Do chimpanzee pant hoots have semantic content?

I read with great interest the exciting study by Leroux et al. [(2021) Anim Behav 179, 49–50] who investigated the nature of pant-hoot–food-call combinations in a community of wild chimpanzees (Pan troglodytes schweinfurthii) at the Budongo Conservation Field Station, Budongo Forest, Uganda. The authors propose, among others, that they reveal the first evidence that wild chimpanzees are able “to combine meaning-bearing units into larger structures” (i.e., that they are capable of semantic compositionality and, by extension, syntax). Their analysis represents an important addition to a growing body of research and discussions on communicational combinatoriality in wild primates and specifically apes, and, by extension, extinct hominins. Incidentally, I have recently published a paper in Animal Cognition in which I also suggested, based on a reanalysis of existing data, that wild chimpanzees can display semantic compositionality and syntax in their communication [Gabrić (2021) Anim Cogn, online ahead of print]. In the present commentary, I argue that Leroux et al.’s (2021) interpretation of the data may be ungrounded given that (1) unlike for food calls, there is currently very little if any indication in the scientific literature that pant-hoots have semantic content (i.e., are meaningful) and given that (2) Leroux et al. (2021) did not investigate their a priori assumption that the observed pant-hoots are in fact semantic. Since pant-hoots feature prominently in the chimpanzee vocal repertoire and the debate on their eventual semanticity is still wide open, this represents a fine opportunity to revisit this issue in the context of Leroux et al.’s (2021) study. Their paper further raises several other less significant questions.

Chimpanzee pant-hoots encode information about individual but not group differences

Vocal learning, the ability to voluntarily modify the acoustic structure of vocalizations based on social cues, is a fundamental feature of speech in humans (Homo sapiens). While vocal learning is common in taxa such as songbirds and whales, the vocal learning capacities of nonhuman primates appear more limited. Intriguingly, evidence for vocal learning has been reported in chimpanzees (Pan troglodytes), for example in the form of regional variation (‘dialects’) in the ‘pant-hoot’ calls. This suggests that some capacity for vocal learning may be an ancient feature of the Pan-Homo clade. Nonetheless, reported differences have been subtle, with inter-community variation representing only a small portion of the total acoustic variation. To gain further insights into the extent of regional variation in chimpanzee vocalizations, we performed an analysis of pant-hoots from chimpanzees in the neighboring Kasekela and Mitumba communities at Gombe National Park, Tanzania, and the geographically distant Kanyawara community at Kibale National Park, Uganda. We observed group differences only among the geographically isolated communities and did not find any differences between the neighboring communities at Gombe. Furthermore, we found differences among individuals in all communities. Hence, the variation in chimpanzee pant-hoots reflected individual differences, rather than group differences. The limited evidences for vocal learning in Pan suggest that extensive vocal learning emerged in the human lineage after the divergence from Pan.

Re-emergence of the leaf clip gesture during an alpha takeover affects variation in male chimpanzee loud calls

Loud calls are used by many species as long-distance signals for group defense, mate attraction, and inter- and intragroup spacing. Chimpanzee loud calls, or pant hoots, are used in a variety of contexts including group coordination and during male contests. Here, we observed an alpha male takeover in wild chimpanzees (Pan troglodytes verus) during which the leaf clipping gesture re-emerged after disappearing for almost two years in this community. Leaf clipping only occurred in males and was observed almost exclusively prior to pant hoot vocalizations, as has been observed in other chimpanzee communities of the Taï forest in Côte d’Ivoire. Consequently, we hypothesized that leaf clipping may be important for male-male competition by affecting variation in the acoustic properties of male chimpanzee loud calls. We therefore investigated whether pant hoots preceded by leaf clipping differed acoustically from those without, while also testing the influence of social context on pant hoot variation, namely male dominance rank and hierarchy instability, i.e., before, during and after the alpha takeover. We found that pant hoots preceded by leaf clipping were longer, contained more call elements and drum beats, and lower fundamental and peak frequencies. Moreover, during the alpha takeover pant hoots were shorter, contained fewer drum beats and higher fundamental frequencies. Additionally, pant hoot and aggression rates were also highest during the alpha takeover with leaf clipping more likely to occur on days when pant hooting rates were high. Overall social rank had limited effects on pant hoot variation. We suggest that elevated arousal and aggression during the alpha takeover triggered the re-emergence of leaf clipping and the associated acoustic changes in pant hoots. Further research should focus on the potential mechanisms by which leaf clipping is connected to variation in pant hoots and cross-population comparisons of the behaviour.

Chimpanzees (Pan troglodytes) modify grouping and vocal behaviour in response to location-specific risk

Chimpanzees have hostile intergroup relations and are reported to use two strategies to reduce risk in the territory periphery: travelling in larger subgroups ('parties'), and travelling silently. We examined data from the Kanyawara chimpanzee community, Kibale National Park, Uganda to test for evidence of these strategies. We compared behaviour in the territory core with two potentially dangerous contexts: the periphery and croplands. Parties that visited the periphery had over twice as many adult males as parties that remained in the core. Analysis of vocal production rate of 249 parties revealed that, controlling for time of day and party composition, chimpanzees produced fewer pant-hoot calls in croplands than in the core. Pant-hoot production varied in different sectors of the periphery, being reduced in three sectors, unchanged in one, and increased in one. Focal follows of 12 males found results similar to party follows, but with rank-related individual variation. Overall, these results indicate that chimpanzees have the ability to modify grouping and vocal behaviour to reduce risk in areas with a high risk of detection. However, rather than consistently remaining silent in the periphery, chimpanzees in this population sometimes increased their vocalization rate, perhaps to advertise territory ownership and coalition strength.

Chorusing and Call Convergence in Chimpanzees: Tests of Three Hypotheses

Male chimpanzees produce a species-typical long distance call known as the pant hoot. Males give acoustically similar pant hoots when calling together during choruses, but the process by which this vocal convergence takes place is unclear. Three potential mechanisms might account for call matching. First, vocal convergence could represent the passive effect of male chimpanzees sharing a common affective state during choruses. Second, matching could result through the active modification of calls. Such modification might occur if males modeled their calls after those produced by their current chorusing partner. Alternatively, actively mediated convergence would result if males mimicked the calls given by high-ranking males. We examined acoustic variation in calls given during choruses and while alone to test these three hypotheses. Results showed that a similar call type is not given repeatedly during successive choruses. In addition, low-ranking individuals did not produce calls that matched those of the alpha male. Chorused calls produced by two dyadic pairs were more similar to each other than they were to calls delivered during other choruses. The higher ranking, yet dependent, member of one of these dyads appeared to give calls that converged on those produced by the other individual. These data are consistent with the hypothesis that male chimpanzees accomodate each other vocally through the active alteration of their calls during choruses. We suggest that a similar form of accomodation takes place in animals from a wide range of other taxa and that the process serves to maintain and strengthen social bonds between individuals.