From summer growth to winter decline: brain size, captive effect, and cognitive outcomes in the common shrew during Dehnel's phenomenon

Through my PhD, I investigated a remarkable phenomenon of brain plasticity in the common shrew, focusing on how its brain adapts structurally and functionally to seasonal changes. The common shrew undergoes what is known as Dehnel’s phenomenon, a seasonal adaptation where brain and body sizes decrease significantly in winter before regrowing in spring. This adaptation likely serves to reduce metabolic demands when food resources are scarce, allowing the shrew to maintain essential physiological functions while conserving energy. Our findings reveal that from summer to winter, the common shrew undergoes an adaptive reduction in brain cell size, not through cell loss but through decreased cell volume. This microstructural change, marked by a decrease in intracellular water and an increase in extracellular water, might enable the brain to conserve energy when resources are scarce, making it more metabolically efficient during challenging winter conditions. I further explored how associative learning and activity levels differ between captive and wild shrews across seasons. In summer, wild-caught shrews demonstrate stronger associative learning abilities, but winter seems to slow cognitive processing. Additionally, activity levels vary with both season and captivity status, indicating that environmental factors strongly influence behavior. Lastly, i discovered that despite a reduction in overall brain volume, shrews retain essential spatial navigation skills in winter, prioritizing survival-related functions such as environmental navigation. By linking structural changes in the brain to functional outcomes, these studies demonstrate how shrews optimize cognitive abilities to adapt to seasonal environmental challenges. This work advances our understanding of brain plasticity and highlights the delicate balance between brain structure, energy efficiency, and cognitive functionality. It underscores how animals adapt their brains for immediate survival and the diverse challenges posed by their environments across different seasons.