Reduced thermoregulatory ability contributes to increased mortality in urban box turtles (Terrapene spp.)

Urban wildlife often shows pronounced differences in ecology, behavior, and survival compared to rural individuals. Given the broad impacts of temperature in ecology, thermal discrepancies between cities and their rural surroundings are likely to play a major role in explaining these differences between urban and rural animals. Although experimental work has demonstrated the impacts of urban temperatures on wildlife, we lack field data on the body temperatures of free-living urban animals and the ecological impacts of these temperatures. In addition, while urban heat islands have directed research towards the impacts of warmer temperatures, relatively little focus has been given to the potential for urban organisms to experience colder temperatures. Here, we use 10 years of data to show that box turtles (Terrapene spp.) in an urban park are less effective thermoregulators than their rural counterparts at both hot and cold temperature extremes. In contrast to the heat island effect, the body temperatures of brumating urban box turtles are colder and more variable in the winter compared to rural turtles, despite similar environmental temperatures. Furthermore, during the hottest months we find no evidence of an urban heat island effect, yet body temperatures of urban turtles are hotter than rural turtles. These body temperature differences suggest that urban turtles have a reduced ability for behavioral thermoregulation, likely due to lower habitat quality and reduced movement capacity in the urban environment. We further show that turtles experiencing extremely cold body temperatures in the winter are less likely to survive the subsequent year, indicating that over-winter thermoregulatory ability could play an under-appreciated role in the fitness and abundance of urban ectotherms. As urbanization continues to degrade landscapes and as climate change increases the occurrence of temperature extremes, we highlight the need to understand the impacts of cold temperatures on wildlife in human-altered environments.