Some small mammals exhibit Dehnel’s Phenomenon, a drastic decrease in body mass, braincase and brain size from summer to winter, followed by regrowth in spring. This is accompanied by a reorganization of the brain and changes in other organs. The evolutionary link between these changes and seasonality remains unclear, although the intensity of change varies between locations as the phenomenon is thought to lead to energy savings during winter.

Here we explored geographic variation of the intensity of Dehnel’s Phenomenon in Sorex araneus. We compiled literature on seasonal changes in braincase size, brain and body mass, supplemented by our own data from Poland, Germany, and the Czech Republic.

We analysed the effect of geographic and climate variables on the intensity of change and patterns of brain reorganization.

From summer to winter the braincase height decreased by 13%, followed by 10% regrowth in spring.

For body mass the changes were -21%/+82%, respectively. Changes increased towards northeast. Several climate variables were correlated with these transformations, confirming a link of the intensity of the changes with environmental conditions. This relationship differed for the decrease vs. regrowth, suggesting that they may have evolved under different selective pressures.

We found no geographic trends explaining variability in the brain mass changes although they were similar (-21%/+10%) to those of the braincase size. Underlying patterns of change in brain organisation in north-eastern Poland were almost identical to the pattern observed in southern Germany. This indicates that local habitat characteristics may play a more important role in determining brain structure than broad scale geographic conditions.

We discuss the techniques and criteria used for studying this phenomenon, as well as its potential presence in other taxa and the importance of distinguishing it from other kinds of seasonal variation.