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; 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. Here, 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 behaviour in C. viridis. Immature, soft-shielded beetles tended to drop more, while mature beetles with a hard shield preferred to duck. 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 green tortoise beetles. In addition, interactions between ontogeny and environment, as well as differences between populations and years influenced the avoidance behaviour. This study emphasises the importance of considering several factors and their interactions to understand the ecology, maintenance and evolution of multiple anti-predator behaviours.