Urban heat and desert wildlife: rodent body condition across a gradient of surface temperatures

Animals in urban areas can be exposed to human-mediated land use change and radiant heat (called urban heat island effect). Few studies have empirically evaluated the effects of urban heat on wild vertebrates. We live-trapped desert wild rodents from a large metropolitan area in the Sonoran Desert, USA, across seven field sites spanning three strata of land surface temperatures. During the summers of 2019 and 2020, we captured 116 adult pocket mice (Chaetodipus spp. and Perognathus spp.) and Merriam’s kangaroo rats (Dipodomys merriami) in mountainous urban parks and open spaces. We measured body condition, proxy for health, using percent body fat (i.e., fat mass divided by body mass). For mammals, this parameter estimates the storage of energy-rich fat, which is important for growth, survival, and reproduction. We measured body condition using a noninvasive quantitative magnetic resonance instrument. Site-level surface temperatures were measured using data loggers and long-term climate data. Results supported the prediction that body condition was greatest in cooler temperature strata compared to the hottest areas. To relate body condition to resource availability, we evaluated vegetation cover and degree of urbanization. Body fat of adult pocket mice was greater in areas with more vegetation cover and where nighttime temperatures and surface temperatures were lower and urbanization was greater. Kangaroo rats had more fat in areas with the lowest strata of surface temperature. These results demonstrate that extreme heat negatively covaries with small mammal body condition, which indicates that urbanization and climate change have the potential to reduce rodent fitness.