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.

Fecal samples were collected from shot moose during the winter 2014/15 in six moose management areas in southern Sweden. DNA was extracted from fecal samples, and then we carried out PCR and Illumina sequencing to identify plant species present. The data set includes proportional data (relative read abundances, RRA) of the 128 identified taxa (molecular operational taxonomic units, MOTU). We also provide metadata for the samples, such as place of origin (moose management area, MMA), moose management unit (MMU), as well as sex and age of the moose individual. Age was determined by analysis of tooth cemental layers. Data included in this data file represent fecal samples in Category 1 and 2 as described in the article. Category 1 are individuals without any traces of any crops rich in non-structural carbohydrates (NCC) in either rumen or faecal samples; Category 2 are individuals with identified NCC in faecal samples.