Time constraints and flexibility of growth strategies: geographic variation in catch-up growth responses in amphibian larvae.

1. As size is tightly associated with fitness, compensatory strategies for growth loss can be vital for restoring individual fitness. However, immediate and delayed costs of compensatory responses may prevent their generalization, and the optimal strategy may depend on environmental conditions. Compensatory responses may be particularly important in high-latitude habitats with short growing seasons, and thus, high-latitude organisms might be more efficient at compensating after periods of unfavourable growth conditions than low-latitude organisms. 2. We investigated geographical differences in catch-up growth strategies of populations of the common frog (Rana temporaria) from southern and northern Sweden in two factorial common garden experiments involving predation risk and two different causes of growth arrest (nutritional stress and low temperatures) to evaluate how the compensatory strategies can be affected by context-dependent costs of compensation. Larval and metamorphic traits, and post-metamorphic performance were used as response variables. 3. Only northern tadpoles exposed to low food completely caught up in terms of metamorphic size, mainly by extending the larval period. Low food decreased survival and post-metamorphic jumping performance in southern, but not in northern tadpoles, suggesting that northern tadpoles have a better ability to compensate after periods of restricted food. 4. Both northern and southern tadpoles were able to metamorphose at the same size as control tadpoles after being exposed to low temperatures, indicating that consequences of variation in temperature and food availability differed for tadpoles. However, the combination of low temperatures and predation risk reduced survival in both southern and northern tadpoles. Also, predation risk decreased energy storage in both experiments. 5. Our results highlight the influence of climatic variation and the type of stressor as selective factors shaping compensatory strategies.