Predictability and variability of association patterns in sooty mangabeys

Abstract

In many group-living animal species, interactions take place in changing social environments, increasing the information processing necessary to optimize social decision-making. Communities with different levels of spatial and temporal cohesion should differ in the predictability of association patterns. While the focus in this context has been on primate species with high fission-fusion dynamics, little is known about the variability of association patterns in species with large groups and high temporal cohesion, where group size and the environment create unstable subgroups. Here, we use sooty mangabeys as a model species to test predictability on two levels: on the subgroup level and on the dyadic level. Our results show that the entirety of group members surrounding an individual is close to random in sooty mangabeys; making it unlikely that individuals can predict the exact composition of bystanders for any interaction. At the same time, we found predictable dyadic associations based on assortative mixing by age, kinship, reproductive state in females, and dominance rank; potentially providing individuals with the ability to partially predict which dyads can be usually found together. These results indicate that animals living in large cohesive groups face different challenges from those with high fission-fusion dynamics, by having to adapt to fast-changing social contexts, while unable to predict who will be close-by in future interactions. At the same time, entropy measures on their own are unable to capture the predictability of association patterns in these groups. Significance statement While the challenges created by high fission-fusion dynamics in animal social systems and their impact on the evolution of cognitive abilities are relatively well understood, many species live in large groups without clear spatio-temporal subgrouping. Nonetheless, they show remarkable abilities in considering their immediate social environment when making social decisions. Measures of entropy of association patterns have recently been proposed to measure social complexity across species. Here, we evaluate suggested entropy measures in sooty mangabeys. The high entropy of their association patterns would indicate that subgroup composition is largely random, not allowing individuals to prepare for future social environments. However, the existence of strong assortativity on the dyadic level indicates that individuals can still partially predict who will be around whom, even if the overall audience composition might be unclear. Entropy alone, therefore, captures social complexity incompletely, especially in species facing fast-changing social environments.

Consistency of Social Interactions in Sooty Mangabeys and Chimpanzees

“…Body contact and proximity showed lower consistency than directed interactions, most likely because a certain level of tolerance in foraging and resting extends to most group members, adding random noise that is not present in directed interactions. For body contact, no clear species-specific pattern emerged, but proximity (3 m distance) was much less consistent in mangabeys than in chimpanzees, a result in line with recent findings regarding high levels of randomness in sooty mangabey spatial association patterns (Mielke et al, 2020a). Just like rare interaction types, common but highly inconsistent interaction types could add noise to social relationship indices or when comparing network overlap.…”

“…Dyadic distributions of aggression and dominance interactions were highly predictable across groups. Spatial proximity was the least predictable aspect for all three groups, but as we have reported before, mangabey association beyond body contact contains large uncertainty (Mielke et al, 2020a). Grooming interactions were less predictable in chimpanzees, indicating more varied grooming partner choice or changes over time.…”

“…All mangabey directed social interactions are expected to show high consistency, as they should be strongly influenced by stable parameters, especially kinship, dominance rank, and sex (Range and Noë, 2002). Association patterns in this species are nearly random (Mielke et al, 2020a), so we predict low consistency for spatial interaction types. Chimpanzees at Taï are similar in that they have been shown to have stable grooming, aggression, and association patterns in both sexes.…”

Impact of dominance rank specification in dyadic interaction models

“…My previous use of absolute rank differences to describe preferred association patterns (Mielke et al, 2020) might have omitted important information about differences in social behaviour between individuals of different ranks. In this study, the effect we previously found for female-female association patterns (preferred spatial association with closely-ranked group members) only held for lower-ranking group members – possibly because high-ranking females associate more closely with males, which were omitted here (Mielke et al, 2020). Social interactions (both socio-positive and negative) occurred largely with closely ranked group members.…”

Accuracy and Precision of Social Relationship Indices

“…How robust are indices if inappropriate behaviours are included in the calculation? For example, spatial proximity is a poor indicator of sooty mangabey social relationships (Mielke et al, 2020), but was previously included in social relationship indices for the species (Mielke et al, 2017) – would the index still be robust? We evaluated the relative robustness of index output by swapping some interaction types with others of identical frequencies and selectivity that are otherwise poor indicators of the interaction probabilities (i.e., ‘uninformative’ behaviours).…”