Each winter, one of Europe’s smallest mammals pulls off a neurological trick that sounds like science fiction. The common shrew shrinks its skull and brain as temperatures drop, then rebuilds that lost tissue when food and warmth return. I see in this extreme cycle of loss and renewal a living experiment in brain resilience that researchers now hope could illuminate new ways to slow or even reverse conditions such as Alzheimer’s disease.
The animal’s seasonal brain remodeling is not a subtle tweak but a dramatic restructuring of organs, bones, and behavior. By watching how this tiny predator dismantles and then restores its own nervous system, scientists are beginning to map out cellular processes that keep neurons alive, flexible, and functional under stress, exactly the qualities that fail in neurodegenerative disease.
Meet the shrew that breaks the rules of mammal biology
The protagonist of this story is the Eurasian common shrew, Sorex araneus, a creature that weighs only a few grams yet burns through energy at a ferocious rate. With a life expectancy of barely a year, the Eurasian insectivore cannot afford to coast through winter, so it has evolved a strategy that defies what I was taught about fixed adult brains, shrinking its skull and nervous system to cut metabolic costs before regrowing them when conditions improve, as detailed for Sorex.
Field biologists have watched these animals ride out harsh seasons by literally changing head size, a phenomenon so counterintuitive that early reports were dismissed as measurement error. Behavioral ecologist Nov Dechmann, working at the Max Planck Institute of Anim, has been among those documenting how the shrew’s skull compresses and then expands again, with the brain following suit, a pattern that led her to describe it as “a crazy animal” when she and colleagues tracked the shrinking brain.
Dehnel’s phenomenon: a seasonal brain that bends but does not break
Researchers now group this reversible remodeling under the term Dehnel’s phenomenon, a label for small mammals that shrink and later regrow parts of their bodies in sync with the seasons. Common shrews are one of the clearest examples, showing a pronounced drop in brain volume as winter sets in, followed by partial restoration in spring, a pattern that has been tracked in wild populations for over 13 years and summarized as reversible brain shrinkage.
What makes this so striking to me is that the shrew does not simply lose mass and limp along. Instead, it appears to reconfigure its entire physiology around the new brain size, adjusting behavior and organ function to match. As winter approaches, some small mammals, including these insectivores, shrink their brains and other organs to survive, an extreme form of brain plasticity that has been described as a way to ride out cold months when food is scarce, with small mammals using this tactic to cut energy demands.
Water, not wasted neurons: what MRI is revealing inside shrew skulls
For years, one of the biggest questions was whether the shrew paid a permanent price for this seasonal downsizing, perhaps by killing off neurons that never truly return. Recent work from the Max Planck Institute of Animal Behavior in Germany has started to overturn that assumption. Postdoctoral researcher Cecilia Baldoni and colleagues used imaging and tissue analysis to show that overall brain shrinkage in these animals is driven largely by water loss rather than massive cell death, with the team reporting that “the cells lost water” when they examined brain cells from winter animals.
In parallel, scientists at the same institute have framed water as the key player behind Dehnel’s phenomenon, arguing that common shrews shrink their brains in winter not by shedding huge numbers of neurons but by changing how water is distributed inside and outside those cells. That work, which highlights water as the main factor involved in the phenomenon, suggests that the animals are using a reversible shift in cellular hydration to compress and later re-expand brain tissue, a mechanism summarized in a report that singles out water as central.
From wild MRI scans to human disease models
To move beyond snapshots of skulls and dissected brains, researchers have turned to non invasive imaging that lets them watch the same animals over time. Using MRI, scientists have followed individual common shrews as their brains shrink heading into winter and then regrow with the return of longer days, mapping which regions change the most and how quickly they rebound. This approach, which relies on repeated MRI scans of live animals, has been highlighted as a way to link structural changes to behavior and to identify molecular pathways that might be harnessed for human diseases.
In one summary of this work, the project is described as using MRI to study shrews that shrink and regrow their brains, with the explicit goal of offering clues for new treatments for neurodegenerative conditions. The same research has been shared in public forums that emphasize how these animals might inform therapies for human brain disorders, with social media posts noting that shrews that shrink and regrow brains could point toward new treatments.
Why a shape shifting shrew matters for Alzheimer’s
The leap from a wild insectivore to a human patient with Alzheimer’s is large, but the logic behind the comparison is straightforward. In Alzheimer’s disease, neurons gradually lose their connections and die, and the adult human brain shows limited capacity to replace that lost tissue. By contrast, the common shrew appears to tolerate repeated cycles of structural loss and regrowth without catastrophic cognitive collapse, a resilience that has led some researchers to frame this animal as a potential model for understanding how to protect or rebuild neural circuits, a link that has been highlighted in coverage of shrews as a pathway for treating human diseases such as Alzheimer.
Scientists studying these animals have been explicit about that ambition. Reports on the common shrew’s seasonal brain cycle describe how this tiny mammal regrows its brain in winter and suggest that the underlying mechanisms could help cure Alzheimer’s, positioning the species as a surprising ally in the search for new therapies. One overview notes that scientists found that this tiny mammal regrows its brain and directly connects that discovery to the hunt for Alzheimer’s cures, presenting the shrew as a bridge between animal behavior and human neurodegeneration in a way that has been picked up by Scientists focused on dementia.
