Cities are often hotter and drier compared with nearby undeveloped areas, but how organisms respond to these multifarious stressors associated with urban heat islands is largely unknown. Terrestrial isopods are especially susceptible to temperature and aridity stress as they have retained highly permeable gills from their aquatic ancestors. We performed a two‐temperature common garden experiment with urban and rural populations of the terrestrial isopod, Oniscus asellus , to uncover evidence for plastic and evolutionary responses to urban heat islands. We focused on physiological tolerance traits including tolerance of heat, cold and desiccation. We also examined body size responses to urban heat islands, as size can modulate physiological tolerances. We found that different mechanisms underlie responses to urban heat islands. While evidence suggests urban isopods may have evolved higher heat tolerance, urban and rural isopods had statistically indistinguishable cold and desiccation tolerances. In both populations, plasticity to warmer rearing temperature diminished cold tolerance. Although field‐collected urban and rural isopods were the same size, rearing temperature positively affected body size. Finally, larger size improved desiccation tolerance, which itself was influenced by rearing temperature. Our study demonstrates how multifarious changes associated with urban heat islands, i.e. heat and aridity, will not necessarily contribute to contemporary evolution in each of the corresponding physiological traits.

isopod_thermal tolerance

This dataset contains 14 columns, including "colonyID", "individual", "isa", "site", "thermal.tolerance.trait", "thermal.tolerance", "tester", "heat.block", "date.tt", "time.of.day", "age.days", "initial.weight", "habitat", and "treatment". Each row of the dataset represents the response of an individual isopod. colonyID – a unique but arbitrary alpha-numeric identifier for each group of isopods reared together in the lab (i.e., a replicate); multiple individual isopods were tested from the same replicate. isa - a continuous numerical value describing the amount of impervious surface area in a given collection site at a 1km resolution based on the Elvidge et al. 2007 dataset. site - a shorthand name given to the collection site from which the F1 isopod's parental generation were collected in the field. thermal.tolerance.trait – a categorical variable describing whether the critical thermal minimum (ctmin) or critical thermal maximum (ctmax) was assessed on the individual isopod. thermal.tolerance – a continuous numeric value describing either the ctmax or ctmin. Values are presented in units of degrees Celsius. tester - initials denoting the person who conducted the thermal tolerance test for each individual isopod. heat.block - denotes which of the two heat blocks (of the same model and specification) were used in conducting the thermal tolerance test for each individual isopod. date.tt – the date (month/day/year) that the thermal tolerance assessment was conducted. time.of.day - the time of day (measured on a 24-hour clock) at which the thermal tolerance tests were conducted. age.days - the number of days an individual isopod spent in a temperature rearing treatment, that is, the number of days between said isopod's birth and thermal tolerance trial. initial.weight - mass in g of each individual isopod just prior to thermal tolerance trials. habitat - a categorical variable denoting whether the individual F1 isopod's parents derived from an urban or rural field collection site. treatment - a categorical variable denoting whether an individual isopod was reared at a 21 or 29-degree temperature rearing treatment. Values are presented in units of degrees Celsius

isopod_desiccation_tolerance

This dataset contains 15 columns, including "colonyID", "individual", "isa", "site", "mins.dead", "tester", "desiccator", "date", "died", "age.days", "time.of.day", "initial.weight", "habitat", "habitat.n", and "treatment". Each row of the dataset represents the response of an individual isopod. colonyID – a unique but arbitrary alpha-numeric identifier for each group of isopods reared together in the lab (i.e., a replicate); multiple individual isopods were tested from the same replicate. isa - a continuous numerical value describing the amount of impervious surface area in a given collection site at a 1km resolution based on the Elvidge et al. 2007 dataset. site - a shorthand name given to the collection site from which the F1 isopod's parental generation were collected in the field. mins.dead – a continuous numeric value describing the amount of time (in minutes) it took an individual isopod to reach its desiccation tolerance endpoint under low humidity conditions. tester - initials denoting the person who conducted the desiccation tolerance test for each individual isopod. desiccator - denotes which of the two desiccators (of the same model and specification) were used in conducting the desiccation tolerance test for each individual isopod. date – the date (month/day/year) that the desiccation tolerance assessment was conducted. died - the date the desiccation tolerance trial was conducted for a given isopod. age.days - the number of days an individual isopod spent in a temperature rearing treatment, that is, the number of days between said isopod's birth and desiccation tolerance trial. time.of.day - the time of day (measured on a 24-hour clock) at which the desiccation tolerance tests were conducted. initial.weight - mass in g of each individual isopod just prior to deisccation tolerance trials. habitat - a categorical variable denoting whether the individual F1 isopod's parents derived from an urban or rural field collection site. habitat.n - a categorical variable with "0" representing the rural habitat and "1" representing the urban habitat. treatment - a categorical variable denoting whether an individual isopod was reared at a 21 or 29-degree temperature rearing treatment. Values are presented in units of degrees Celsius

combined_age_body_mass

This dataset contains 5 columns, including "colonyID", "age.days", "initial.weight", "habitat", and "treatment". Each row of the dataset represents the response of an individual isopod. colonyID – a unique but arbitrary alpha-numeric identifier for each group of isopods reared together in the lab (i.e., a replicate); multiple individual isopods were tested from the same replicate. age.days - the number of days an individual isopod spent in a temperature rearing treatment, that is, the number of days between said isopod's birth and its tolerance trial. initial.weight - mass in g of each individual isopod just prior to tolerance trials. habitat - a categorical variable denoting whether the individual F1 isopod's parents derived from an urban or rural field collection site. treatment - a categorical variable denoting whether an individual isopod was reared at a 21 or 29-degree temperature rearing treatment. Values are presented in units of degrees Celsius