Macro-nutritional balancing in a circumpolar boreal ruminant under winter conditions

1. Differences in botanical diet compositions among a large number of moose faecal samples collected during winter correlated with the nutritional differences identified in the same samples (Mantel r = 0.89, p = 0.001), but the nutritional differences were significantly smaller (p < 0.001).2. Nutritional geometry revealed that moose mixed Scots pine Pinus sylvestris and Vaccinium spp. as nutritionally complementary foods to reach a nutritional target resembling Salix spp. twigs, and selected for Salix spp. browse (Jacob's D > 0).3. Available protein (AP) and total non-structural carbohydrates (TNC) were significantly correlated in observed diets but not in hypothetical diets based on food availability.4. The level of Acetoacetate in moose serum (i.e. "starvation') was weakly negatively associated with digestibility of diets (p = 0.08) and unrelated to increasing AP:TNC and AP:NDF ratios in diets (p > 0.1).5. Our study is the first to demonstrate complementary feeding in free-ranging moose to attain a nutritional target that has previously been suggested in a feeding trial with captive moose. Our results add support to the hypothesis of nutritional balancing as a driver in the nutritional strategy of moose with implications for both the management of moose and food resources.