Cities can be used as open-sky replicates to assess the responses of organisms to environmental changes, such as increased heat or pollution. In contrast with the numerous communitylevel studies, population-level studies are rare, hugely biased towards birds, and rarely based on multiple city replicates. They are also mostly based on solitary species, despite some social insects being ecologically dominant and economically important. Our study aimed at assessing whether colonies of the ant Temnothorax nylanderi from forest and urban populations exhibit distinct responses to pollution.We collected colonies from four pairs of forest and urban habitats and reared them in the laboratory under normal or cadmium-supplemented diets. We then measured the emergence rate and size of newlyproduced workers as well as the mortality rate of adult workers. We also assessed the effect of some social factors such as colony size or size of field workers. In all four forest/city replicates, urban colonies were less negatively affected by cadmium than forest colonies. We did not find any effect of social factors on cadmium resistance, in contrast with what was found in other eusocial insects. We discuss the potential adaptive nature of this differential response.

Urbanization brings new pressures for individuals. Among them, trace elements, such as cadmium, are important stressors. A recent study highlights a weaker negative effect of cadmium on city colonies relative to their forest counterparts in the ant Temnothorax nylanderi. Here, we aim to test whether the better tolerance of city colonies in this species results from a better ability of workers to rear larvae despite stressful conditions and/or a better ability of larvae to develop properly despite stressful conditions. We performed a cross-fostering experiment of workers and larvae from city and forest colonies, in common garden conditions in the laboratory. Colonies were fed using cadmium-enriched or cadmium-free food for 2 months, and we measured four life-history traits. As expected, cadmium had a negative impact on all traits. Unexpectedly, we did not observe a better tolerance of city colonies to cadmium, contrary to our previous study, which prevented us from disentangling the respective contributions of workers and larvae to cadmium tolerance. Interestingly, forest colonies seemed to be of better quality in our laboratory conditions. Finally, colony size increased adult survival, but only in the absence of cadmium, suggesting that social buffering could collapse with strong external disturbances.