Fat and protein reserves of wintering and prebreeding rock ptarmigan from south Hudson Bay

The proximate body composition and muscle and organ composition of rock ptarmigan (Lagopus mutus rupestris) were measured in birds collected during late winter at Winisk, Ontario, and just prior to breeding after their migration to southern Baffin Island. The neutral fat reserve was less than 4% of the body weight in male and female ptarmigan from both locations. This small lipid-energy reserve, coupled with the catabolism of labile protein, would not maintain the existence-energy requirements of rock ptarmigan for longer than 2 days during fasting winter conditions.The ovary weight and liver weight were not correlated with the neutral fat load of prebreeding females, but hypertrophy of the liver was apparent. In the absence of food the small amount of neutral fat and labile protein could help form only a fraction of the normal clutch size, but the possible role of these reserves in buffering short-term dietary shortages during egg formation is outlined. The small size of the energy reserves emphasizes th...