Population-specific social dynamics in chimpanzees

Abstract

Understanding intraspecific variation in sociality is essential for characterizing the flexibility and evolution of social systems, yet its study in nonhuman animals is rare. Here, we investigated whether chimpanzees exhibit population-level differences in sociality that cannot be easily explained by differences in genetics or ecology. 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. Results indicated temporally stable, population-level differences in dyadic-level sociality. Moreover, group cohesion measures capturing network characteristics beyond dyadic interactions (clustering, modularity, and social differentiation) showed population-level differences consistent with the dyadic indices. 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. 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.

Keywords: animal culture; behavioral diversity; chimpanzees; social learning; sociality.

Fig. 1.

Party size across four neighboring populations of semiwild chimpanzees, 2011–2013 (populations 1–4: n = 765, 911, 635, and 691 observations, respectively). Medians are represented by the bold, horizontal lines within the boxes. The boxes represent the interquartile range (IQR); the vertical lines attached to the boxes represent Q1 − 1.5 IQR (lower) and Q3 + 1.5 IQR (upper).

Fig. 2.

Dyadic (A) proximity and (B) grooming associations per population. The association values (black dots) are the twice-weight indices [x/(x + 2y_AB + y_A + y_B)] for all dyads (populations 1–4: n = 300, 1,081, 91, and 78, respectively). Medians are represented by the bold, horizontal lines within the boxes. The boxes represent the interquartile range (IQR); the vertical lines attached to the boxes represent Q1 − 1.5 IQR (lower) and Q3 + 1.5 IQR (upper).

Fig. 3.

(A and B) Social network attributes across the four study populations. Significant differences (P < 0.0001) were found for all attributes except “strength,” some of which were between the larger and smaller populations (e.g., eigenvector centrality) and others independent of population size (e.g., clustering coefficient); also see SI Appendix, Tables S3 and S4. Ranges are represented by the boxes (IQR); medians are indicated by the bold, horizontal lines within the boxes.

Fig. 4.

Visual representation of population differences in sociality across the four populations at the Chimfunshi Wildlife Orphanage. The icons refer to the following characteristics of each group: party size, proximity, grooming, clustering, modularity, and social differentiation. Proximity and grooming represent the aggregation of all dyadic twice-weight association indices. Clustering, modularity, and social differentiation represent population structure in terms of social cohesiveness, based on proximity. The height of the grids indicates the range of a given characteristic across the four populations. The position of each icon on the grid’s vertical axis indicates the relative position of the group in relation to the total range of the characteristic across all populations.