Optimal toxicity in animals: Predicting the optimal level of chemical defences

1. Prey species that defend themselves with toxins operate under constraints that affect both the benefits and costs associated with this trait. To date, there has been only limited work on understanding these benefits and costs. 2. We present a mathematical model that accounts for costs incurred by toxic animals throughout their life cycle, using known parameters from the ecological literature. These costs are traded against the benefit of a reduced predation risk. Factors involved in modifying this risk are discussed, particularly the relationship to population density. 3. Explicit, quantitative predictions of the model include: an increase in the optimal level of toxicity as predation pressure increases; a decrease in the optimal level of toxicity as cost increases; and an increase in optimal toxicity as the length of juvenile life stages increases. 4. We show that toxicity will not be favourable under a combination of high cost and high predation, and cite examples to support this case.