Dynamic models allowing for flexibility in complex life histories accurately predict timing of metamorphosis and antipredator strategies of prey

1. The development of antipredator defences in the larval stage of animals with complex life cycles is likely to be affected by costs associated with creating and maintaining such defences because of their impact on the timing of maturation or metamorphosis. 2. Various theoretical treatments have suggested that investment in defence should both increase or decrease with increasing resource availability, but a recent model predicts investment in defences should be highest at intermediate resource level and predator density. 3. Previous models of investment in defence and timing of metamorphosis provide a poor match to empirical data. Here we develop a dynamic state‐dependent model of investment in behavioural and morphological defences that enables us to allow flexibility in investment in defences over development, the timing of metamorphosis and the size of the organism at metamorphosis that were absent from previous theory. 4. We show that the inclusion of this flexibility results in different predictions to those of the fixed investment approach used previously, especially when we allow metamorphosis to occur at the optimal time and state for the organism. 5. Under these more flexible conditions, we predict that morphological defences should be insensitive to resource level whilst behavioural defences should either increase or decrease with increasing resources depending on the predation risk and the magnitude of the fitness benefits of large size at metamorphosis. 6. Our work provides a formal framework in which we might progress in the study of how the use of antipredator defences is affected by their costs. Most of the predictions of our model in are in good accord with empirical results, and can be understood in terms of the underlying biological assumptions. The reasons why simpler models failed to match empirical observations can be explained, and our predictions that are a poor match help to target the circumstances which warrant future study.