State- and context-dependent applications of an energetics model in free-ranging bighorn sheep
2021
Denryter, Kristin | German, David W. | Stephenson, Thomas R. | Monteith, Kevin L.
Energy is fundamentally important to life and when energy supplies cannot support energy demands, animals may reduce energy deficits through behavioral compensation—a phenomenon that can vary with physiological state (state dependent) or environmental conditions (context dependent). We built an energetics model to estimate seasonal and annual energy expenditures by female Sierra Nevada bighorn sheep (Ovis canadensis sierrae; hereafter, Sierra bighorn) and evaluated whether energy expenditures by Sierra bighorn were consistent with behavioral compensation. We parameterized the model with values from the literature for energy expenditures for metabolism, feeding, locomotion, tissue synthesis, and reproduction in bighorn sheep and populated the model with empirical data (body mass, ingesta-free body fat, reproductive state, and daily movements) from female Sierra bighorn. Annual energy expenditures by reproductive females (7,182 MJ × yr⁻¹) were ~42% greater than annual energy expenditures by non-reproductive females (5,066 MJ × yr⁻¹). Regardless of reproductive state, female Sierra bighorn expended less energy on activity in winter compared with summer, which was consistent with context-dependent (environmental) behavioral compensation. Results for state-dependent behavioral compensation were ambiguous, but daily movements and associated energy expenditures decreased over winter, coincident with declines in body fat. Our energy model can be adapted to any free-ranging ungulate, if annual body mass and fat dynamics, metabolic rates, reproductive state, and daily movements are known. Coupling our model with estimates of food supplies may generate deeper insights regarding energetic motivations of migration and habitat use.
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