Energy use efficiency may mediate metabolic thermal adaptation in Daphnia magna

Thermal adaptation in ectotherms is a critical response to global warming. Resting metabolic rate (RMR), reflecting the energy required for body maintenance, is a key factor of thermal adaptation due to its fundamental role in regulating energy acquisition and allocation within other biological functions. Yet, the importance of thermal adaptation of RMR in controlling individual response to warming still requires to be elucidated. Here, we investigated physiological mechanisms underlying thermal adaptation in two Daphnia magna lines with contrasting thermal preferences, focusing on RMR but also energy use efficiency (EUE) over a large thermal gradient. Using structural equation modelling we showed that adaptation of EUE between the cold and warm lines was able to mitigate the consequences of RMR thermal variability on individuals growth rate. These findings highlight the pivotal role of EUE in ectotherm thermal adaptation, offering key insights to predict the metabolic response of these ectotherms to climate change. Under a context of global warming, ectotherms metabolic response is critical as energy balance and allocation within individuals have repercussions on energy transfers within food web. Accurate predictions of metabolic thermal response of ectotherms may thus constitute an important step toward predicting ecosystem functioning under warming.