Female caribou subsist primarily on lichens and some senescent browse during winter when demands for fetal growth add to costs of thermoregulation and mobility. Lichens, although potentially high in digestible energy, contain less protein than required for maintenance by most north-temperate ungulates. To understand the adaptations of caribou to the nutritional constraints of their primary food resource, we fed captive female caribou a sequence of 3 diets designed to resemble decreasing quality of forages during early, mid-, and late winter, respectively: high energy–high protein (HIGH), medium energy–low protein (MEDIUM), and low energy–low protein (LOW). In vitro digestibility of dry matter declined from 94% (HIGH) in November, to 66% (MEDIUM) in December and January, and to 53% (LOW) from February to April. Dietary protein averaged 19.8% in November and 4.3% from December to April. We used measures of body condition, stable isotopic signatures, and concentrations of nitrogen (N) metabolites to define protein dynamics in the animals. Subcutaneous rump fat declined between October and April from 2.3 cm ± 0.3 SE to <0.5 cm as intake of digestible energy declined from 44.0 ± 2.0 MJ/day to 16.3 ± 3.2 MJ/day. In erythrocytes, increasing enrichment of carbon (13C) throughout winter suggested that caribou reused body lipids, and increases in 15N during January and February indicated that they also recycled amino-N. Urinary N was primarily urea with an isotopic signature that tracked dietary 15N through late winter. Plasma urea-N declined from 44.0 ± 2.6 mg/dl to 8.5 ± 1.2 mg/dl as nitrogen intake declined from 91.5 ± 5.3 g N/day to 14.1 ± 0.9 g N/day. Examination of these data suggests that caribou catabolized dietary C and N in preference to endogenous fat reserves and body protein. Female caribou appear to tolerate low intakes of protein and energy in winter by minimizing net loss of body protein and reapportioning body reserves to support fetal growth.
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