Adaptive functional morphology in the high-Arctic Svalbard rock ptarmigan (Lagopus muta hyperborea): Snowshoes minimise foot sinking depth during locomotion on snow

Svalbard rock ptarmigan ( Lagopus muta hyperborea ) are endemic to the high-Arctic archipelago of Svalbard, Norway, where climatic conditions can be extreme and the ground is snow-covered for much of the year. In preparation for Arctic winters, these birds double their body mass through large deposits of body fat that act as an emergency energy reserve. Despite this, Svalbard rock ptarmigan have a lower cost of locomotion in winter relative to their lighter summer mass. Additionally, they regrow thick feathers on the foot prior to winter, to which their genus Lagopus (translating literally to hare-foot) is reference. To investigate whether feathered feet may aid winter locomotion, we tested the effect of foot feathering on foot sink depth in snow across a range of snow stiffnesses. We used a force rig which pressed feathered feet into snow across varying degrees of snow hardness, to assess the relationship between foot sink depth and the stiffness of snow in the winter feet of Svalbard rock ptarmigan and its two southern-more relatives, willow ( Lagopus lagopus ) and mainland rock ptarmigan ( Lagopus muta ). Comparing foot surface area of feathered and unfeathered (taken via high-definition images), our findings indicate that the greater foot feathering of the Svalbard rock ptarmigan reduced foot sink depth in snow relative to the other ptarmigans. Specifically, the foot load of feathered Svalbard rock ptarmigan feet was significantly lower than that of the other (sub)species, indicating that the rich regrowth of feathers is a primary driver of the reduced foot load in the Svalbard rock ptarmigan. Given foot load is a strong predictor of the metabolic cost of locomotion in snow, the feathered winter feet are suggested to be a biomechanical energy-saving adaptation of the Svalbard ptarmigan, alongside their thermal insulating properties.