Trade‐off mediated by pyrrolizidine alkaloids predicts alternative reproductive tactics in ithomiine butterflies

1. Trade‐offs between male mating success and survival are the basis of alternative male reproductive tactics. 2. Adult male Greta morgane butterflies acquire protective pyrrolizidine alkaloids (PAs) from plants and use PA‐derived pheromones to attract females. Adult females acquire PAs from male spermatophores, thereby obtaining chemical defence against predation while avoiding costs of PA toxicity and sequestration. 3. We found that males that fed longer on PA‐containing plants were more successful at mating, but males fed a diet containing PAs had a shorter lifespan than males fed a diet without PAs. Together, these results demonstrate a trade‐off between survival and reproduction in relation to male feeding behaviour. 4. As predicted by the alternative reproductive tactics hypothesis, we found that some males fed preferentially on PA‐containing plants while others specialised on PA‐free plants.