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

Abstract

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-level diet variation is recognized, such variation 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 individual foragers should be more selective when they have greater energy reserves to invest in searching for high-quality foods. We tested these predicted mechanisms of individual niche differentiation 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 individuals' realized dietary niches (taxonomic richness and composition) using DNA metabarcoding of fecal samples collected repeatedly from 15 GPS-collared animals (range 6-14 samples per individual, median 12). Bushbuck diets were structured by spatial heterogeneity and constrained by individual condition. 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 structure (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 Ryan A. Long and Robert M. Pringle contributed equally.

Foraging history of individual elephants using DNA metabarcoding

“…We considered alternative predictions about seasonal variation in the contributions of individuals to group foraging through the lenses of Optimal Foraging Theory and the Niche Variation Hypothesis. Evidence suggests that populations of diverse species differ in the degree to which they match the predictions of both theories [ 1 , 2 ], and that large mammalian herbivores have highly plastic diets that may generally be aligned with predictions based on Optimal Foraging [ 14 , 35 ]. Our data matched the expectation that elephant diets are plastic but departed from both sets of predictions as dietary richness either expanded (Artists) or did not reveal a strong correlation (Royals) with increasing NDVI, while familial cohesion levels were relatively strong in dry seasons (figures 3 and 4 ).…”

“…Social foraging may foster dietary cohesion within groups, but individuals that forage together may often still obtain different diets if they vary in dominance, age, reproductive state or other characteristics that alter their nutritional needs, preferences or access to resources [ 12 , 13 ]. The growing awareness that individual diets differ compels us to understand individual-level dietary variation [ 14 ], though fine-grained timeseries on individuals' diets remain sparse and difficult to obtain.…”

Coursing the mottled mosaic: Generalist predators track pulses in availability of neonatal ungulates

“…We quantified search behavior of coyotes using the intensity of use metric (Almeida et al, 2010 ; Walker et al, 2022 ). Intensity of use is directly proportional to the time spent per unit area and will increase when coyotes exhibit complex paths or move slowly through an area (Almeida et al, 2010 ; Walker et al, 2022 ).…”

“…We quantified search behavior of coyotes using the intensity of use metric (Almeida et al, 2010 ; Walker et al, 2022 ). Intensity of use is directly proportional to the time spent per unit area and will increase when coyotes exhibit complex paths or move slowly through an area (Almeida et al, 2010 ; Walker et al, 2022 ). Consequently, intensity of use represents the activity of coyotes within their home ranges and allowed us to test the prediction that coyotes exhibit search behavior during mule deer parturition.…”

“…Optimal foraging theory provides the context for understanding animal space use and behavior; and why resource pulses should be sought after by consumers. Animals that forage optimally should switch resources when the benefits of capturing that resource outweigh the costs (Charnov, 1976 ; Emlen, 1966 ; Stephens & Krebs, 1986 ; Walker et al, 2022 ). Increased energetic expenditure associated with elevated search effort, movement costs, or handling time are costs that may be incurred by tracking resource pulses.…”