Reply to Farine and Aplin: Chimpanzees choose their association and interaction partners

Abstract

Farine and Aplin (1) question the validity of our study reporting group-specific social dynamics in chimpanzees (2). As alternative to our approach, Farine and Aplin advance a "prenetwork permutation" methodology that tests against random assortment (3). We appreciate Farine and Aplin's interest and applied their suggested approaches to our data. The new analyses revealed highly similar results to those of our initial approach. We further dispel Farine and Aplin's critique by outlining its incompatibility to our study system, methodology, and analysis. First, when we apply the suggested prenetwork permutation to our proximity dataset, we again find significant population-level differences in association rates, while controlling for population size [as derived from Farine and Aplin's script (4); original result, P < 0.0001; results including prenetwork permutation, P < 0.0001]. Furthermore, when we exchange "population size" for "density" (i.e., population size/enclosure size) in our analyses, as suggested by Farine and Aplin, we continue to find strong evidence for population-level differences in association rates [generalized linear mixed model (GLMM) with population size, P < 0.0001; GLMM with density, P < 0.0001]. Interestingly, whereas population size significantly explains variation in association indices (P = 0.012; see ref. 2), density does not (P = 0.912), indicating that chimpanzees intentionally navigate their social world. Here, we note that this argument is consistent with decades of previous research that has shown that chimpanzees are intentional animals who selectively associate rather than "bump into" their conspecifics (5-8), even in small zoo enclosures (9, 10). Second, we address the incorrect assumptions integrated into Farine and Aplin's simulation approach. Farine and Aplin's rule for deriving "proximity" from the simulated data bears no resemblance to the heuristic used in our original study (2). Where Farine and Aplin

Chimpanzee communities differ in their inter- and intrasexual social relationships

“…However, in such cases, ruling out factors such as ecological and genetic variation, among other explanations, remains difficult. Based on our findings involving chimpanzee groups in shared ecological environments and with comparable genetic composition, we conclude that male and female social bonds that may be shaped by the social environment, in line with previous work on the CWO chimpanzees (Rawlings et al, 2014 ; Van Leeuwen et al, 2012 ; van Leeuwen et al, 2014 , 2018 , 2019 ). It is important to note however, that other factors we have not considered here such as levels of within-group aggression and personality types (Massen & Koski, 2014 ; Rawlings et al, 2020 ), or polymorphic variation in receptor genes that are related to the expression of social behaviour in chimpanzees (Staes et al, 2014 ) may also impact bonding in chimpanzees.…”

“…We applied a standard regression method capable of accounting for repeated measures of individuals as well as controlling for influential variables, while assessing the strength of the predicted variables on the response (Baayen, 2008 ; Bolker et al, 2009 ). Given that we worked with observational data collected on different groups, with inherent biases regarding the selecting and therefore the assessment of certain individuals (e.g., less neophobic individuals, or individuals with high gregariousness; Farine & Whitehead, 2015 ; Whitehead, 2008 ), we additionally treated the inputted datastream (i.e., the data used as response for the GLMMs) in order to minimize the influence of such biases on the inferential framework (Farine & Aplin, 2019 ; van Leeuwen et al, 2019 ). This treatment has been proposed to benefit from permutations before the data are condensed into network indices (Bejder et al, 1998 ), hence the name “prenetwork” or “datastream” permutations (Farine, 2017 ).…”

Cofeeding tolerance in chimpanzees depends on group composition: A longitudinal study across four communities

“…Our results are also based on chimpanzees living with supplemented human care. While our study groups live in large outdoor enclosures, enabling individuals to display natural fission-fusion dynamics for the majority of the day (see van Leeuwen et al., 2018 , 2019 ), they are provisioned twice a day. As a result—and contrary to the natural conditions of wild chimpanzees—the four groups experience a constant and stable level of resource availability.…”

Early Trauma Leaves No Social Signature in Sanctuary-Housed Chimpanzees (Pan troglodytes)

“…At Chimfunshi, the chimpanzees have access to all five of these opportunities. Relatedly, the chimpanzees at Chimfunshi live in spacious environments, which allows the chimpanzees to choose which social partners they want to interact with [54]. In conjunction, these features might have facilitated chimpanzees' social recovery from their experienced trauma (also see [55]).…”