New research has discovered the genes behind Dehnel’s phenomenon, which refers to the ability of shrews and other animals to increase and decrease their brain size seasonally.
To survive seasonal food shortages in winter, the common shrew performs a biological trick that sounds like science fiction. This small mammal reduces the size of its brain by up to 30%, conserving energy when resources are scarce, but when spring arrives, the brain returns to its original size, with neurons fully restored and without apparent cognitive damage.
Scientists have discovered new clues about how this remarkable process evolved and what genes make it possible. Known as Dehnel phenomenon, it was first described by Polish zoologist August Dehnel in the mid-20th century. It is extremely rare among mammals, but is not exclusive to shrews, with moles, weasels and European stoats also seasonally reducing their brain size.
What these animals have in common is a accelerated metabolism and the fact that they do not hibernate, which forces them to find alternative ways to reduce energy expenditure during winter. In a new paper published in Molecular Biology and Evolution, ecologist William Thomas of Stony Brook University and his colleagues mapped the complete genome of the common shrew and compared it with that of other mammals that exhibit Dehnel’s phenomenon. The work builds on the team’s previous research, which examined seasonal changes in gene activity in different regions of the shrew’s brain.
By combining genetic and gene expression data, researchers have identified a gene set linked to the production of brain cells that have been consistently shown to be more active in species capable of reversible brain shrinkage. In the common shrew, one gene in particular—the VEGFAassociated with the permeability of the blood-brain barrier — showed increased activity. This can help the brain better detect and manage nutrients during times of scarcity.
The shrew genome also showed genes involved in DNA repair and longevity, suggesting intrinsic protective mechanisms against damage during repeated cycles of shrinkage and growth. Meanwhile, increased activity in water regulation genes reinforces the idea that changes in brain volume occur mainly through water lossand not by the permanent loss of brain cells.
Taken together, the findings point to what researchers describe as a “finely tuned system” that allows brain size to fluctuate without triggering neurodegeneration. The work highlights potential biomarkers and therapeutic targets for neurodegenerative diseases, although caution is still needed when extrapolating results obtained in shrews to humans.
