A test of the thermal coadaptation hypothesis with ultimate measures of fitness in flour beetles

Whole-organism performance of ectotherms depends on body temperature, which is tightly linked to environmental temperatures. Individuals attempting to optimize fitness must thus select appropriate temperatures. The thermal coadaptation hypothesis posits that To for traits closely linked to fitness should match temperatures selected by a species (Tset) and should coevolve with Tset. To may mismatch Tset if the thermal reaction norm for fitness is asymmetric. In this study, we examined six traits related to fitness in red and in confused flour beetles (Tribolium castaneum and T. confusum, respectively), including longevity, lifetime reproductive success, reproductive rate, and development time at four temperatures between 23 and 32°C. For reproductive traits, To matched Tset whereas for longevity To was lower than Tset. Tribolium species have a strongly r-selected life history strategy, therefore reproductive traits are likely more tightly linked to fitness than longevity due to high predation rates at early life stages. We therefore provide support for the thermal coadaptation hypothesis for reproductive traits that are tightly linked to fitness. Our results highlight the importance of knowing the relationships of traits to fitness when studying thermal physiology. •Optimal temperature in performance is linked to underlying thermal physiology.•Optimal temperature should match preferred temperature in traits linked to fitness.•Experiments with flour beetles show link between fitness and preferred temperature.•Lifetime reproductive success is linked to fitness, longevity is not.