On plasticity of aggression: influence of past and present predation risk, social environment and sex

Behaviours are phenotypically very plastic traits, allowing fast changes and fine-tuning of trait expression in response to situational demands. Aggression is a behaviour for which the right decisions can have immediate fitness consequences. Hence, the right behavioural decision is expected to be influenced by both past experience and the current environment. Using a factorial common garden experiment, we tested how past and presently perceived predation risk, as well as the social environment during development, and sex affected two components of intraspecific aggression (viz. hesitation to attack and attack intensity) in a social fish species, the three-spined stickleback (Gasterosteus aculeatus). Exposure to predator cues during development affected how fish responded to actual predation risk: exposed fish hesitated more, while predator-naive fish attacked more in the presence of predators. Sociality also had a strong effect: fish grown in pairs hesitated less than fish grown in solitude or in shoals, whereas solitary-grown fish attacked more than conspecific-experienced (i.e. pair or shoal reared) fish. Female aggression was even across the different developmental environments and lower than that of males. Predator- and conspecific-naive males were more aggressive than other males, suggesting that males have high innate aggression levels that are mediated by environmental risk. The results demonstrate complex underpinnings to plasticity-driven behavioural variation and draw attention to the fact that interpretations of presence/absence of an expected behavioural response may be difficult if it is influenced by both past and present environmental conditions.