In the high-stakes environment of winter survival, the Eurasian common shrew employs a biological strategy that seems to defy the laws of mammalian anatomy. While most animals either migrate or enter a deep dormant state to survive food scarcity, this small rodent undergoes a dramatic physical transformation known as Dehnel’s phenomenon: it literally shrinks its body mass, skull, and internal organs to conserve energy, only to regrow them as spring arrives.
Researchers from Stony Brook University, in collaboration with institutes across Germany and Denmark, are now decoding the molecular blueprints of this process. By understanding how the shrew manages this seasonal regression and subsequent regeneration, scientists believe they may identify a pathway toward brain regeneration treatments for humans suffering from neurodegenerative diseases.
The research, published across two separate studies in Genome Research and Molecular Biology and Evolution, suggests that the shrew’s ability to reshape essential organs—including the brain—is not merely a curiosity of nature, but a sophisticated energy-saving mechanism. By reducing the amount of tissue that requires metabolic maintenance during the leanest months of the year, the shrew ensures its survival until resources replenish in the spring.
The Mechanics of Dehnel’s Phenomenon
Unlike hibernation, where an animal’s metabolic rate drops and they enter a state of torpor, shrews undergoing Dehnel’s phenomenon remain active. This is largely due to their exceptionally high metabolic rate; for a creature with a lifespan of approximately one year, spending a quarter of that life asleep would be an evolutionary disadvantage.
Liliana M. Dávalos, a professor in the Department of Ecology and Evolution and the principal investigator of both papers, describes the scale of this change as extreme. “If your head shrank by 25%, everyone around you would notice it right away,” Dávalos noted, highlighting the profound physiological shift the animals undergo.
To track this process, researchers captured wild shrews across five distinct developmental stages: juveniles before shrinkage, during the shrinkage phase, after shrinkage is complete, adults during the regrowth phase, and finally, adults after full regrowth. This allowed the team to observe the precise timing of metabolic and genetic shifts. Because common shrews are difficult to breed in laboratory settings, the team relied on field studies and carefully maintained captive specimens, a method supported by Jeremy Searle, a professor at Cornell University’s Department of Ecology and Evolutionary Biology.
The Metabolic Shift: Fueling the Shrinkage
The research reveals that the shrew’s body undergoes a complete metabolic overhaul to survive the winter. Analysis of blood samples and liver weights confirmed several key regulatory changes:
- Oxidative Phosphorylation: The shrews showed significant regulatory changes in the final step of cellular respiration, which produces adenosine triphosphate (ATP), the primary energy currency of the cell.
- Fatty Acid Metabolism: There was a marked increase in the breakdown of fatty acids to provide a steady stream of ATP.
- Gluconeogenesis: During autumn and early winter, the shrews exhibited their highest rates of producing glucose internally, a critical adaptation for surviving the starvation typical of winter months.
The Genetic Key: FOXO1 Signaling and Lifespan
At the center of this biological transformation is a protein known as Forkhead box protein O1, or FOXO1. This signaling pathway is responsible for regulating metabolism, energy balance, and overall body size. In the autumn, as the shrews begin to shrink, FOXO1 signaling peaks; as they regrow in the spring, this signaling drops.
William Thomas, the lead author of both studies and a former postdoctoral research associate at Stony Brook, suggests that this ability comes with a steep evolutionary price. The researchers hypothesize that the metabolic cost of regulating FOXO1 and undergoing such drastic physical changes may be what limits the shrew’s lifespan to just one year.
Thomas relates this to the concept of “terminal investment,” where an organism increases its reproductive effort as its chances of long-term survival decrease. The shrew shrinks to survive one winter, regrows to mate in the spring, and produces offspring, but the physiological toll ensures it will not survive to see a second winter.
| Feature | Hibernation | Dehnel’s Phenomenon |
|---|---|---|
| Activity Level | Dormant/Torpid | Remains Active |
| Physical Change | Minimal structural change | Shrinkage of skull and organs |
| Energy Strategy | Reduced metabolic rate | Reduced tissue maintenance |
| Primary Goal | Survive food scarcity | Survive food scarcity |
Implications for Human Neurodegeneration
The most provocative aspect of the study is the potential application of these findings to human medicine. Because the Eurasian common shrew is able to regenerate brain tissue during its spring regrowth, it provides a living model for neuroplasticity and regeneration that is largely absent in humans.
The team has developed a new, more contiguous chromosomal reference for the shrew’s genome, replacing a fragmented version from over a decade ago. This genomic map allows scientists to identify the specific proteins and genes expressed during the regrowth of the brain.
“Can we mimic what the shrews are doing to shrink and regrow their brains and facilitate humans [who are undergoing neurodegenerative processes] do the same thing?” Thomas asked. The goal is to eventually identify genes and proteins that could help humans regain neurological function as the brain decreases in size during the progression of Alzheimer’s disease or Huntington’s disease.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Please consult a healthcare professional for diagnosis and treatment of neurodegenerative disorders.
The next phase of this research involves utilizing the newly sequenced genome as a resource for the broader scientific community to explore how FOXO1 signaling affects lifespan and tissue regeneration across different species. As researchers continue to isolate the specific proteins responsible for the shrew’s regrowth, the hope is to transition these findings from evolutionary biology into clinical therapeutic targets.
We invite readers to share their thoughts on these findings in the comments below and share this story with others interested in the intersection of evolutionary biology and medical science.
