Predator-induced plasticity on the life-history and antipredator defenses of the aposematic wood tiger moth larva

Abstract

Defense mechanisms in organisms evolve as a response of predator-prey interactions, reducing prey mortality. Flexibility in antipredator strategies due to heterogeneous environments can be explained by phenotypic plasticity. This plasticity can be important for aposematic organisms where variation in the warning signal within a population is considered puzzling. In aposematism, monomorphism is expected because the predator better learns to avoid the unprofitable prey associated with a conspicuous signal and tends to generalize the negative experience to nearby stimulus. Thus, selection should favour the most common and conspicuous warning signal leading to positive frequency-dependent survival selection. I examined predator-induced plasticity on antipredator defenses of the aposematic wood tiger moth larva, Arctia plantaginis. The main defense of the larva is the continuous warning signal comprising a hairy black body and an orange patch on the dorsal part. A large orange patch has been related with a more efficient antipredator function, while a small patch with efficiency in thermoregulation and immunity response. Given this, I hypothesized that predation would induce a plastic response in warning signal size. Due to the potential importance or interaction of other antipredator defenses such as body size, development time and behaviour I also studied their plastic response to predation. I expected a positive plastic response from the larvae under pr edation risk. Based on general theory and past results in the system I also predicted that under predation larvae would develop a smaller body size, a faster development time with less instars and slower escape behaviour. I reared larvae from 20 families in a split-design experiment. Individuals from each family were evenly split and reared in two environments. In one, the larvae were exposed to a simulated bird attack, whereas in the other, the larvae were left to develop in normal conditions. Overall I found predation induced a significant plastic response in the orange warning signal size, body size and number of moulting events. It also affected overall survival. However, predation did not have a significant effect on the larval development time. The escape behaviour was also plastic and there was a significant interaction between families and the treatment, suggesting that not all organisms responded similarly to predation. I also find a significant family effect, meaning that families in the study expressed differences in the mean value for a specific trait. In conclusion, the traits showed to differ in the plastic responsiveness to predation. This provides an insight about the selection pressures that constraint the plastic response. For instance, the development time showed to be a trait less sensitive, suggesting that for organisms that are seasonally constrained, time to metamorphosis is more critical than the risk of predation.