Tadpole foraging history affects the degree of induced offenses in salamander hatchlings

Abstract

To better understand phenotypic plasticity from an adaptive perspective, it is crucial to examine how specific environmental signals influence phenotype expression. How biological signals emitted by prey into an aquatic environment are perceived by predators can be context-dependent, and the food type consumed by the prey can alter the chemical cues it releases by affecting its physiological traits, such as elemental composition and excretion patterns. These diet-driven differences in emitted chemical cues can, in turn, influence how predators receiving these cues adjust their phenotype in early developmental stages. Ecological stoichiometry provides a useful framework for exploring this relationship, as the elemental composition of organisms varies across trophic levels and changes with diet. In this study, we investigated how the food consumed by Ezo brown frog Rana pirica tadpoles (the prey) influences the plasticity of growth rate in hatchling larvae of the Ezo salamander Hynobius retardatus (the predator), which modulates their growth in response to water-mediated chemical signals from tadpoles. We reared omnivorous tadpoles for 1 month on either an animal-based diet (Chironomid larvae) or a plant-based diet (pasture grass). Tadpoles with an animal-based diet grew larger than those with a plant-based diet but maintained a constant body C/N ratio despite differences in dietary C/N ratios. This suggests that tadpoles excrete waste with a lower C/N ratio when consuming an animal-based diet, altering the chemical signals they release. We then examined the growth response of salamander hatchlings raised with tadpoles from different foraging histories. Hatchlings grew significantly larger and faster when reared with tadpoles that had an animal-based food type. Given that newly hatched salamanders cannot yet prey on tadpoles, these results suggest that hatchlings adjust their growth rate in anticipation of future prey availability or condition, highlighting a potential link between diet-mediated signal variation and phenotypic plasticity.