Much like women who change their shoes from boots in the winter, to sandals in the summer, Caribou hooves change with the season.
Picture from Tumblr
In the summer, when the tundra is soft and wet, the footpads become sponge-like and provide extra traction. In the winter, the pads shrink and tighten, exposing the rim of the hoof, which cuts into the ice and crusted snow to keep it from slipping. This also enables them to dig down through the snow to their favorite food.