Body composition of the George River caribou (Rangifer tarandus caribou) in fall and late winter

The fall and late winter body composition of George River caribou was determined based on a sample of 14 calves, 9 yearlings, and 41 adult females. In fall, the fattest individuals were the adult nonlactating females with 10.62 ± 2.65% ingesta-free body fat. Ingesta-free body fat was 5.25 ± 2.47% for lactating females, 7.76 ± 0.11% for yearlings, and 3.44 ± 0.57% for calves. There was no evidence of depletion of fat reserves between fall (October–December) and mid-April in any age-class, and no change in body weights of calves. However, the ingesta-free body weights of pregnant females in April were 19.8 and 9.5% less than those of nonlactating and lactating females in fall, respectively. These differences were due mainly to a loss of water (9.8 and 7.3 kg) and protein (4.6 and 2.6 kg). This protein loss of the ingesta-free body was not compensated for by growth of the gravid uterus, which accounted for 3.5% (0.53 kg) of the total protein mass of pregnant females in mid-April. These results suggest that, between fall and late winter, the George River caribou were affected more by a deficiency in protein than a negative energy budget and that increased attention should be given to monitor protein reserves. However, in summer and early fall, these caribou accumulated only limited fat reserves as compared with other northern ungulates and the possibility of an energy deficiency during summer should be investigated.