This may be interpreted in terms of an inability to form social bonds with all individuals in large populations, not necessarily in terms of relatively low propensities to be in proximity to others. To understand this pattern better, we reran the analysis with population size as fixed effect (instead of offset term), finding that the population difference in proximity probability could be explained by differences in population size (LRT “population size”: χ 2 = 10.29, df = 1, P < 0.002 estimate ± SD = −4.30 ± 2.73 LRT “population”: χ 2 = 0.01, df = 1, P = 0.98). Whereas in the two smaller populations each possible dyad was observed to be in proximity at least once, in the two larger populations there were dyads who never associated (population 1: ∼15% of possible dyads population 2: ∼41% of possible dyads). The probability for two population members to have associated in close proximity over the course of the study period significantly differed across populations (binomial part LRT for “population”: χ 2 = 37.29, df = 1, P < 0.0001). Based on the current status of cultural primatology, or the study of culture in nonhuman animals more generally, we view this as a pressing question in the study of psychological and behavioral diversity: Beyond isolated accounts of tradition formation in nonhuman animals, is there any indication that nonhuman animals exhibit intraspecific population-level variation in their everyday social interactions that might be instigated by cultural processes? Consequently, it seems conceivable that intraspecific, population-level differences in general sociality (e.g., interaction tendencies) could also emerge by means of learning processes, hence extending the ecological and phylogenetic determinism of sociality postulated by socioecological theory. These behavioral differences extend beyond material culture to “social conventions” or “traditions” without apparent function ( 20, 21, 24, 26– 30). With the advent of cultural primatology ( 12– 16), and the identification of numerous learned behavioral differences across groups in great apes ( 17– 21) and monkeys ( 22– 24), the propensity of primates to develop population-specific behaviors has been well-established (but see ref. We conclude that substantial variation in social behavior exists across neighboring populations of chimpanzees and that this variation is in part shaped by cultural processes. Subsequently, we explored whether the observed intraspecific variation in sociality could be attributed to cultural processes by ruling out alternative sources of variation including the influences of ecology, genetics, and differences in population demographics. Moreover, group cohesion measures capturing network characteristics beyond dyadic interactions (clustering, modularity, and social differentiation) showed population-level differences consistent with the dyadic indices. Results indicated temporally stable, population-level differences in dyadic-level sociality. We compared social proximity and grooming tendencies across four semiwild populations of chimpanzees living in the same ecological environment over three consecutive years, using both linear mixed models and social network analysis. Here, we investigated whether chimpanzees exhibit population-level differences in sociality that cannot be easily explained by differences in genetics or ecology. Understanding intraspecific variation in sociality is essential for characterizing the flexibility and evolution of social systems, yet its study in nonhuman animals is rare.
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