Adaptation to Urban Heat Islands Enhances Thermal Performance Following Development under Chronic Thermal Stress but Not Benign Conditions in the Terrestrial Isopod Oniscus asellus

The effects of chronic thermal stress during development on thermal performance traits are not well characterized under urban heat islands, despite these conditions being biologically relevant for how organisms experience the urban environment and the often strong linkages between thermal performance traits and fitness. Here we use the terrestrial isopod Oniscus asellus to examine the effects of chronic thermal stress during development on voluntary running speed of urban and rural isopods. We used a laboratory common-garden experiment design with two developmental acclimation temperature treatments (21 degrees C, a benign treatment, and 29 degrees C, a stressful treatment) and three test temperatures (19 degrees C, 31 degrees C, 40 degrees C); we tested running speed of individuals from urban and rural populations under each of the temperature combinations. We found that for both urban and rural isopods, running speed across three test temperatures was reduced under developmental acclimation conditions of 29 degrees C compared with 21 degrees C. Importantly, however, urban isopods had a running speed advantage over the rural isopods under the 29 degrees C developmental acclimation conditions at the lower two test temperatures. No population differences were detected under benign developmental acclimation conditions of 21 degrees C. The evolution of higher heat tolerance in urban isopods further supported the interpretation of adaptation to heat stress. Convergence of urban and rural isopod running speed at the highest test temperature, however, suggests potential limits or constraints on adaptation. Our results indicate that thermal adaptation to urban heat islands can mitigate negative effects of chronic developmental thermal stress, even when overall performance is reduced compared with benign conditions.