Variation in aggressive behaviour in the poeciliid fish Brachyrhaphis episcopi: population and sex differences

Aggression is often positively correlated with other behavioural traits such as boldness and activity levels. Comparisons across populations can help to determine factors that promote the evolution of such traits. We quantified these behaviours by testing the responses of wild-caught poeciliid fish, Brachyrhaphis episcopi, to mirror image stimuli. This species occurs in populations that experience either high or low levels of predation pressure. Previous studies have shown that B. episcopi from low predation environments are less bold than those that occur with many predators. We therefore predicted that fish from high predation populations would be more aggressive and more active than fish from low predation populations. However, we found the opposite - low predation fish approached a mirror and a novel object more frequently than high predation fish suggesting that 'boldness' and aggression were higher in low predation populations, and that population-level boldness measures may vary depending on context. When tested individually, low predation fish inspected their mirror image more frequently. Females, but not males, from low predation sites were also more aggressive towards their mirror image. Variation in female aggression may be driven by a trade-off between food availability and predation risk. This suggests that the relationship between aggression and boldness has been shaped by adaptation to environmental conditions, and not genetic constraints.