Influences of predation risk and energy maximization on food selection by beavers (Castor canadensis)

We developed deterministic models to generate predictions for three hypotheses: beavers (Castor canadensis) select food on the basis of (1) maximization of energy return, (2) minimization of predation risk, or (3) minimization of the ratio of predation risk to energy return. To test predictions, beavers were provided with a choice of two sets of quaking aspen, 1.5 or 3.0 cm in diameter, with the 3.0-cm stems placed at distances from the pond equal to or greater than those of the 1.5-cm stems. The models predicted that beavers using strategy 1 should select mainly large aspen, beavers using strategy 2 should select mainly small aspen, and the preferences of beavers using strategy 3 should depend on the relative distances of large and small aspen. Selection of experimental aspen by beavers was consistent with hypothesis 3. Beavers primarily stripped bark of aspen for consumption in water, and the stripping rate increased as the water temperature decreased. Size-distance relationships in selection of aspen and the stripping location of experimental aspen in relation to water temperature were consistent with the hypothesis that beavers were trading off maximization of profitability against minimization of predation risk.