The present report describes the spontaneous use of sticks as tools by youn
g adult gorillas (Gorilla gorilla gorilla) in a social group at the San Die
go 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 no
t climb due to electric wire, to knock down leaves and seeds. Two of the th
ree gorillas selected sticks that were more suitable (i.e, longer or thicke
r sticks) for throwing. Moreover, they looked up at the target (i.e. the fo
liage of the tree) before throwing and grasped the stick at a position appr
opriate 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 t
o 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 gori
llas used much longer sticks for branch-pulling than for stick-throwing, in
dicating that he was capable of task-dependent selection of sticks. Analyzi
ng 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.