Heat limits scale with metabolism in ectothermic animals.

Area of Science:

• Physiology
• Ecology
• Climate Change Biology

Background:

• Ectotherms show limited increases in critical thermal limits despite acclimation to warmer environments.
• This modest plasticity raises concerns for ectotherm resilience to climate warming.
• Heat tolerance depends on both thermal stress magnitude and exposure duration, suggesting rate processes are involved.

Purpose of the Study:

• To investigate if metabolic rate and its thermal scaling can explain the limited plasticity of ectotherm heat tolerance.
• To develop a new approach integrating temperature, time, and metabolic rate to understand critical temperature adjustments.

Main Methods:

• Re-evaluated a large thermal tolerance dataset for diverse ectotherms.
• Developed a 'metabolic rescaling' approach by computing cumulative metabolic expenditure ('metabolic currency') during heating.
• Integrated temperature, time, and the exponential relationship between metabolic rate and temperature.

Main Results:

• Metabolic rescaling dramatically improved explanations for variation in heat limits.
• Heating tolerance was found to be effectively fixed within species; heat limits at any acclimation temperature could be predicted accurately from measurements at other temperatures.
• Heating rate strongly and consistently influenced heat limits.

Conclusions:

• Warmer-acclimated ectotherms have marginally higher critical temperatures because they expend a fixed energy budget for heating at a faster rate.
• This challenges the notion of hard physiological boundaries, suggesting a unification of thermal tolerance and metabolic scaling theory.
• The findings offer a new perspective on ectotherm responses to warming and climate change.