Seasonal variation in the thermal responses to changing environmental temperature in the world's northernmost land bird.

Arctic homeotherms counter challenges at high latitudes using a combination of seasonal adjustments in pelage/plumage, fat deposition and intricate thermoregulatory adaptations. However, there are still gaps in our understanding of their thermal responses to cold, particularly in Arctic birds. Here, we have studied the potential use of local heterothermy (i.e. tissue cooling that can contribute to significantly lower heat loss rate) in Svalbard ptarmigan ( Lagopus muta hyperborea ) - the world's northernmost land bird. We exposed birds kept under simulated Svalbard photoperiod to low ambient temperatures ( T a ; between 0 and -30°C) during three seasons (early winter, late winter, summer), whilst recording resting metabolic rate (RMR), core temperature ( T c ) and several cutaneous temperatures. Leg skin temperature varied the most, but still only by up to ∼15°C, whereas body trunk skin temperature changed <1°C when T a decreased from 0 to -30°C. At the same time, T c increased by 0.9°C, concomitant with increased RMR. This was probably driven by the triggering of cerebral thermosensors in response to cooling of the poorly insulated head, the skin of which was 5.4°C colder at -30°C than at 0°C. Thermal conductance in winter was higher in yearlings, probably because they were time/resource constrained from acquiring a high-quality plumage and sufficient fat reserves as a result of concomitant body growth. In conclusion, Svalbard ptarmigan do not employ extensive local heterothermy for cold protection but instead rely on efficient thermogenesis combined with excellent body insulation. Hence, cold defence in the world's northernmost land bird is not mechanistically much different from that of its lower latitude relatives.