Body size shapes average behaviour, social impact, and social responsiveness in agonistic interactions

Social interactions shape behavioural phenotypes and fitness, and are therefore major drivers of evolution. However, the precise mechanisms through which behavioural dynamics mediate the evolutionary consequences of social interactions remain poorly understood. Plastic responses during social interactions ("social plasticity") arise through two causal pathways: individuals can elicit phenotypic change in their social partners ("social impact") and individuals can adjust their own phenotype to the phenotypes of others ("social responsiveness"). Crucially, functional links among average behaviour, social impact, and social responsiveness may have unanticipated evolutionary consequences, especially when mediated by non-behavioural or morphological traits. Here, we assessed the effects of adult body size on the three components of flexible contest behaviour in southern field crickets (Gryllus bimaculatus). Small animals were on average less aggressive, elicited more aggression in their opponents, and showed greater responsiveness by reducing their aggression more strongly when facing large opponents. Thus, average behaviour, social impact, and social responsiveness were all size-dependent. If genetic, such links may constrain or facilitate evolutionary responses to selection in ways not previously recognised. Our finding that traits mediating social interactions are inherently multivariate highlights the importance of distinguishing distinct pathways of social plasticity to accurately predict micro-evolutionary trajectories of social systems.