Aim We assessed the phenotypic responses of body size and organ weights of two small rodent species ( Myodes glareolus and Apodemus flavicollis ) to elevation, a surrogate for temperature and other environmental conditions. We expected not only an increase in body size (Bergmann's rule), but also an increase in relative heart weight with increasing elevation (Hesse's rule). Location The Bohemian Forest, south‐eastern Germany....