Social relationships and greetings in wild chimpanzees (Pan troglodytes): use of signal combinations

Social negotiation and "accents" in Western lowland gorillas' gestural communication

Recent findings on chimpanzee infants' gestural development show that they use some gesture types flexibly and adjust them depending on their interaction partner and social context, suggesting that gestural communication is partly learnt and partly genetically determined. However, how gesture types are shaped by social and demographic factors remains unclear. We addressed this question by focusing on gesture type morphology and conducted a fined-grained analysis of gestural form during intraspecific social-play interactions in two captive groups of Western lowland gorillas (Gorilla gorilla gorilla). We focused on the most frequent gesture types (BEAT CHEST, SLAP BODY, SLAP GROUND and TOUCH BODY) produced by subadults (infants, juveniles and adolescents). We considered twelve morphological gesture characteristics (e.g., horizontal and vertical hand trajectories, fingers flexion and spread). Our multifactorial investigation shows that morphological characteristics of distinct gesture types can be shaped by social factors, namely signaller's sociodemographic characteristics (group and kinship), signaller's behavioural characteristics (body posture) and context-related characteristics (recipient's sex, attentional state and position in the signaller's visual field). We nurtured the lively debate concerning gesture origins by revealing the existence of "accents" in non-verbal communication and the highly variable adjustment of gestural form to different conspecifics and interactional characteristics, which supports the revised social negotiation hypothesis.

Chimpanzees show the capacity to communicate about concomitant daily life events

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.

Social uncertainty promotes signal complexity during approaches in wild chimpanzees (Pan troglodytes verus) and mangabeys (Cercocebus atys atys)

The social complexity hypothesis for the evolution of communication posits that complex social environments require greater communication complexity for individuals to effectively manage their relationships. We examined how different socially uncertain contexts, reflecting an increased level of social complexity, relate to variation in signalling within and between two species, which display varying levels of fission-fusion dynamics (sympatric-living chimpanzees and sooty mangabeys, Taï National Park, Ivory Coast). Combined signalling may improve message efficacy, notably when involving different perception channels, thus may increase in moments of high social uncertainty. We examined the probability of individuals to emit no signal, single or multisensory or combined (complex) signals, during social approaches which resulted in non-agonistic outcomes. In both species, individuals were more likely to use more combined and multisensory signals in post-conflict approaches with an opponent than in other contexts. The clearest impact of social uncertainty on signalling complexity was observed during chimpanzee fusions, where the likelihood of using complex signals tripled relative to other contexts. Overall, chimpanzees used more multisensory signals than mangabeys. Social uncertainty may shape detected species differences in variation in signalling complexity, thereby supporting the hypothesis that social complexity, particularly associated with high fission-fusion dynamics, promotes signalling complexity.

The social dynamics of complex gestural communication in great and lesser apes (Pan troglodytes, Pongo abelii, Symphalangus syndactylus)

Gestures play an essential role in primate communication. However, little is known about how complexity of gestural use (in terms of repertoire size, intentional use, flexibility and use of gestural sequences) relates to individual and dyadic measures of sociality and whether more complex gestural use is more effective in eliciting a response. We observed 19 captive chimpanzees (Pan troglodytes), 16 Sumatran orangutans (Pongo abelii) and 18 siamangs (Symphalangus syndactylus) to assess the complexity and effectiveness of their gestural use. We found that, beyond interspecies variation, the number of gesture types used in a dyad was higher when individuals had stronger social bonds; the probability of accounting for others' attention increased with age, especially for visual gestures; and sequences were more likely used by younger or socially less integrated individuals. In terms of effectiveness, older individuals and those using fewer sequences were more likely to be responded to, while across dyads, the probability of obtaining a response was higher when both individuals accounted for the other's attention and when they used fewer sequences. Overall, this confirms the link between sociality and complex gestural use and suggests that more complex forms of communication, at least in terms of intentional use, may be more effective at achieving communicative goals. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Greetings among female chimpanzees in Mahale, Tanzania

