Predator avoidance behaviour in green tortoise beetles depends on environment and shield ontogeny, but varies over time and space

A prey species can have multiple predator defence mechanisms and these may be used in context-dependent ways to increase survival. How predator avoidance behaviours vary within a species, and how this variation is maintained, are important ecological and evolutionary questions where factors such as the physical state of the individual or the environmental setting may play a role. We investigated spatial and temporal intraspecific variation in two anti-predator behaviours in the green tortoise beetle Cassida viridis: ducking under their shield (expanded pronotum and elytra) or dropping from their host plants, by simulating a predator attack in the beetles' natural environment. We found that the beetle ontogeny (measured as shield hardness and maturity) and the substrate underneath the host plant (water or land) influenced the anti-predator response in C. viridis. Immature, soft-shielded beetles were more likely to drop than duck, and mature beetles with a hard shield preferred to duck, whereas immature beetles with a hard shield did not show a preference. Beetles situated on host plants above water ducked more, but no preference for either behaviour was found for beetles situated above land. These results indicate a cost-benefit balance in the response of the anti-predator defences in our study populations of C. viridis. In addition, the effect of ontogeny varied between populations and years, and a tendency for such variation was also found for substrate. This study emphasises the importance of considering several factors and their interactions to understand the ecology, maintenance, and evolution of multiple anti-predator behaviours.