Cognitive skills of common shrews (Sorex araneus) vary with seasonal changes in skull size and brain mass.

In a rare phenomenon, shrews and a few other species cope with seasonal environments by reducing and regrowing brain size, potentially at the cost of changes in cognitive abilities. Here, we confirm an extensive seasonal shrinkage (21.4%) and regrowth (17.0%) of brain mass in winter and spring, respectively, in the common shrew (Sorex araneus L.) in Southern Germany. In a spatial learning task experiment, individuals with reduced winter brain size covered larger distances to find food, compared with the relatively large-brained summer juveniles and regrown spring adults. By reducing their brain mass, these shrews may reduce their energetic demands, but at the cost of cognitive performance, implying a complex trade-off for coping with seasonally fluctuating resources. These results are relevant for our understanding of evolution and the dynamics of mammalian nervous systems in response to environmental changes.