Chimpanzees show the capacity to communicate about concomitant daily life events

Abstract

One universal feature of human language is its versatility in communicating about juxtapositions of everyday events. Versatile combinatorial systems of communication can be selected for if (a) several vocal units are flexibly combined into numerous and long vocal sequences and (b) vocal sequences relate to numerous daily life events. We propose (b) is more likely during simultaneous or serial (concomitant) events than single events. We analyzed 9,391 vocal utterances across the repertoire of wild chimpanzees and their events of production. Chimpanzees used vocal sequences across a range of daily life events and twice as often during concomitant than single events. Also, utterance diversity correlated positively with event diversity. Our results show the potential of chimpanzee vocal sequences to convey combined information about numerous daily life events, a step from which generalized combinatoriality could have evolved.

Keywords: biological sciences; evolutionary biology; zoology.

Graphical abstract

Figure 1

Frequency of each event eliciting vocalizations across the dataset Concomitant events are ordered alphabetically, not in the sequential order of the event. See Table 3 for definitions of two-letter codes.

Figure 2

Proportion of vocal sequences as a function of single or concomitant events (A and B) (A) Includes individuals of all ages (0–55 years; n = 9,391 utterances, 98 individuals) and (B) only adults (>10 years; n = 5,399 utterances, 53 individuals). The y axis depicts the likelihood of the utterance to be a sequence (a combination of different vocal units, as opposed to a single vocal unit). Shown are medians (thin horizontal lines), quartiles (25% and 75%; boxes), percentiles (2.5% and 97.5%; vertical lines), as well as the fitted model and its 95% confidence intervals (thick horizontal lines). Each dot represents the value per individual under each category, and its area is proportionate to the total number of utterances recorded per chimpanzee and predictor category (mean: 47.91, range: 1–188). ∗∗∗p < 0.001, p values were determined using a binomial GLMM test.

Figure 3

Relation between the diversity of utterances and events (A and B) (A) Relation between diversity of utterances and diversity of events per chimpanzee. (B) Modelled relation between the residuals of each variable (on log scale), fitted against logged sample size (the total number of utterances per chimpanzee and field season) for individuals of all ages (model 2a, N = 152) (see Figure S1 for details). The line represents the fitted model and the polygon its 95% confidence limit. Each dot depicts an individual per season and their area is proportionate to the total number of utterances per chimpanzee and field season (mean: 61.78, range: 1–214).

Figure 4

An example case of the proportion of utterances of each length produced during three different events: a single event (“feed”), two and three concomitant events of different lengths Concomitant events are ordered alphabetically, not in the sequential order of the event. The color on the bars represents the number of different vocal units per utterance. N represents the total number of utterances per event in the dataset.