Inquiring Pant-Hoots in Wild Chimpanzees and the Role of Social Bonds and Group Cohesion

The evolution of human language remains a puzzle, with comparative approaches focusing on vocalizations, gestures, bimodal combinations, and, most recently, social interaction and turn-taking. The latter is characterized by cooperative, reciprocal exchanges of alternating short bursts of information among interactants. Some hallmarks of human conversational turn-taking have been found in other primate species, suggesting a possible specialization of great apes in gestural rather than vocal turn-taking. However, relatively little is known about the vocal turn-taking abilities of great apes. Here, we conducted a systematic, quantitative study on vocal exchanges of adult male chimpanzees (Pan troglodytes) living in a habituated community in the Loango National Park, Gabon. We focused on pant-hoots, the typical long-distance calls of chimpanzees, which have been argued to function in some contexts as question-and-answer-like exchanges, referred to as "inquiring pant-hoots" (IPHs), a term coined by Goodall (1986). We collected a comprehensive data set over a period of 16 months (January-May 2019; November 2019-November 2020) resulting in a total of 1747 pant-hoots of ten adult males. We analyzed the data with a special focus on general pant-hoot patterns, criteria for IPHs, social factors, and temporal organization. Overall, general calling frequency was highest in males with high social ranks, in larger parties, and during periods of increased fission and fusion. Twenty percent of calls qualified as IPHs and were positively correlated with smaller party size, higher fission-fusion rates, and the absence of close social partners. Temporal patterns were influenced by social bond strength, the presence of drumming, and an avoidance of overlap. Our findings add to the growing evidence of complex vocal turn-taking abilities in nonhuman primates, contradicting the notion of a specialization in gestural rather than vocal turn-taking for chimpanzees and possibly other great apes. They also emphasize the role of long-distance vocalizations for species in fission-fusion societies and visually dense environments.