Elevated temperatures are expected to rise beyond what the physiology of many organisms can tolerate. Behavioural responses facilitating microhabitat shifts may mitigate some of this increased thermal selection on physiology, but behaviours are themselves mediated by physiology, and any behavioural response may trade-off against other fitness-related activities. We investigated whether experimental evolution in different thermal regimes (Cold: 15°C; Hot: 31°C; Intergenerational fluctuation 15/31°C; Control: 23°C) resulted in genetic differentiation of standard locomotor activity in the dung fly Sepsis punctum. We assessed individual locomotor performance, an integral part of most behavioral repertoires, across eight warm temperatures from 24°C to 45°C using an automated device. We found no evidence for generalist-specialist trade-offs (i.e. changes in the breadth of the performance curve) for this trait. Instead, at the warmest assay temperatures, hot-selected flies showed somewhat higher maximal performance than all others, especially cold-selected flies, overall more so in males than females. Yet, the flies’ temperature optimum was not higher than that of the cold-selected flies, as expected under the ‘hotter-is-better’ hypothesis. Maximal locomotor performance merely weakly increased with body size. These results suggest that thermal performance curves are unlikely to evolve as an entity according to theory and that locomotor activity is a trait of limited use in revealing thermal adaptation.

We assessed individual locomotor activity or performance, an integral part of most behavioral repertoires, across eight warm temperatures from 24°C to 45°C using an automated device (LAMs; Trikinetics). Locomotor activity was automatically measured by placing individual Male or Female flies into one of 32 rack slots of two different Locomotor Activity racks (LAMs; Trikinetics). Experimental flies of the dung fly species Sepsis punctum (Diptera: Sepsidae) stemmed from four experimental evolution treatments (Cold=constant 15C, Hot=constant 31°C; Kontrol=wildtype 23°C; Inter=switch from 15°C to 31°C from one generation to next; see Berger et al. 2014 JEvolBiol 10.1111/jeb.12452). Multiple runs (replicates) per treatment combination were assessed to obtain locomotor activity/performance Thermal Performance Curves. The hind tibia length (mm) of each fly was measured after the experiment.