An animal’s brain structure and size arise from developmental mechanisms that are shaped by socioecological adaptations and life history. 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. But maintenance and function of brain tissue require large amounts of energy, using up a substantial proportion of individuals’ metabolic budget. Therefore, 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.

Habitat seasonality provides predictable fluctuations in the environment and resource availability, which impact energy budget allocation. Consequently, seasonal changes in physiology and behavior of animals are often observed.

The best known examples of seasonal brain plasticity are found in some songbirds, where the size of song control nuclei changes in anticipation to the breeding season. For example, in canaries the volume of the HVC increases by 50% from fall to spring, and the robust nucleus of the arcopallium (RA) increases by 43%, leading to a change in overall brain mass of 13%. In food hoarding black-capped chickadees, the hippocampus decreases by 17% during the non-hoarding season, when the spatial cognitive demands of food caching are reduced. Similar processes can also be found in food-hoarding mammals. The males of Richardson’s ground squirrel store food before hibernation, and hippocampal size increases by 15% during that period. Similarly, hippocampus volume increases during the caching period in gray squirrels and chipmunks. Notably, during hibernation ground squirrels exhibit a reversible decrease in hippocampal dendrite arbors, as well as in the number and size of dendritic spines.

The brains of some species of red-toothed shrews (Sorex spp.) decrease in mass from summer to winter by 20% or more, followed by regrowth of ca. 15%.

The change in overall brain size dramatically affects brain architecture across seasons; in a Russian population of shrews, neocortex and hippocampus show the most profound winter decrease compared to other brain regions, while other regions remain stable in size or grow in the spring.