Developmental threshold model challenged by temperature

Question: Does the L-shape of the reaction norm for age and size at ontogenetic transitions arise irrespective of temperature? The developmental threshold model suggests that this shape arises due to a threshold in body size to undertake the transition.Data studied: Ages and sizes at metamorphosis measured in a common garden experiment in Rana temporaria originating from six populations along a latitudinal gradient. Individuals were exposed to three temperature treatments and two levels of food.Search method: Population-specific age and size at metamorphosis were plotted against each other, and the shape of the reaction norm across populations was further illustrated by an exponential decay function. The observed patterns were further explored via linear mixed-effect models.Conclusion: The L-shape of the reaction norm depends on temperature, suggesting that at low temperature a developmental threshold in age rather than in size regulates the process of metamorphosis.