Seasonal changes in body composition of mature female caribou and calves (Rangifer tarandus groenlandicus) on an arctic island with limited winter resources

Abstract

Twelve collections of mature female caribou and calves (Rangifer tarandus groenlandicus) were conducted between June 1982 and June 1984 on Coats Island, Northwest Territories, Canada, to study seasonal changes in body composition in this winter mortality limited population. Mature females depleted reserves of dissectible fat and muscle considerably during both winters of the study, particularly the second, when nearly all dissectible fat and 32% of estimated fall muscle mass were lost. Recovery of fat and muscle was rapid during the two summers, because of good quality forage and little environmental disturbance. Lactation appeared to slow fattening in early summer 1983, but by October females achieved fatness similar to that in 1982, when a majority of females in summer and fall were nonlactating. Low rumen fill and consistently high fat and muscle levels in fall 1982 and 1983 suggested that mature females then approached "set points" in body fat and muscle content. Calves grew rapidly in summer; most of this growth was lean tissue, and their losses of body fat and muscle were severe during winter. Mature females and calves increased rumen fill substantially over winter to compensate for highly fibrous food. This made total body weight a much poorer predictor of condition than carcass weight. The liver, kidneys, and empty rumen were heaviest in summer in response to high forage quality. Poor condition of females was associated with light fetuses in May 1984.