The genus name Lagopus means hare-foot, and the reason is visible on any winter bird. Dense feathers cover the tarsus and continue over the toes to the claws, something no other bird group does to the same degree. The covering grows longer and denser as autumn advances and is reduced again in spring.
Two functions overlap. The feathering increases the effective surface area of the foot, so the bird sinks less into powder snow, and it insulates tissue that would otherwise be pressed against ice for months. Ptarmigan also burrow into snow to roost, using the snowpack itself as shelter from wind, and their winter plumage turns white for camouflage against predators such as gyrfalcons and arctic foxes.
The seasonal change is often described as one moult. It is not; rock ptarmigan go through a sequence of moults across the year, and males and females do not switch on the same schedule.