Dynamics of body protein and the implications for reproduction in captive muskoxen (Ovibos moschatus) during winter.

Muskoxen are considered to be obligate capital breeders because they rely exclusively on endogenous stores to reproduce. We studied 14 captive female muskoxen (not pregnant, n = 9; pregnant, n = 5) in February-June 2007 to assess changes in body composition and isotopic correlates of protein status (proportions of amino acid [p-AN] and urea N [p-UN] derived from body N). We measured body mass, body composition, and N metabolites in blood and urine between midgestation in February and early lactation (postcalving). All muskoxen lost body mass (-6% to -12%) and fat (-22% to -24%) over the winter, and pregnant muskoxen lost body protein (-6%) in late gestation. Nonpregnant animals maintained stores of body protein (+6%) in late winter. Losses of body protein in pregnant muskoxen (255 +/- 71.5 mg protein kg(-0.75) d(-1)) were similar to the amount of protein deposited in reproductive tissues (319 +/- 33.4). Plasma urea concentration increased (27-59 mg dL(-1)) with p-UN (0.13-0.33), which indicated oxidation of amino N during late winter. High estimates of p-AN (0.72 +/- 0.07) indicated that amino N from body protein was reutilized in late winter. Muskoxen conserve the capital of body protein stores for reproductive investment while using income of dietary protein for maintenance functions. We conclude that variation in protein supplies from body stores and the diet explain a large part of the variation in productivity of Arctic ungulates.