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Recent research by Dr. Bryan McLean, a biology professor at UNC Greensboro, alongside his team, sheds light on the unique survival adaptations of the masked shrew, a small, mole-like mammal native to the Appalachian Mountains. The study reveals that these creatures exhibit a remarkable ability to decrease both their body size and braincase volume during the winter months as a strategy to conserve energy.
Published in the May 2025 edition of The American Naturalist, the findings indicate that the masked shrew (Sorex cinereus) can reduce its body weight by as much as 13 percent during colder periods. Remarkably, as spring arrives and environmental conditions improve, the shrew increases in size once again. The research also identified seasonal variations in the height of the braincase and the length of the femur, key indicators of physical adaptation.
“This reduction in size is an innovative survival mechanism,” McLean remarks. “Understanding such strategies is crucial as mammals navigate the challenges of an ever-changing environment.”
This phenomenon, known as Dehnel’s phenomenon, has been documented in various mammals but is particularly prevalent among shrews, which are small insectivorous animals distinct from rodents. It exemplifies extreme “phenotypic plasticity,” allowing organisms to change their physical characteristics in response to different environmental factors.
“While we cannot ascertain how widespread Dehnel’s phenomenon is among mammals, it is certainly less common than other energy conservation methods such as hibernation,” McLean noted.
McLean and his research team, consisting of both graduate and undergraduate students, examined 125 masked shrews captured in North Carolina’s Pisgah National Forest between 2021 and 2023. Using specialized “pitfall traps” hidden beneath leaf litter, the researchers collected the shrews and weighed them in the field. The specimens were later taken to UNCG’s Joint School of Nanoscience and Nanotechnology for detailed microCT scans to analyze various skeletal measurements. All specimens and corresponding data have been archived in the UNCG Mammal Collection, with contributions from colleagues at Georgia Southern University.
“Our study represents the southernmost population of masked shrews analyzed for these attributes,” McLean stated. “The femur measurements we gathered are the first to document the extent of seasonal changes in these long bones, highlighting the rapid remodeling capabilities of shrews’ skeletons.”
Most prior research related to Dehnel’s phenomenon has focused on European populations. To provide broader context, the team performed a meta-analysis that included 74 studies from across the Northern Hemisphere, integrating their findings with existing data to pinpoint the factors influencing this phenomenon. McLean’s team crafted statistical models that successfully predicted the observed body size reductions based solely on the climatic conditions present at the study site in North Carolina.
“Our analysis illustrates the prevalence of Dehnel’s phenomenon among Sorex shrews,” McLean elaborates. “In various populations across three continents, we found that the most significant reductions in body mass and braincase height occur in regions experiencing the lowest temperatures during the cool season. This indicates a predictive relationship between fall and winter temperatures and the manifestation of Dehnel’s phenomenon in these animals.”
“Phenotypic plasticity plays a vital role in how shrews and numerous other species adapt to temperature fluctuations,” McLean emphasizes. “By advancing our understanding of these adaptive processes, we can gain insights into how mammals may cope with rapidly shifting climatic conditions.”
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