In the natural world, the capacity for brain plasticity—the ability to remodel and regenerate neural tissue—is rarely observed at the scale seen in the common shrew (Sorex araneus). These diminutive mammals exhibit a remarkable biological phenomenon known as Dehnel’s phenomenon, characterized by a seasonal, reversible shrinking and regrowth of their brains. While this rare process has been documented for decades, the underlying mechanisms enabling such dramatic brain volume fluctuations without permanent damage have long remained an enigma for neuroscientists and ecologists alike.
Recent research employing advanced non-invasive magnetic resonance imaging (MRI) techniques has illuminated this mystery, revealing a crucial role of water regulation within shrew brain cells. The study, conducted by a collaborative team of researchers mostly affiliated with the Max Planck Institute of Animal Behavior and published in Current Biology, presents groundbreaking evidence that the brains of common shrews lose approximately nine percent of their volume during the winter months, not due to cell death as would be expected in humans, but rather as a result of controlled water expulsion from brain cells.
This discovery disrupts long-held assumptions on brain tissue shrinkage. Typically, cellular dehydration leads to irreversible damage and cell death; however, in common shrews, brain cells survive and even increase in number during the shrinking phase. Dr. Cecilia Baldoni, the study’s first author, emphasizes the uniqueness of this process: “The cells lost water but remained alive, which is a stunning departure from what we observe in pathological brain volume loss in humans.”
Water homeostasis in the brain is tightly regulated, with aquaporin proteins playing a pivotal role. The protein aquaporin 4, abundant in the brain’s astrocyte cells, is widely recognized for facilitating bidirectional water transport across cell membranes. In shrews, aquaporin 4 levels appear elevated during brain shrinkage, suggesting an active role in orchestrating the removal of intracellular water to reduce brain volume seasonally. This molecular mechanism starkly parallels conditions found in diseased human brains, such as those affected by Alzheimer’s or Parkinson’s diseases, where aquaporin dysregulation and water imbalance contribute to neurodegeneration.
Understanding Dehnel’s phenomenon thus not only answers fundamental biological questions but also potentially charts a novel path toward therapeutic strategies for human neurodegenerative diseases. As Associate Professor John Nieland of Aalborg University explains, “Our findings illustrate that shrews experience similar brain volume reductions as patients with neurodegenerative diseases—but crucially, shrews possess innate biological machinery to reverse this loss and restore brain tissue.” This suggests that studying shrews could unlock the regenerative secrets currently missing from human medicine.
The research meticulously compared brain scans across seasons, capturing live common shrews during summer and then recapturing the same individuals in winter. This longitudinal, within-subject design allowed for direct observation of brain plasticity in action. The MRI data was complemented by microscopic analyses of brain tissue, which confirmed the remarkable preservation and even proliferation of neural cells, despite significant shrinkage.
One intriguing aspect uncovered is the regional specificity of brain shrinkage. Not all areas of the brain contract equally—regions such as the neocortex and cerebellum, critical for cognitive functions and motor control, maintain stable water balances and volume, while other parts shrink substantially. This selective preservation ensures that vital neurological capabilities remain intact, enabling shrews to continue complex behaviors like navigation and predator evasion through the resource-scarce winter months.
This precise modulation of brain volume is analogous to adjusting power usage within a house, where heating is sustained in essential rooms while less critical areas are conserved. The selective strategy preserves key functions while achieving metabolic economy, offering new insights into how brain plasticity aligns with ecological demands.
The ecological implications of Dehnel’s phenomenon are profound. Shrews have extremely high metabolic rates and must consume food frequently to survive, regardless of seasonal fluctuations. By shrinking their brains, they reduce their energetic burden during winter scarcity, providing a striking example of energy trade-offs in mammalian physiology.
This discovery raises compelling questions about the neurobehavioral effects of brain shrinkage. Does a smaller brain impede the shrew’s cognitive or navigational abilities? Can shrews compensate behaviorally for lost neural volume? Ongoing research aims to unpack these functional consequences and understand how shrews maintain performance despite seasonal anatomical changes.
From a neurological standpoint, the possibility of replicating or harnessing similar mechanisms in humans holds transformative potential. Many debilitating brain diseases involve irreversible loss of neurons and brain volume, exacerbated by water imbalance and cellular death. Unlocking the shrew’s biological blueprint for fluid brain volume modulation and regeneration could inspire new classes of therapies to halt or reverse neurodegeneration.
The next frontier in this research trajectory focuses on the regrowth phase, where shrew brains restore volume from late winter into spring. Detailed molecular and cellular investigations during this regenerative window may reveal triggers and gene expression pathways capable of stimulating neuronal proliferation and tissue repair—critical knowledge for developing regenerative medicine.
The implications extend beyond medical applications; uncovering how mammalian brains can safely undergo cyclical shrinkage and regrowth may redefine our understanding of brain plasticity’s limits and inform conservation biology for species facing environmental challenges impacting resource availability.
This study exemplifies the power of interdisciplinary research, combining ecological fieldwork, cutting-edge imaging technology, and molecular biology to unravel complex physiological processes. By bridging animal biology and human medicine, it offers a rare glimpse into nature’s solutions for brain health and resilience.
In summary, the common shrew’s seasonal brain shrinkage via regulated water loss—and absence of cell death—reveals a heretofore hidden dimension of neural plasticity. These small mammals demonstrate a unique physiological adaptation with significant ramifications for neuroscience and biomedical research, illuminating new paradigms on brain regeneration and disease intervention strategies. As investigations progress, the humble shrew may well unlock transformative pathways for treating currently incurable human brain disorders.
Subject of Research: Animals
Article Title: Programmed seasonal brain shrinkage in the common shrew via water loss without cell death
News Publication Date: 1-Sep-2025
Web References: https://www.cell.com/current-biology/fulltext/S0960-9822(25)01081-4
References: DOI: 10.1016/j.cub.2025.08.015
Image Credits: Christian Ziegler / Max Planck Institute of Animal Behavior
Keywords: Brain plasticity, Dehnel’s phenomenon, common shrew, MRI imaging, aquaporin 4, brain shrinkage, brain regeneration, neurodegenerative disease, water homeostasis, neuroimaging, seasonal adaptation, regenerative neuroscience
