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Abstract
The seasonal changes in brain size of some shrews represent the most drastic reversible transformation in the mammalian central nervous system known to date. Brain mass decreases 10–26% from summer to winter and regrows 9–16% in spring, but the underlying structural changes at the cellular level are not yet understood. Here, we describe the volumetric differences in brain structures between seasons and sexes of the common shrew (Sorex araneus) in detail, confirming that changes in different brain regions vary in the magnitude of change. Notably, shrews show a decrease in hypothalamus, thalamus, and hippocampal volume and later regrowth in spring, whereas neocortex and striatum volumes decrease in winter and do not recover in size. For some regions, males and females showed different patterns of seasonal change from each other. We also analyzed the underlying changes in neuron morphology. We observed a general decrease in soma size and total dendrite volume in the caudoputamen and anterior cingulate cortex. This neuronal retraction may partially explain the overall tissue shrinkage in winter. While not sufficient to explain the entire seasonal process, it represents a first step toward understanding the mechanisms beneath this remarkable phenomenon.
Highlights
An animal’s brain structure and size arise from developmental mechanisms that are shaped by socioecological adaptations and life history (Striedter 2005)
Our results confirm the seasonal pattern of change in the volume of overall brain hemispheres of red-toothed shrews in Southern Germany that was previously reported from Russia and Poland (Bielak and Pucek 1960; Yaskin 1994)
We found a decrease of 16.1% from summer juveniles to winter subadults and a subsequent increase by 9.8% in spring-summer adults
Summary
An animal’s brain structure and size arise from developmental mechanisms that are shaped by socioecological adaptations and life history (Striedter 2005). The brain functions related to behavior and cognitive processes, in particular, provide individuals with the capacity to adapt to environmental changes over the life span. The actual size of an animal’s brain and each of its constituent parts is likely the result of a trade-off between the advantages of higher computational capacity and the costs of energetic demands. Of particular interest for understanding these adaptive and energetic trade-offs are species exhibiting large variation of brain size between individuals or over the life span. Habitat seasonality provides predictable fluctuations in the environment and resource availability, which impact energy budget allocation. In canaries the volume of the HVC increases by 50% from fall to spring,
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