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

1. Accurately predicting the responses of organisms to novel or changing
environments requires the development of ecologically-appropriate
experimental methodology and process-based models. 2. 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 thermal performance curves predictively. Further, thermal
dependence varies across traits, and performance is also limited by
additional abiotic factors, such as oxygen availability. 3. 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 thermal performance curve
for whole-organism performance to become more symmetrical. 4. We
quantified thermal performance curves for two traits often used as fitness
proxies, sprint speed and aerobic scope, in lizards under conditions of
normoxia and high-elevation hypoxia. 5. In line with the predictions of
HMTL, anaerobically-fueled 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. 6. Following these results, we present a mathematical
framework, which we call Temperature-Oxygen Performance Surfaces (TOPS),
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.