The importance of preferential associations and group cohesion: constraint or optimality

Abstract

Many animals form social groups, but the adaptive value of being gregarious often differs between individuals. It has been suggested that individuals who gain fewer benefits may only join groups due to constraints on resources. These individuals would prefer to associate loosely with a group, associate strongly with a few preferred individuals and avoid despotic individuals. Alternatively, if joining a group is the optimum decision due to advantages gained from cohesion, individuals may exhibit decreased preferences regarding with whom they associate. We tested these ideas by analysing the social networks of wild winter groups of black-capped chickadees (Poecile atricapillus) in relation to experimental manipulation of resource distribution: three feeders 5 m apart (individuals can maintain contact with the group while also following their social preferences), 40 m apart (increased risk of losing contact while following preferences) and 130 m apart (extremely difficult to follow preferences without losing cohesion). If preferential associations are more important than cohesion, we predicted fewer but stronger connections with decreasing distance between feeders. Dynamic network analysis revealed that distance treatments did change the number of above average strength associations. However, stability of individual centrality was found to decrease when resources were close together and network structure remained similar overall. This suggests individuals maintain cohesion even when alternative foraging choices are available nearby but associate more selectively when given the opportunity. Our results indicate that while individuals will attempt to follow their preferential associations where possible, these are insufficient to cause breakdown in cohesion, possibly due to costs of losing contact with the group. Animals must compromise between associating with preferred individuals and following group consensus. The importance of social preferences can help infer animals’ reasons for joining a group. If group cohesion brings benefits, following group consensus may be more important than individuals’ social preferences. Alternatively, if groups form simply due to lack of resources, following social preferences may be more important than maintaining group cohesion, depending on the costs of losing contact with the group. We examined this by looking at social network structure of black-capped chickadees in relation to resource distribution, expecting changes in network structure as it became easier or harder to exhibit social preferences. We found individuals had fewer strong associations when resources were closer, meaning that networks were more differentiated than when resources were further apart. This suggests that individuals associated more selectively when possible, though this did not appear to change overall network structure.