Integrating Physiological Rates of Thermal Stress and Repair Predicts Heat Failure During Temperature Fluctuations

The Thermal Death Time (TDT) model has enabled quantification of heat injury in ectothermic organisms across variable thermal stress intensities and durations. Using experimental data from Drosophila suzukii, here we extend this framework to include a quantitative assessment of repair of thermal injury. We first demonstrate that heat injury accumulates additively and exponentially at stressful temperatures and subsequently show how repair of heat injury also depends on both temperature and duration across permissive temperatures. We find repair to be additive across different permissive temperatures, with the highest repair rate at intermediate temperatures. Importantly, integration of injury and repair rates enables accurate predictions of thermal failure during simple temperature fluctuations alternating between stressful and permissive temperature ranges. Our data therefore support an extension of the TDT framework to integrate injury-repair dynamics. With further validation for other traits and taxa, these dynamics will be critical for assessing thermal vulnerability in a warming climate.