Animals representing a wide range of taxonomic groups are known to select specific food combinations to achieve a nutritionally balanced diet. The nutrient balancing hypothesis suggests that, when given the opportunity, animals select foods to achieve a particular target nutrient balance, and that balancing occurs between meals and between days. For wild ruminants who inhabit landscapes dominated by human land use, nutritionally imbalanced diets can result from ingesting agricultural crops rich in starch and sugar (non-structural carbohydrates, NC), which can be provided to them by people as supplementary feeds. Here, we test the nutrient balancing hypothesis by assessing potential effects that the ingestion of such crops by Alces alces (moose) may have on forage intake. We predicted that moose compensate for an imbalanced intake of excess NC by selecting tree forage with macro-nutritional content better suited for their rumen microbiome during wintertime. We applied DNA metabarcoding to identify plants in faecal and rumen content from the same moose during winter in Sweden. We found that the concentration of NC-rich crops in faeces predicted the presence of Picea abies (Norway spruce) in rumen samples. The finding is consistent with the prediction that moose use tree forage as a nutritionally complementary resource to balance their intake of NC-rich foods, and that they ingested P. abies in particular (normally a forage rarely eaten by moose) because it was the most readily available tree. Our finding sheds new light on the foraging behaviour of a model species in herbivore ecology, and on how habitat alterations by humans may change the behaviour of wildlife.

At northern latitudes, large spatial and temporal variation in the nutritional composition of available foods poses challenges to wild herbivores trying to satisfy their nutrient requirements. Studies conducted in mostly captive settings have shown that animals from a variety of taxonomic groups deal with this challenge by adjusting the amounts and proportions of available food combinations to achieve a target nutrient balance. In this study, we used proportions-based nutritional geometry to analyse the nutritional composition of rumen samples collected in winter from 481 moose (Alces alces) in southern Sweden and examine whether free-ranging moose show comparable patterns of nutrient balancing. Our main hypothesis was that wild moose actively regulate their rumen nutrient composition to offset ecologically imposed variation in the nutritional composition of available foods. To test this, we assessed the macronutritional composition (protein, carbohydrates, and lipids) of rumen contents and commonly eaten foods, including supplementary feed, across populations with contrasting winter diets, spanning an area of approximately 10 000 km2. Our results suggest that moose balanced the macronutrient composition of their rumen, with the rumen contents having consistently similar proportional relationship between protein and non-structural carbohydrates, despite differences in available (and eaten) foods. Furthermore, we found that rumen macronutrient balance was tightly related to ingested levels of dietary fiber (cellulose and hemicellulose), such that the greater the fiber content, the less protein was present in the rumen compared to non-structural carbohydrates. Our results also suggest that moose benefit from access to a greater variety of trees, shrubs, herbs and grasses, which provides them with a larger nutritional space to manoeuvre within. Our findings provide novel theoretical insights into a model species for ungulate nutritional ecology, while also generating data of direct relevance to wildlife and forest management, such as silvicultural or supplementary feeding practices.

The nutrient balancing hypothesis proposes that, when sufficient food is available, the primary goal of animal diet selection is to obtain a nutritionally balanced diet. This hypothesis can be tested using the Geometric Framework for nutrition (GF). The GF enables researchers to study patterns of nutrient intake (e.g. macronutrients; protein, carbohydrates, fat), interactions between the different nutrients, and how an animal resolves the potential conflict between over-eating one or more nutrients and under-eating others during periods of dietary imbalance. Using the moose (Alces alces L.), a model species in the development of herbivore foraging theory, we conducted a feeding experiment guided by the GF, combining continuous observations of six captive moose with analysis of the macronutritional composition of foods. We identified the moose’s self-selected macronutrient target by allowing them to compose a diet by mixing two nutritionally complementary pellet types plus limited access to Salix browse. Such periods of free choice were intermixed with periods when they were restricted to one of the two pellet types plus Salix browse. Our observations of food intake by moose given free choice lend support to the nutrient balancing hypothesis, as the moose combined the foods in specific proportions that provided a particular ratio and amount of macronutrients. When restricted to either of two diets comprising a single pellet type, the moose i) maintained a relatively stable intake of non-protein energy while allowing protein intakes to vary with food composition, and ii) increased their intake of the food item that most closely resembled the self-selected macronutrient intake from the free choice periods, namely Salix browse. We place our results in the context of the nutritional strategy of the moose, ruminant physiology and the categorization of food quality.