Braincase shrinkage during winter was firstly described in shrews by Dehnel in 1949, and is known as the Dehnel’s Phenomenon. Recently, Dechmann et al. investigated the seasonal size variation in the skulls of shrews (Sorex araneus) and least weasels (Mustela nivalis). They measured skull length and braincase depth on specimens previously collected from a Polish National Park, sampling all seasons. Both species showed an initial juvenile growth until the first winter, followed by shrinkage until spring in the adults, and a subsequent re-grow on the second summer, though never reaching the initial size. Heat maps built from high resolution CT scans demonstrated that size changes also involved changes in shape and in bone thickness, with the thinnest skulls coinciding with the smallest braincase size. Interestingly, these patterns differed between sexes, especially in weasels as only males were observed to re-grow. Despite phylogenetically distant, both species have similar life histories, having short life spans and high metabolisms, and inhabiting an environment with seasonal fluctuation of resources availability. Winter shrinkage would reduce energetic requirements and prepare individuals for the harder conditions, and re-growth during the resources-abundant season would prepare the males for reproduction while females would allocate the energy into caring for the offspring. The authors conclude these seasonal reversible size changes are genetically fixed and exclusive of animals with such life histories, as an adaptation to extreme environmental conditions. Future investigation shall clarify the potential drivers and consequences of this phenomenon, including how the variation in size affects brain size and reorganization.