Chimpanzees spontaneously take turns in a shared serial ordering task

Abstract

Social coordination can provide optimal solutions to many kinds of group dilemmas, and non-human subjects have been shown to perform single actions successively or simultaneously with partners to maximize food rewards in a variety of experimental settings. Less attention has been given to showing how animals are able to produce multiple (rather than single) intermixed and co-regulated actions, even though many species' signal transmissions and social interactions rely on extended bouts of coordinated turn-taking. Here we report on coordination behaviour in three pairs of chimpanzees (mother/offspring dyads) during an experimentally induced turn-taking scenario. Participants were given a "shared" version of a computer-based serial ordering task that they had previously mastered individually. We found that minimal trial-and-error learning was necessary for the participants to solve the new social version of the task, and that information flow was more pronounced from mothers toward offspring than the reverse, mirroring characteristics of social learning in wild chimpanzees. Our experiment introduces a novel paradigm for studying behavioural coordination in non-humans, able to yield insights into the evolution of turn-taking which underlies a range of social interactions, including communication and language.

Figure 1

Still images illustrating progressive stages of a single trial of the serial ordering task. Two chimpanzees sit in adjacent booths and participate in a joint turn-taking task on a single touch-panel with a transparent barrier separating the two halves of the screen. The top panel shows the beginning of the trial, where the chimpanzee on the right is about to touch the first numeral in the sequence (1). In the bottom three panels, the numerals 2, 3 and 4 are being selected by the chimpanzee on the left, left and right, respectively. For temporal analysis, touch types were divided into “stays” (bottom middle) where the individual selecting a numeral does so after having selected the preceding numeral as well, and “switches” (bottom right), where the preceding numeral had been selected by the partner.

Figure 2

Accuracy in the eight-numeral ordering task. (A) Accuracy rates (percent of trials correct per session) of the three chimpanzee pairs over the 32 sessions of the eight-numeral task. (B) Total number of correct and incorrect responses for the 2240 trials given to each pair.

Figure 3

Accuracy and response latency comparisons. (A) Accuracy. Percent correct trials in the first and last eight blocks (out of the total of 32 blocks) of the 8-numeral social task are shown on the left. Percent correct trials during the first eight blocks of the subsequently introduced automated condition are shown on the right. Accuracy is calculated for each subject out of the total number of trials a subject received that were not terminated due to mistakes of their partner. (B) Response latency. Latencies reflect the length of time it took subjects to touch a target numeral in either of the following scenarios: (1) a “stay”, i.e., a touch made to a numeral by an individual after the same individual had touched the preceding numeral in the sequence, and (2) a “switch”, i.e., a touch made to a numeral by an individual after its partner had touched the preceding numeral in the sequence. Data (means ± s.e.m.) shown on the left are from the first eight and the final eight blocks of the 8-numeral social task (i.e., Phase 4 in the Supplementary Methods), and data shown on the right are from the eight blocks of the automated condition.