Post-metamorphic carry-over effects of larval digestive plasticity

1. For animals with complex life cycles, conditions in the larval environment can have important effects that persist after metamorphosis. These carry-over effects may influence juvenile growth plasticity and have important fitness consequences. 2. Small juvenile red-eyed treefrogs, Agalychnis callidryas, grow faster than larger ones. We examined to what extent this growth pattern is due to carry-over effects of intraspecific larval competition. In particular, we assessed larval gut plasticity and determined whether carry-over effects could persist given the extensive gut remodelling that occurs when herbivorous larvae transition to carnivorous juveniles. 3. We reared larvae in mesocosms at low, medium and high densities and measured the size of both larval and juvenile guts, livers and fat bodies. We also monitored the timing of the onset of juvenile feeding post-metamorphosis and, after the onset of feeding, we measured intake rate and mean diet retention time. Finally, we measured juvenile metabolic rates to determine whether any organ size plasticity contributed to metabolic carry-over effects. 4. Larval density had strong effects on larval morphology with higher densities increasing gut length and decreasing liver and fat body sizes. The effects of this plasticity carried over post-metamorphosis. High larval densities produced smaller juveniles with proportionately longer guts and extremely small livers and fat bodies. There were no apparent carry-over effects on size-specific metabolic rate. 5. Differences in larval density were also associated with differences in post-metamorphic feeding. Small juveniles from high larval densities began feeding even before metamorphosis was complete, whereas large juveniles from low larval densities experienced a significant 2-week delay. Although juvenile body mass varied over threefold across treatments, once feeding was initiated, neither intake nor mean diet retention time scaled with body size. 6. Overall, high larval densities produced small juveniles with very low lipid reserves that may have stimulated hyperphagia relative to larger juveniles. Longer guts carried over from the larval stage could facilitate this by allowing small juveniles to elevate intake without sacrificing diet retention time. Patterns of intake coupled with differences in the onset of feeding explain the size-dependent growth pattern previously reported in this and other species.