Spread of arbitrary conventions among chimpanzees: a controlled experiment

Abstract

Wild chimpanzees (Pan troglodytes) have a rich cultural repertoire--traditions common in some communities are not present in others. The majority of reports describe functional, material traditions, such as tool use. Arbitrary conventions have received far less attention. In the same way that observations of material culture in wild apes led to experiments to confirm social transmission and identify underlying learning mechanisms, experiments investigating how arbitrary habits or conventions arise and spread within a group are also required. The few relevant experimental studies reported thus far have relied on cross-species (i.e. human-ape) interaction offering limited ecological validity, and no study has successfully generated a tradition not involving tool use in an established group. We seeded one of two rewarded alternative endpoints to a complex sequence of behaviour in each of two chimpanzee groups. Each sequence spread in the group in which it was seeded, with many individuals unambiguously adopting the sequence demonstrated by a group member. In one group, the alternative sequence was discovered by a low ranking female, but was not learned by others. Since the action-sequences lacked meaning before the experiment and had no logical connection with reward, chimpanzees must have extracted both the form and benefits of these sequences through observation of others.

Figure 1

Illustration of the experimental set-up in the chimpanzee compounds at the field station of the Yerkes National Primate Research Center. Chimpanzees are rewarded from the observation tower for depositing tokens into either of two receptacles.

Figure 2

Differential spread of the bucket and pipe conventions in two groups of chimpanzees. (a) FS1 group and (b) FS2 group. Grey circles indicate≥1 deposit to bucket and black circles indicate≥1 deposit to pipe. Numbers within the circle indicate tokens deposited. Horizontal bars indicate that no deposit was made during that session; X indicates that the individual was kept indoors. Individuals are listed, from bottom to top, in the order of first deposit. Individuals GG and ER were the trained models.