Squirrels’ brains grow 15% larger in the autumn to remember where they buried up to 10,000 nuts (they have an 80%+ success rate finding them) and smaller in the summer since they don’t need to remember so much.
Tree squirrels have larger brains than ground squirrels because they have to navigate a 3D environment, jumping from tree to tree, as opposed to a 2D environment of staying on the ground.