Aerobic scope and climate warming: Testing the “ plastic floors and concrete ceilings ” hypothesis in the estuarine crocodile ( Crocodylus porosus )

Abstract Ectotherms are predicted to show a reduction in absolute aerobic scope (AAS = maximum − standard metabolic rates) if habitat temperatures surpass optima. However, thermal phenotypic plasticity may play a protective role in the maintenance of AAS. In fishes, resting physiological rates (“ physiological floors ,” e.g., standard metabolic rates [SMR]) are typically thermally phenotypically plastic whilst maximum physiological rates (“ physiological ceilings ,” e.g., maximum metabolic rate [MMR]) are typically fixed. This observation led to the “ plastic floors and concrete ceilings ” hypothesis. The applicability of this hypothesis to nonavian reptiles remains untested, despite this group being at risk of climate warming‐induced extinction. We tested this hypothesis in juvenile estuarine crocodiles ( Crocodylus porosus ) by maintaining animals at a water temperature indicative of current summer conditions (28°C) or at a water temperature reflecting a high magnitude of warming (34°C; i.e., thermal acclimation treatments) for 6 months. Metabolic traits (SMR, MMR, and AAS) were subsequently quantified between 28–36°C. A twofold increase in SMR was observed between 28°C and 36°C in both thermal acclimation treatments (pooled Q 10  = 3.2). MMR was thermally insensitive between 28°C and 36°C in 28°C‐acclimated crocodiles but doubled between 28°C and 36°C in 34°C‐acclimated crocodiles. These findings demonstrate thermal phenotypic plasticity in a “ physiological ceiling ” (MMR) and rigidity in a “ physiological floor ” (SMR), showing the opposite pattern to many fishes. Overall, crocodiles displayed impressive aerobic capacity at temperatures reflecting climate warming scenarios. AAS remained unchanged across an 8°C temperature range in 28°C‐acclimated animals and doubled in 34°C‐acclimated animals.