. the end of the stick). They were more likely to throw sticks when particular adult group members were not nearby. These two gorillas also pulled branches of trees toward themselves by using sticks to capture them (branch-pulling), and one of these two beat a branch with a stick to knock down leaves and seeds (branch-beating. One of these two gorillas used much longer sticks for branch-pulling than for stick-throwing, indicating that he was capable of task-dependent selection of sticks. Analyzing the spontaneous use of sticks as tools by gorillas in captivity can lead to a better understanding of not only their cognitive ability but also of their social relationships, which may otherwise be concealed.