Plasticity in age and size at metamorphosis of Crinia georgiana tadpoles: responses to variation in food levels and deteriorating conditions during development

Adaptive phenotypic plasticity in life-history traits is predicted to evolve when populations occur in heterogeneous environments. Anuran larvae of many species cannot escape their aquatic environment until metamorphosis and therefore should show plasticity in response to conditions experienced as tadpoles. In this study, we manipulated the aquatic environments of Crinia georgiana tadpoles in the laboratory to mimic variation among ponds in resources and drying conditions in nature. This species breeds in very shallow water in winter and ponds frequently dry between bouts of rain, especially towards spring. Tadpoles kept in constant conditions at different levels of food metamorphosed at different body sizes but showed no plasticity in metamorphic, timing. Tadpoles fed only lettuce metamorphosed at sizes similar to those of field-collected tadpoles, whereas tadpoles fed a more protein-rich food metamorphosed at unusually large sizes, indicating that the seeps where C. georgiana tadpoles occur are poor in nutrients. When we decreased food and water levels, tadpoles at later developmental stages were able to accelerate development and metamorphose earlier than tadpoles kept under constant conditions. Furthermore, tadpoles in very shallow water with no access to food metamorphosed earlier and at smaller body sizes than tadpoles with a more moderate decrease in depth that were able to continue feeding. Rapid development and the ability to accelerate metamorphosis in C. georgiana tadpoles are consistent with adaptation in a heterogeneous environment where larvae are under strong time constraints.