Warm acclimation reduces the sensitivity of Drosophila species to heat stress at ecologically relevant scales.

Thermal acclimation is presumed to affect heat tolerance, though it is unclear how this should impact populations under realistic natural conditions. In this study, we quantified how thermal acclimation affects heat tolerance landscapes in Drosophila and, as a consequence, their predicted mortality in the field based on modelling with a dynamic thermal tolerance algorithm. We measured the thermal tolerance of four Drosophila species (D. repleta, D. hydei, D. simulans and D. virilis) acclimated to five constant temperatures covering a range from 18 to 30°C. We then combined this information with field temperatures to construct dynamic tolerance landscapes for these species and examine how survival varies over the course of a year. Our analyses reveal the effect of acclimation on an ecologically relevant scale, specifically through the study of cumulative mortality under natural thermal regimes. We explore how different species respond to thermal challenges during acclimation, generally showing an increase in critical temperature (CT_max) while either reducing or maintaining constant thermal sensitivity (z). Furthermore, we show that while acclimation presents a relatively modest improvement in thermal tolerance during short ramping laboratory trials, this response becomes stronger when tolerance estimates are translated into ecologically relevant timescales, such as annual survival. Our results indicate that acclimation to warm conditions can substantially increase Drosophila thermal tolerance, contradicting the idea that thermal acclimation in ectotherms has only a minor effect. Our work applies novel approaches to studying thermal tolerance and aims to highlight the role of acclimation in ameliorating the impact of global warming.