Youngsters do not pay attention to conversational rules: is this so for nonhuman primates?

Abstract

The potentiality to find precursors of human language in nonhuman primates is questioned because of differences related to the genetic determinism of human and nonhuman primate acoustic structures. Limiting the debate to production and acoustic plasticity might have led to underestimating parallels between human and nonhuman primates. Adult-young differences concerning vocal usage have been reported in various primate species. A key feature of language is the ability to converse, respecting turn-taking rules. Turn-taking structures some nonhuman primates' adult vocal exchanges, but the development and the cognitive relevancy of this rule have never been investigated in monkeys. Our observations of Campbell's monkeys' spontaneous vocal utterances revealed that juveniles broke the turn-taking rule more often than did experienced adults. Only adults displayed different levels of interest when hearing playbacks of vocal exchanges respecting or not the turn-taking rule. This study strengthens parallels between human conversations and nonhuman primate vocal exchanges.

Figure 1. Observations.

The proportion of spontaneous inappropriate contact calls uttered in relation to Campbell's monkeys' age. **P≤0.01

Figure 2. Playback experiments.

Visual attention (i.e. duration of looking towards the speaker in the 30 seconds ‘after' minus 30 seconds ‘before' playback) paid to appropriate and inappropriate vocal exchanges (respecting or not the turn-taking rule) in relation to listener's age. *P≤0.05, ^NSP>0.05.