Rise and fall: increasing temperatures have nonlinear effects on aggression in a tropical fish

Environmental conditions are fundamental drivers of animal behaviour. Aggression in ectotherms is a marked example and is often considered positively correlated with temperature. However, many studies exploring this relationship, and its important consequences, focus on a few temperatures and implicitly assume a linear relationship between the two variables. This may limit our understanding of the influence of temperature on aggression. We measured levels of aggression in Neolamprologus pulcher, a cichlid fish commonly used as a model in studies of aggression-based social hierarchy, across a range of temperatures in a mirror aggression test. We used temperatures expected from their natural range with a 2 °C extension on either end. As predicted by thermal performance curves, rates of aggression increased with temperature at the lower end of the temperature range. After reaching a peak, any further increase in temperature resulted in a considerable drop in rates of aggression. Exploring responses at a finer temporal scale, we found that the influence of high temperatures changed dramatically during the 25 min trials. Early in the trials the frequency of aggression increased linearly with temperature, across all temperatures. The initially high levels of aggression at the higher temperatures were not sustained and dropped with increasing duration of exposure to the mirror at high temperatures. Together these findings provide a more comprehensive understanding of the influence of short-term exposure to different temperatures on aggression, such as acute exposure to increased temperatures caused by global warming and associated rapid thermal fluctuations, in African rift lake fishes and in ectotherms more generally. Moreover, our results highlight the importance of measuring aggression across a range of temperatures or otherwise account for nonlinear changes in thermal performance.