Greeting behaviors have been reported in several primate species, although their forms and context may vary across species. Chimpanzees (Pan troglodytes) greet each other in various ways when they meet; however, many recent studies of greetings in chimpanzees have mostly focused on pant grunt vocalizations, which are often viewed as equivalent to submissive signals. As most greetings in chimpanzees are directed toward adult males, either from other males or females, relatively few studies have focused on female-female greetings. Thus, the primary aim of this study was to describe the greetings (not limited to pant grunts) between chimpanzee females in the Mahale Mountains National Park based on long-term observational data. I observed 405 female-female greeting events (10.9 instances per 100 observation hours [obsn. h]) between 1994 and 2018, of which 242 were pant grunts (6.5 instances per 100 obsn. h); 42.3% of greetings were nonaudible, such as tactile or gestural greetings. Most pant grunts were directed toward older females; females under 20 years of age were generally responsible for this trend, as they were the most frequent greeters among females and tended to perform pant grunts toward older females. Nonetheless, among females 20 years of age or older, pant grunts from an older to a younger female were not rare (37%). Compared to previous studies in Mahale, pant grunts between females were an order of magnitude less than those directed toward males. There may also be a large difference in the frequencies of female-female pant grunts across study sites, which may be attributed to differences in female gregariousness.

Exploring greetings and leave-takings: communication during arrivals and departures by chimpanzees of the Bossou community, Guinea

In human fission-fusion societies, ritualized non-linguistic signal exchanges that include gestures, vocalizations, and facial expressions are regularly observed at both arrivals (greetings) and departures (leave-takings). These communicative events play an important role in the formation and maintenance of social relationships. Wild chimpanzees also form large communities that split into smaller fluid parties during daily activities, with individuals moving freely between them. However, in chimpanzees only greetings have been reported. This study explores signal exchanges in the Bossou chimpanzee community during fissions (departures) and fusions (arrivals) given an individual's social rank, kinship, position as traveller or party-member, the level of potential threat, and the party size and presence of mature males. We analysed three time periods (1993-1994; 2003-2004; 2013-2014) during which the composition and social hierarchy of the community varied. We show that the occurrence and form of communication during fission and fusion events are mediated by social factors, including rank, kinship, and party size and composition. Individuals were more likely to communicate during fusions than during fissions, communication was more likely to be produced towards a higher-ranking individual and to non-kin individuals, but the tendency to communicate in general increased with an increase in social rank. The presence of more individuals, and in particular mature males, decreased the likelihood of communication. Communication during fusions supported patterns reported in previous studies on greetings, and our results support the argument that, if present, leave-takings are not a common feature of chimpanzee social interactions. Current methodological difficulties regarding the function of declarative signals hinder our ability to discriminate potential parting rituals within communication before departures. Given similar methodological difficulties, we also provide a note of caution in the interpretation of all signals produced during fusions as 'greetings'.

The founder sociality hypothesis

In this review, we propose that the social dynamics of founder populations in novel and newly available environments can have critical effects in shaping species' sociality and can produce long-lasting changes in social structure and behavior. For founder populations which expand into an underexploited niche separated from the parent population, the necessity of bond formation with strangers, lack of clear territories, and initial abundance of resources can lead to altered initial social dynamics to which subsequent generations adapt. We call this the founder sociality hypothesis. After specifying the theoretical reasoning and mechanism of effect, we focus on three particular cases where the social dynamics of founder populations may have a central role in explaining their modern behavioral ecology. In particular, we develop and review evidence for three predictions of the founder sociality hypothesis in territorial, mixed-sex group forming species: relatively stronger social bonds in the dispersing sex with relatively weaker bonds in the nondispersing sex, reduced territoriality, and increased social tolerance. We briefly touch on the implications for human evolution given our species' evolutionary history marked by frequent expansion and adaptation to novel environments. We conclude by proposing several experiments and models with testable predictions following from the founder sociality hypothesis.