The Brain That Shrinks and Regrows: Seasonal Plasticity in Shrews
Lázaro J, Hertel M, Sherwood CC, Muturi M, Dechmann DKN. Profound seasonal changes in brain size and architecture in the common shrew. Brain Struct Funct 2018;223(6):2823-2840. doi: 10.1007/s00429-018-1666-5.
Background: The common shrew (Sorex araneus) exhibits one of the most extreme reversible reductions in brain size among mammals, shrinking its brain mass by up to 26% from summer to winter and regrowing it in spring. This phenomenon, known as Dehnel’s phenomenon, has been linked to adaptations for energy conservation during periods of resource scarcity. However, the structural and cellular mechanisms underlying these seasonal brain size fluctuations remained unclear before this study.
Hypothesis: This study hypothesized that the seasonal changes in overall brain size of the common shrew are accompanied by differential volumetric and cellular alterations across brain regions, driven by adaptive functional and energetic demands rather than changes in cell number.
Methods: The authors trapped shrews across three seasonal age groups in southern Germany, processed their brains histologically and reconstructed the volumes of multiple brain regions using Neurolucida and quantified them with Neurolucida Explorer. Golgi-stained neurons were traced to assess soma and dendritic morphology in the anterior cingulate and somatosensory cortices and the caudoputamen. Statistical analyses compared volumetric and neuronal parameters across seasons and sexes.
Results: Total hemisphere volume decreased by 16.1% from summer to winter and regrew by 9.8% in spring. The hypothalamus and thalamus showed the largest reversible changes, while the neocortex and striatum shrank without regrowth. The hippocampus and olfactory bulb exhibited partial recovery, and several regions showed sex-specific differences. Neuronal analyses revealed reduced soma size and dendritic volume, particularly in the caudoputamen and anterior cingulate cortex, consistent with neuronal retraction.
Conclusions: The study concluded that profound and region-specific seasonal brain plasticity in shrews is linked to structural remodeling of neurons rather than changes in cell number. These reversible changes likely reflect adaptive responses to seasonal energy constraints and shifting ecological demands.
