Despite substantial changes in group composition, chimpanzees adhered for 12 years to arbitrary group-specific greetings not explained by genetics or the environment; seems that our conventions are rooted in our evolutionary history

Temporal stability of chimpanzee social culture. Edwin J. C. van Leeuwen. Royal Society Biology Letters, May 26 2021. https://doi.org/10.1098/rsbl.2021.0031

Abstract: Culture is a hallmark of the human species, both in terms of the transmission of material inventions (e.g. tool manufacturing) and the adherence to social conventions (e.g. greeting mannerisms). While material culture has been reported across the animal kingdom, indications of social culture in animals are limited. Moreover, there is a paucity of evidencing cultural stability in animals. Here, based on a large dataset spanning 12 years, I show that chimpanzees adhere to arbitrary group-specific handclasp preferences that cannot be explained by genetics or the ecological environment. Despite substantial changes in group compositions across the study period, and all chimpanzees having several behavioural variants in their repertoires, chimpanzees showed and maintained the within-group homogeneity and between-group heterogeneity that are so characteristic of the cultural phenomenon in the human species. These findings indicate that human culture, including its arbitrary social conventions and long-term stability, is rooted in our evolutionary history.

4. Discussion

For a behaviour to be labelled ‘cultural’, scholars typically evaluate the behaviour for its (i) emergence through social learning, (ii) sharedness among group members (and absence or difference for members of other groups) and (iii) longevity [14,39,40]. While a plethora of studies have documented socially learned behavioural traditions in animals [41,42], to my knowledge, there is a paucity with respect to evidencing the stability of traditions.

One established example of cultural persistence in animals concerns tool use in wild chimpanzees: over a period of 25 years, group differences with respect to tool-material selection for nut cracking remained highly similar, despite a large number of (female) migrations [43,44]. Material culture has plausibly been part of chimpanzees' repertoire for thousands of years [45], yet social culture remains to be systematically documented in animals. Recently, cultural transmission of social customs has been suggested for traditions in great apes ([9,10,46]; also see [14,47]). Moreover, social culture in terms of socially learned patterns of association and interaction that result in group-specific sociality has been implied as an explanatory mechanism for non-random social dynamics in animals (e.g. [48–54]). In these studies, some forms of stability were documented—for instance, neighbouring groups of meerkats (Suricata suricatta) differed consistently in the time of day on which they emerged from their burrows across a decade [54], and reef sharks (Carcharhinus amblyrhynchos) were found to gather in the same close association cliques annually across four consecutive years, presumably to benefit from public information regarding food patches [52]. Arguably the best-documented case of social culture in animals comes from observations on troops of wild olive baboons (Papio anubis) living in the Masai Mara Reserve of Kenya. Owing to the simultaneous deaths of a substantial number of aggressive adult males (caused by a selective outbreak of tuberculosis), the respective ‘Forest Troop’ became a group of relatively socially tolerant and non-aggressive members in comparison to another group living in the same reserve (Talek troop) and the Forest Troop pre-dating the deaths [48,53]. Despite a substantial influx of new group members, across a period of at least 10 years, the Forest Troop remained markedly characterized by what the authors called a culture of ‘pacifism’—relatively high rates of grooming and affiliation, relaxed dominance hierarchies and an overall tendency of non-aggressive interactions, even between resident females and newly immigrated males [48].

Here, I report the multiyear (12 years) stability of a variational cultural practice [24], which is plausibly devoid of any ecological relevance. Whereas material cultures [7,55], but also culturally induced social foraging [52] and dyadic interactions [48,53] are behaviours with clear adaptive value [56], the precise variant by which chimpanzees engage in the handclasp grooming does not bear any relevance to survival or fitness. As such, stability in variant preference might be even less expected given the lack of eliciting affordances in the environment (e.g. the presence of termites might (re-)trigger termite fishing). In analogy to human culture: whereas the motivation for bridging social distances gives rise to greeting behaviours universally, the exact manner in which the greeting gesture emerges (and perpetuates) is highly culture specific [57,58]. The here reported group differences are also difficult to explain based on genetics: the study groups do not systematically differ in their subspecies composition, and whereas the handclasp behaviour an sich could be hypothesized to be under positive genetic selection [59] (i.e. its function is still largely unknown), the relative style preferences by which the groups choose to handclasp seem harder to place in such a selectionist framework (also see [11,24]). The finding that female dyads engaged more in palm clasping while male dyads engaged more in wrist clasping could be due to the fact that chimpanzee males may use the handclasp as a means to confirm or assert dominance over the partner. The subject's wrist position allows the partner to support the weight of the subject's clasping arm, which can be viewed as a prosocial act by the partner [11]. Albeit plausible, more research is needed to investigate this conjecture, including how such configurations are initiated.

Between-group heterogeneity is expected to gradually transition toward homogeneity owing to factors like drift, the natural cycle of births and deaths, and migrations, unless there are mechanisms in place to prevent this, like in humans [2]. Despite such changes of group composition in the studied chimpanzees, the group-specific variant preferences remained, suggesting the workings of stability-fostering mechanisms (also see [43,60]). One potent mechanism promoting group-level cultural stability is conformity—the tendency to copy the behaviour of the majority [2,28]. Whether chimpanzees are conformists remains disputed [43,61–64]—yet, the findings of this study warrant scrutiny of any chimpanzee behaviour that could bolster within-group cultural homogeneity across extended periods of time. In any case, where the minimal genetic and environmental variation across groups allows for inferring the cultural nature of the handclasp styles by means of the method of exclusion (also see [24,25]), the observed temporal stability of group-specific style preferences despite substantial population turnover provides a positive indication of the cultural hypothesis.

Recapitulating, chimpanzees retained group-specific grooming style preferences across a 12-year study period in which a substantial number of individuals replaced original group members owing to births, deaths and translocations. This stability of cultural variants indicates that (i) preliminary findings on social culture in chimpanzees are robust, (ii) animals can develop and maintain cultural preferences in the domain of arbitrary, non-fitness-related phenomena, much like the human species and (iii) animal cultures can possess the necessary ingredients in terms of variant adherence and longevity to be a potent force in gene–culture coevolutionary dynamics, thus shaping both phenotypes and genotypes in animals [12,13].