How absolute biomass intake can alter nutrient profile interpretation in free-ranging species: The case of protein intake in brown bears

If an animal eats a large amount of a diet of a low content of a
macronutrient, it can still ingest a considerable amount of that
macronutrient. Various animals have been shown to balance nutrient content
with intake in this way. We use the brown bear (Ursus arctos) as a model
species, given their recent classification as ‘low protein omnivores’, to
show how taking biomass intake into account can affect macronutrient
intake interpretation. We 1) calculated absolute protein intake from
published results of self-selection diet studies with bears in captivity
and 2) modelled absolute protein intake of Swedish brown bears in autumn
while binging on a berry diet of low protein concentrations. In feeding
experiments in captivity, the self-selected macronutrient composition of
brown bear diets are characterized by protein percentages (both on a dry
matter and metabolizable energy basis) that appear low when compared to
diets of carnivores. However, when taking into account absolute food
intake and expressing this as daily protein intake per metabolic body mass
(33-117 g/kg0.75/d), protein intake was considerably higher than
established minimum requirements for domestic dogs and cats (2.6-3.8
g/kg0.75/d) – carnivores one would not consider ‘low protein specialists’.
Our hypothetical berry model yielded a protein intake of 3.2-9.7
g/kg0.75/d, which is lower than the outcomes from the self-selection
trials but still comparable to established requirements of domestic dogs
and cats. Instead of perceiving bears as low-protein consumers, it might
be more accurate to perceive them as temporary hyperphagia specialists for
which low protein concentrations are necessary to avoid dramatically
overshooting protein requirements. Including absolute food intake in diet
determination offers important nuances in result interpretation. When
coining labels to categorize animals, it may be advisable to not only
consider nutrient concentrations but also absolute intake.