I love squirrels, seeing them often in my garden, and was fascinated to hear recently that their brains may grow in autumn and shrink again in spring.
The reason for this might be that the task of storing a winter food supply in a way that's easy to find again takes a lot of mental effort, to organise and memorise the cache locations.
Other mammals adapt for winter in significant ways; stoats and mountain hare change the colour of their coats; hedgehogs and bears hibernate. There is also evidence that shrews shrink both body and brain, to survive with shrunken resources.
So the idea about squirrels is plausible, and it's supported by research findings, but of course it's hard to be sure and there is still debate.
There's a good article here about a leading researcher and her liking and fascination with these clever little beasts.
Studies of squirrel brain regeneration may reveal clues about how to slow Alzheimer's disease, because in at least one form, the cells squirrels seem to regenerate are the ones that sufferers appear to be losing? Could mental activity, such as doing puzzles, help? It's often been suggested and there is some evidence in favour, I recall. It's an alluring possibility but no more at this stage.