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

Social interactions shape behavioural phenotypes and fitness, and therefore represent major drivers of evolution. Increasingly, it is recognised that plasticity in response to social partners (“social plasticity”) operates through two causal pathways: an individual’s impact on and responsiveness to partner phenotypes. Functional links among an individual’s average behaviour, social impact, and social responsiveness may have unanticipated evolutionary consequences. Body size is likely to be a key driver of such links, as size often determines the outcome of social interactions. This is particularly true in competitive contexts, where larger contestants generally have an advantage and individuals are expected to decide whether to escalate a contest or retreat based on the difference between their own body size and that of their rivals. We quantified the extent to which the three components, average behaviour, social impact and responsiveness, vary as a function of body size in competitive interactions, using southern field crickets (Gryllus bimaculatus) as a study system. We found that smaller animals were on average less aggressive, elicited more aggression in their opponents, and were more responsive by reducing their aggression more strongly when interacting against large opponents. Thus, all three social behavioural components—average aggression, social impact and social responsiveness¬—were size-dependent. If heritable, such associations in traits mediating aggression may either constrain or facilitate evolutionary responses to selection in ways not previously recognised. Our findings provide the first empirical evidence for hidden relationships among the key components of social traits, highlighting the need for a multivariate perspective to better predict the evolution of social behaviour.