In a nutshell, a reciprocal transplant experiment reveals local adaptation and fitness trade-offs in response to urban evolution in an acorn-dwelling ant

Urban-driven evolution is widely evident, but whether these changes confer fitness benefits and thus represent cases of adaptive urban evolution is less clear. We performed a multi-year field reciprocal transplant experiment of acorn-dwelling ants across urban and rural environments. Fitness trade-offs via survival were consistent with local adaptation: we found a survival advantage of the ‘home’ treatments compared to the ‘away’ treatments. Seasonal bias in survival was consistent with evolutionary patterns of gains and losses in thermal tolerance traits across the urbanization gradient, such that rural ants in the urban environment were more vulnerable in the summer, putatively due to low heat tolerance and urban ants in the rural environment were more vulnerable in winter, putatively due to an evolved loss of cold tolerance. The results for fitness via fecundity were more complex. Fecundity differences were present in the rural environment, but not the urban environment, and could reflect mismatched cues for the seasonal production of reproductives. To broadly contextualize our results, we performed a multi-species meta-analysis of urban adaptive evolution studies and found general support for local adaptation. The rural adaptation signal was stronger than for urban adaptation, consistent with the relative differences in time for adaptation to occur.