Dehnel's Phenomenon in Mammals

ABSTRACT Introduction Though Dehnel's phenomenon is intuitively fascinating, it remains poorly understood. It entails seasonal, summer‐to‐winter shrinking and then regrowth of many tissues and organs, including significant changes in the skull and brain in a small handful of species with year‐round activity and high metabolic rates. Aim The availability of new methods has led to renewed interest and resulting knowledge making a review timely. Methods We carried out a comprehensive review of the literature, resulting in a complete dataset for all currently known species showing Dehnel's phenomenon. Results Based on this review, both long‐term climate and short‐term environmental variables do not only drive patterns but also modify the extent of the phenomenon. New methods such as modern imaging, metabolomics, and genomics have given us a deeper understanding of Dehnel's phenomenon and open up avenues for understanding the evolution of this phenomenon and applied medical research. Discussion We conclude with recommendations about future directions of research, including a checklist for traits to identify species that might potentially show this phenomenon.