Seasonal Body Size Plasticity and the Generality of Dehnel’s Phenomenon in <i>Sorex</i> Shrews

AbstractDehnel's phenomenon describes a seasonal and reversible winter decrease in body size, which is a trait that predicts total energy demand. However, the phenomenon remains less well studied than common energy-saving or energy-seeking strategies of mammals. Here, we explore the generality of Dehnel's phenomenon in <i>Sorex</i> shrews on three continents. First, we use new field sampling to document seasonal phenotypic change in masked shrews (<i>Sorex cinereus</i>) in North America at the lowest latitude yet investigated for this species (35.7°). This includes the first documentation of appendicular skeleton remodification in <i>Sorex</i>. Summer-to-winter decreases in <i>S. cinereus</i> body mass, braincase height, and femur length were 13%, 11.5%, and 8.7%, respectively, with subsequent increases of each in second-year individuals. Second, we compile a comprehensive dataset of studies relevant to Dehnel's phenomenon to test whether seasonal plasticity in <i>Sorex</i> globally is related to climate, demonstrating that body and braincase plasticity are functions of cold season temperatures. Meta-analytical models for both of these traits generalized by (<i>a</i>) applying at both inter- and intraspecific scales and (<i>b</i>) predicting the seasonal change newly observed for <i>S. cinereus</i>. Our results support body size plasticity as an environmentally responsive innovation in these very small homeothermic mammals.