Tadpole plasticity shapes the potential benefits of egg feeding to poison frog parents

Feeding offspring is a form of care common in animals. How food shapes offspring fitness, and thus selection on parents, is limited by plasticity in offspring growth and development, and such limitation may shape the evolution of parental care. We compared plastic responses to egg feeding, food availability in the nursery, and nursery size in larvae of two poison frogs: Ranitomeya imitator, regularly fed trophic eggs by their mothers in the wild, and R. variabilis tadpoles, which only rarely have the opportunity to consume eggs. Hand-reared R. imitator, but not R. variabilis, were heavier at metamorphosis when offered eggs. Although R. imitator ate more of the eggs we offered, perhaps explaining this proximately, the greater plasticity of R. imitator persisted when we manipulated the quantity of an egg-free diet meant to approximate food in a natural nursery. Egg feeding did not influence the length of the larval period, and only tadpole R. variabilis took longer to complete metamorphosis in more food-rich nurseries, as is predicted for amphibians generally. In large nurseries, individuals of both species spent longer as tadpoles and were heavier at metamorphosis, an effect independent of food, and one that reflects the complex relationship between ecology and amphibian development. These species differences highlight how tadpole traits might constrain the evolution of egg feeding: R. imitator feed eggs to their tadpoles, and their tadpoles, unlike R. variabilis (a presumptive proxy for the non-feeding ancestor), offer high returns on such investment. Comparative work will offer the exciting opportunity to assess the extent to which egg feeding selected for this growth response to food availability and/or this plasticity facilitated the evolution of care.