Mechanisms of individual variation in large herbivore diets: Roles of spatial heterogeneity and state-dependent foraging

Many populations of consumers consist of relatively specialized
individuals that eat only a subset of the foods consumed by the population
at large. Although the ecological significance of individual
specialization is recognized, it is difficult to document and its
underlying mechanisms are poorly understood. Optimal foraging theory
provides a useful framework for predicting how individuals might select
different diets, positing that animals balance the ‘opportunity cost’ of
stopping to eat an available food item against the cost of searching for
something more nutritious; diet composition should be contingent on the
distribution of food, and foragers should be more specialized when
individuals have high energy reserves to spend searching for high-quality
foods. We tested these predicted mechanisms of individual niche
differences by quantifying environmental (resource heterogeneity) and
organismal (nutritional condition) determinants of diet in a widespread
browsing antelope (bushbuck, Tragelaphus sylvaticus) in an African
floodplain-savanna ecosystem. We quantified individual diet breadth and
composition using DNA metabarcoding of fecal samples collected repeatedly
from 15 GPS-collared animals (6–14 samples per individual, median 12).
Bushbuck diets were structured by spatial heterogeneity and constrained by
individual conditions. We observed significant individual-level
partitioning of food plants by bushbuck both within and between two
adjacent habitat types (floodplain and woodland). Individuals with home
ranges that were closer together and/or had similar vegetation structures
(measured using LiDAR) ate more similar diets, supporting the prediction
that heterogeneous resource distribution promotes individual
differentiation. Individuals in good nutritional condition had
significantly narrower diets (fewer plant taxa), searched their home
ranges more intensively (intensity-of-use index), and had higher-quality
diets (percent digestible protein) than those in poor condition,
supporting the prediction that animals with greater endogenous reserves
are more specialized because they can invest time in searching for
nutritious foods. Our results support predictions from optimal foraging
theory about the energetic basis of individual specialization and provide
a potentially generalizable framework for understanding how individual
niche width is governed by animal behavior and physiology in heterogeneous
landscapes.