From performance curves to performance surfaces: Interactive effects of temperature and oxygen availability on aerobic and anaerobic performance in the common wall lizard

Accurately predicting the responses of organisms to novel or changing environments requires the development of ecologically-appropriate experimental methodology and process-based models. For ectotherms, thermal performance curves (TPCs) have provided a useful framework to describe how organismal performance is dependent on temperature. However, this approach often lacks a mechanistic underpinning, which limits our ability to use TPCs predictively. Furthermore, thermal dependence varies across traits, and performance is also limited by additional abiotic factors, such as oxygen availability. We test a central prediction of our recent Hierarchical Mechanisms of Thermal Limitation (HMTL) Hypothesis which proposes that natural hypoxia exposure will reduce maximal performance and cause the TPC for whole-organism performance to become more symmetrical. We quantified TPCs for two traits often used as fitness proxies, sprint speed and aerobic scope, in lizards under conditions of normoxia and high-elevation hypoxia. In line with the predictions of HMTL, anaerobically fuelled sprint speed was unaffected by acute hypoxia while the TPC for aerobic scope became shorter and more symmetrical. This change in TPC shape resulted from both the maximum aerobic scope and the optimal temperature for aerobic scope being reduced in hypoxia as predicted. Following these results, we present a mathematical framework, which we call Temperature–Oxygen Performance Surfaces, to quantify the interactive effects of temperature and oxygen on whole-organism performance in line with the HMTL hypothesis. This framework is transferrable across traits and levels of organization to allow predictions for how ectotherms will respond to novel combinations of temperature and other abiotic factors, providing a useful tool in a time of rapidly changing environmental conditions.info:eu-repo/semantics/publishedVersio

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 Q  = 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

Diving beyond aerobic limits: effect of temperature on anaerobic support of simulated predator avoidance dives in an air-breathing ectotherm

Diving optimality models predict air breathers to routinely dive within aerobic limits, but predator avoidance dives may be an exception. Lengthening submergence times during a predation threat may enhance survival probability, and we therefore hypothesized that predator avoidance dives in juvenile estuarine crocodiles (Crocodylus porosus) would be partially anaerobically fueled. We also predicted that reliance on anaerobic metabolism would increase at elevated temperatures to offset the faster depletion of body oxygen stores. Crocodiles were maintained at 28 degrees and 34 degrees C for 60 d and subsequently underwent simulated predator avoidance dive trials at two test temperatures (28 degrees and 34 degrees C). Blood was sampled immediately on surfacing to measure plasma lactate concentrations relative to nondiving (control) values. Aerobic dive limits (cADL; min) were also calculated using known body mass and oxygen storage relationships and rates of diving oxygen consumption and compared with observed dive durations. Postdive plasma lactate levels were elevated beyond resting levels at both test temperatures, indicating that aerobic thresholds were surpassed during simulated predator avoidance dives. Similarly, >= 90% of dive durations exceeded cADLs at both test temperatures. Postdive plasma lactate concentrations were independent of water temperature and thermal acclimation treatment. Together, these findings suggest that reliance on anaerobiosis during simulated predator avoidance dives is important regardless of temperature

Diving in a warming world: the thermal sensitivity and plasticity of diving performance in juvenile estuarine crocodiles (Crocodylus porosus)

Air-breathing, diving ectotherms are a crucial component of the biodiversity and functioning of aquatic ecosystems, but these organisms may be particularly vulnerable to the effects of climate change on submergence times. Ectothermic dive capacity is thermally sensitive, with dive durations significantly reduced by acute increases in water temperature; it is unclear whether diving performance can acclimate/acclimatize in response to long-term exposure to elevated water temperatures. We assessed the thermal sensitivity and plasticity of 'fright-dive' capacity in juvenile estuarine crocodiles (Crocodylus porosus; n = 11). Crocodiles were exposed to one of three long-term thermal treatments, designed to emulate water temperatures under differing climate change scenarios (i.e. current summer, 28 degrees C; 'moderate' climate warming, 31.5 degrees C; 'high' climate warming, 35 degrees C). Dive trials were conducted in a temperature-controlled tank across a range of water temperatures. Dive durations were independent of thermal acclimation treatment, indicating a lack of thermal acclimation response. Acute increases in water temperature resulted in significantly shorter dive durations, with mean submergence times effectively halving with every 3.5 degrees C increase in water temperature (Q(10) 0.17, P < 0.001). Maximal dive performances, however, were found to be thermally insensitive across the temperature range of 28-35 degrees C. These results suggest that C. porosus have a limited or non-existent capacity to thermally acclimate sustained 'fright-dive' performance. If the findings here are applicable to other air-breathing, diving ectotherms, the functional capacity of these organisms will probably be compromised under climate warming