1. Global climate change is expected to have pervasive effects on the diversity and distribution of species, particularly ectotherms whose body temperatures depend on environmental temperatures. However, these impacts remain difficult to predict, in part because ectotherms may adapt or acclimate to novel conditions or may use behavioral thermoregulation to reduce their exposure to stressful microclimates.

2. Here we examine the potential for physiological and behavioral changes to mitigate effects of environmental warming on five species of ants in a temperate forest habitat subject to urban warming.

3. We worked in eight urban and eight non-urban forest sites in North Carolina, USA; sites experienced a 1.1°C range of mean summer air temperatures. At each site, we documented species-specific microclimates (ant operative temperatures, T_e) and ant activity on a transect of 14 bait stations at three times of day. In the laboratory, we measured upper thermal tolerance (CT_max) and thermal preference (T_pref) for each focal species. We then asked whether thermal traits shifted at hotter sites, and whether ants avoided non-preferred microclimates in the field.

4. CTmax and T_pref did not increase at warmer sites, indicating that these populations did not adapt or acclimate to urban warming. Consistent with behavioral thermoregulation, four of the five species were less likely to occupy baits where T_e departed from T_pref. Apparent thermoregulation resulted from fixed diel activity patterns that helped ants avoid the most inappropriate temperatures but did not compensate for daily or spatial temperature variation: Hotter sites had hotter ants.

5. This study uses a novel approach to detect behavioral thermoregulation and sublethal warming in foraging insects. The results suggest that adaptation and behavior may not protect common temperate forest ants from a warming climate, and highlight the need to evaluate effects of chronic, sublethal warming on small ectotherms.