Nutrient intake and its possible drivers in free-ranging European brown bears (Ursus arctos arctos)

The dietary nutrient profile has metabolic significance and possibly contributes to species' foraging behavior. The brown bear (Ursus arctos) was used as a model species for which dietary ingredient and nutrient concentrations as well as nutrient ratios were determined annually, seasonally and per reproductive class. Brown bears had a vertebrate- and ant-dominated diet in spring and early summer and a berry-dominated diet in fall, which translated into protein-rich and carbohydrate-rich diets, respectively. Fiber concentrations appeared constant over time and averaged at 25% of dry matter intake. Dietary ingredient proportions differed between reproductive classes; however, these differences did not translate into a difference in dietary nutrient concentrations, suggesting that bears manage to maintain similar nutrient profiles with selection of different ingredients. In terms of nutrient ratios, the dietary protein to non-protein ratio, considered optimal at around 0.2 (on metabolizable energy basis), averaged around 0.2 in this study in fall and around 0.8 in spring and summer. We introduced the minimal non-fat to fat ratio necessary for efficient maintenance metabolism. This ratio varied across seasons but never fell beneath the theoretically estimated minimum to ensure metabolic efficiency. This population thus managed to ingest diets that never exerted a lack of glucogenic substrate, suggesting that metabolic efficiency may either be a driver of active diet selection or that natural resources available to bears did not constitute a constraint in this respect. Given the considerable proportion of fiber in the diet of brown bears, the relevance of this nutrient and its role in foraging behavior might be underestimated.

Automated summary

The nutrient composition in the diet of brown bears has metabolic significance and is related to their foraging behavior. The study found that brown bears have a diet dominated by vertebrates and ants in spring and early summer, and berries in fall, resulting in protein-rich and carbohydrate-rich diets, respectively. Although there were differences in dietary ingredient proportions between reproductive classes, the nutrient concentrations in the diet were similar, indicating that bears are able to maintain a consistent nutrient profile by selecting different ingredients. The study also showed that the bears' diets provided sufficient glucogenic substrate for efficient metabolism throughout the year, suggesting either a driving force of metabolic efficiency in diet selection or abundant natural resources available to the bears.