In a groundbreaking study that pushes the boundaries of our understanding of mammalian adaptation, researchers at the University of North Carolina Greensboro have unveiled remarkable insights into the seasonal plasticity of the masked shrew (Sorex cinereus). This diminutive, insectivorous mammal, native to the Appalachian Mountains and widespread across northern North America, exhibits an extraordinary survival mechanism: it physically shrinks its body and braincase during the harsh winter months to conserve energy. Published in the May 2025 issue of The American Naturalist, this research not only confirms the presence of Dehnel’s phenomenon in a southernmost population but also sheds light on its underlying biological processes through detailed morphometric and microCT investigations.
The phenomenon, named after the Polish biologist August Dehnel who first documented it in the early 20th century, revolves around the intriguing concept of phenotypic plasticity—where organisms dynamically alter their physical form in response to environmental pressures. In the case of the masked shrew, this plasticity manifests as a reversible reduction in body mass and skeletal dimensions, effectively making the animal smaller during colder seasons and allowing it to survive periods of limited food availability and lower temperatures. Dr. Bryan McLean and his team at UNCG meticulously analyzed 125 individual shrews collected over three years, employing state-of-the-art micro-computed tomography (microCT) scanning techniques to capture detailed images of the animal’s morphology beyond what traditional methods have revealed.
A key aspect of this study is the quantification of shrinkage not only in overall body mass but also in specific skeletal elements, including the braincase height and femur length. While body mass is an easily observable metric, the dynamic remodeling of skeletal structure suggests a far more complex physiological adaptation than previously appreciated. The femoral measurements, particularly, mark a novel contribution to the literature, demonstrating for the first time that long bones undergo considerable seasonal change, which may reflect alterations in locomotion or metabolic demands. These observations challenge the assumption that skeletal morphology remains largely static in adult mammals and open new avenues for exploring the mechanistic bases of such plasticity.
The methodology involved capturing shrews using pitfall traps strategically embedded in leaf litter within Pisgah National Forest, North Carolina. Each captured specimen was weighed promptly in the field before being transported to the Joint School of Nanoscience and Nanotechnology at UNCG for high-resolution microCT scanning. This approach allowed for non-destructive, three-dimensional imaging of the skeletal structure, providing unprecedented accuracy in measuring subtle yet significant changes in bone dimensions across seasons. Importantly, all specimens and associated datasets have been archived in the UNCG Mammal Collection, ensuring that this resource will facilitate future comparative studies on mammalian phenotypic plasticity.
Beyond the empirical data collected from the North Carolina population, Dr. McLean’s team synthesized findings from 74 additional studies spanning the Northern Hemisphere to contextualize their results and construct predictive models. This extensive meta-analysis revealed a robust correlation between the magnitude of seasonal shrinkage and climatic variables, particularly minimum temperatures during the cool seasons. Their models accurately predicted the degree of body and braincase diminution observed in the southern population solely based on local climate data. These findings underscore the universality of Dehnel’s phenomenon among Sorex shrews and reinforce the hypothesis that extreme environmental conditions serve as a selective pressure driving this adaptation.
The implications of this research extend well beyond the masked shrew itself, highlighting phenotypic plasticity as a fundamental survival strategy for small mammals confronted with fluctuating and often harsh environments. Unlike hibernation, which involves profound metabolic suppression and dormancy, the physical shrinking observed in these shrews represents an active remodeling of anatomy that optimizes energy expenditure during resource-scarce periods. This strategy may be more flexible and adaptive, allowing for rapid responses to unexpected environmental changes, a factor especially relevant in the context of global climate variability.
From an evolutionary perspective, the study’s findings provoke questions about the genetic and cellular mechanisms enabling such remarkable morphological changes. It remains unclear how skeletal tissues—that are generally considered rigid and fixed in adult organisms—can undergo cyclical reductions and regrowth. Hypotheses include alterations in bone resorption and formation rates mediated by hormonal triggers linked to photoperiod and temperature cues. Investigating these pathways could unveil conserved molecular circuits governing phenotypic plasticity and inspire biomedical advances in understanding tissue regeneration and remodeling.
Furthermore, the discovery of seasonal femur length fluctuation potentially signals a broader impact on the shrew’s biomechanics and survival strategies. Shorter limb bones during winter may reduce energy costs during movement or alter the animal’s locomotor agility, balancing the trade-offs between mobility and conservation. The extent to which these skeletal changes influence behavior and ecological interactions remains an exciting frontier for future research. Integration of biomechanical analyses and field observations could clarify the adaptive significance of this plasticity beyond mere size reduction.
It is notable that prior research on Dehnel’s phenomenon has predominantly focused on European populations of shrews, rendering this study the first comprehensive examination of its occurrence in a North American context, particularly at a southern latitudinal range. Such geographic breadth permits the exploration of how regional climate variations modulate biological responses and adaption strategies. It also raises questions about the potential impacts of climate change on these finely tuned phenotypic shifts. Understanding whether the frequency or magnitude of shrinkage might alter in response to rising temperatures will be critical for forecasting species resilience.
This research contributes significantly to the broader discourse on how mammals cope with environmental uncertainty. As global climate instability accelerates, phenotypic plasticity may become a pivotal factor determining species survival, especially for small-bodied mammals with high metabolic rates like shrews. The demonstration that cool-season temperatures effectively predict the degree of morphological adaptation illuminates potential biomarkers for monitoring population health and responses to ongoing ecological disruptions.
Finally, Dr. McLean emphasizes the collaborative nature of this work, which involved both graduate and undergraduate students, fostering new generations of scientists trained in cutting-edge imaging technologies and ecological modeling. The partnership with researchers from Georgia Southern University further enriched the study’s scope and depth. By combining detailed anatomical analyses with broad-scale climate data synthesis, this research exemplifies the power of interdisciplinary approaches to uncover hidden complexities within natural systems.
In sum, the investigation into seasonal body size plasticity and Dehnel’s phenomenon in Sorex shrews not only expands our understanding of mammalian adaptation mechanisms but also offers a compelling narrative of resilience in the face of environmental extremes. Through innovative microCT imaging and comprehensive climatic modeling, this study opens new pathways for exploring how small mammals endure the relentless challenges of seasonality and climate change, with broader implications for conservation biology and evolutionary physiology.
Subject of Research: Animals
Article Title: Seasonal Body Size Plasticity and the Generality of Dehnel’s Phenomenon in Sorex Shrews
News Publication Date: 27-Mar-2025
Web References: http://dx.doi.org/10.1086/735018
Image Credits: Sean Norona, UNCG Photographer
Keywords: Dehnel’s phenomenon, phenotypic plasticity, Sorex cinereus, masked shrew, seasonal body size change, microCT imaging, skeletal remodeling, climate adaptation, Appalachian mammals, energy conservation, bone resorption, evolutionary physiology
