Skull measurements Sorex araneus were collected between 1953 and 2004 in Białowieża National Park, preserved in alcohol and stored in the zoological collection of the Mammal Research Institute, Polish Academy of Sciences, in Białowieża. The skull height, length and width (mm) of 502 S. araneus were measured using X-ray images of the alcohol-preserved specimens, as described by Lázaro et al. (2017). Climate parameters in Białowieża between 1952 and 2004 Mean temperatures (degrees Celsius), rainfall (mm), and the number of days with snow cover are routine measurements from the meteorological station in Białowieża (52.7070 N, 23.8479 E). We computed the soil moisture deficit from monthly temperatures and precipitation using the Watbug program (Willmott 1977). Lazáro, J., Dechmann, D.K.N., LaPoint, S., Wikelski, M., & Hertel, M. (2017). Profound reversible seasonal changes of individual skull size in a mammal. Current Biology, 27, R1089-R1107. Willmott, C. J. (1977). WATBUG: a FORTRAN IV algorithm for calculating the climatic water budget. CW Thornthwaite Associates Laboratory of Climatology, Publications in Climatology, 30, 1-55.

We assesed the impact of the changes in climate on the overall skull size (the proxy of the overall body size) and the seasonal changes of skull height (Dehnel’s phenomenon) in skulls of the common shrew, Sorex araneus, collected over 50 years in the Białowieża Forest, E Poland. Overall skull size decreased, along with increasing temperatures and decreasing soil moisture, which determined the availability of the shrews’ main food source, earthworms. The magnitude of Dehnel’s phenomenon increased over time, indicating an increasing selection pressure on animals in winter. Two files include the data on 1) the size of the skulls of Sorex araneus collected in the Białowieża Forest between 1953 and 2004; and 2) the meteorological data from Białowieża, from 1952 to 2004.