Multi-modal use of a socially directed call in bonobos

Abstract

'Contest hoots' are acoustically complex vocalisations produced by adult and subadult male bonobos (Pan paniscus). These calls are often directed at specific individuals and regularly combined with gestures and other body signals. The aim of our study was to describe the multi-modal use of this call type and to clarify its communicative and social function. To this end, we observed two large groups of bonobos, which generated a sample of 585 communicative interactions initiated by 10 different males. We found that contest hooting, with or without other associated signals, was produced to challenge and provoke a social reaction in the targeted individual, usually agonistic chase. Interestingly, 'contest hoots' were sometimes also used during friendly play. In both contexts, males were highly selective in whom they targeted by preferentially choosing individuals of equal or higher social rank, suggesting that the calls functioned to assert social status. Multi-modal sequences were not more successful in eliciting reactions than contest hoots given alone, but we found a significant difference in the choice of associated gestures between playful and agonistic contexts. During friendly play, contest hoots were significantly more often combined with soft than rough gestures compared to agonistic challenges, while the calls' acoustic structure remained the same. We conclude that contest hoots indicate the signaller's intention to interact socially with important group members, while the gestures provide additional cues concerning the nature of the desired interaction.

Figure 1. Representative spectrographic illustration of a contest hoot performed by Fizi.

The acoustic structure is composed of A: introductory phase, B: escalation phase with N = 14 stereotyped units and C: let-down phase.

Figure 2. Percentage of strong reactions elicited by uni- and multi-modal contest hoots.

Black bars: alpha male (α) signallers, grey bars: other male signallers. NS: non-significant, *P<0.05.

Figure 3. Percentage of contest hoots given by male signallers towards recipients of different relative social rank.

Signaller's rank are represented as alpha (α), high and intermediate. Recipients' ranks were calculated relative to the signaller (higher ranking females and males, equal and lower ranking males and females).

Figure 4. Mean percentage of strong reactions elicited from preferred and all other targets for each signaller.

Only seven males participated to the data set, the 3 others elicited no or too few strong reactions.

Figure 5. Spectrographic illustrations of contest hoot calls produced during the challenge (1) and play (2) contexts.

Calls were produced by three subadult males; A: Api; B: Dilolo; C: Lomami.

Figure 6. Frequency of production of rough and soft signals in multi-modal sequences with contest hoots.

Light grey bars: expected values; black bars: observed values in the challenge context; dark grey: observed values in the play context. ***P<0.001