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

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 Q₁₀ = 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.