Winter energy balance and activity of female caribou on Coats Island, Northwest Territories: the relative importance of foraging and body reserves

Reduced availability of forage in winter is the dominant limiting factor for the isolated, predator-free caribou (Rangifer tarandus groenlandicus) population on Coats Island, Northwest Territories. Pregnant females in this population typically begin winter with large fat reserves but catabolize most of them by spring. We modelled net energy requirements of a pregnant female during two winters (1982 – 1983 and 1983 – 1984) to evaluate energy requirements for maintenance, activity, and pregnancy, and to estimate the contribution of body reserves in supplying these requirements. A secondary objective was to determine whether winter activity budgets were related to energy balance, quality of winter diets, or body condition. Estimated net energy requirements were lowest during midwinter (16 MJ/d for an average female) but increased rapidly toward spring. Maintenance was at all times the largest component of requirements, but gestation costs increased to 12 – 14% of total requirements by winter's end, and activity costs tended to increase from 3.8–4.0 MJ/d (23–25% of requirements) at the beginning of winter to 4.6–4.8 MJ/d (26 – 29% of costs) in late winter. Body reserves supplied an estimated 14.2% (1982 – 1983) and 19.2% (1983 – 1984) of overall winter requirements. The contribution from body reserves varied from 9 to 24% during different portions of the two winters and, in late winter 1982 – 1983, reserves may have been a crucial supplement at a time of decreased forage availability and rising energy costs. Caribou were least active in early winter, when they were fattest, and most active at winter's end when their energy needs were increasing. These patterns were not consistent with activity budgets of most mainland caribou, and winter activity budgets were not consistently related to either diet quality or body condition.