Geographic Variation in the Bioclimatic Niche, Hibernation, and Body Size of Anatolian Ground Squirrels

Geographic variation in phenotypic traits offers key insights into how organisms adapt to diverse environmental conditions. In this study, we studied how elevational and associated environmental gradients shape geographic variation in the bioclimatic niche, hibernation, and body size of Anatolian ground squirrels ( Spermophilus xanthoprymnus ). Specifically, we used presence data (170 out of 538 present records) from across the species’ range, body temperature data from 51 free-living individuals in two natural populations located 880 km apart, and body size data from 167 individuals across 10 populations to explore geographic variation in bioclimatic niche, hibernation, and body size across elevational and associated environmental gradients. Our results revealed that the bioclimatic niches of two deeply divergent mitochondrial (mt)DNA lineages (i.e. the western and eastern lineages) are distinct. However, this pattern appears to result from the underlying bioclimatic differences between the regions the western and eastern lineages inhabit. Anatolian ground squirrels from the eastern population, which inhabits a higher-elevation, colder, wetter, and more seasonally variable environment, exhibit longer hibernation periods, spend a higher proportion of this period in torpor bouts, and achieve deeper reductions in body temperature than conspecifics from the western population. Adult males exhibit shorter hibernation periods and spend a smaller proportion of this period in torpor bouts than the other age-sex classes. Anatolian ground squirrels from the eastern lineage, which inhabits areas at higher elevations with colder, wetter, and more seasonally variable environments, are morphologically larger than conspecifics from the western lineage, particularly among males. Overall, our results demonstrate that spatial and environmental gradients shape phenotypic variation in Anatolian ground squirrels through lineage-, population- and demographic-level responses. By integrating bioclimatic niche, hibernation, and body size, this study highlights the importance of combining multiple trait dimensions to improve our understanding of eco-evolutionary divergence in hibernating mammals.