Risk-induced changes in prey behavior appear in many predator-prey communities, often triggering cascades to additional trophic levels when prey alter intake rates or diet selection; however, the fitness consequences of predation risk remain unclear.  Natural colonization by cougars (Puma concolor) on the National Bison Range (Montana, USA) enabled the testing of predictions from foraging theory that assess the fitness consequences of predation-sensitive foraging decisions by ungulate prey.  Diets of herbivores prior to colonization by cougars matched expectations based on an energy maximization foraging goal predicted by a linear programming model.  Here, behavioral observations and new field data were used to re-parameterize the foraging models to predict diets for three prey species that differ in body size and anti-predator behavior: mule deer (Odocoileus hemionus) white-tailed deer (O. virginianus) and elk (Cervus elaphus).  The models were solved for two goals - energy maximization or time minimization - that form the endpoints of a continuum.  The latter would be expected if prey reduce their feeding to minimum requirements in order to devote more time to anti-predator behaviors. 

Results/Conclusions In response to the increase in predation risk, none of the prey switched to a time-minimization foraging strategy.  The largest and least vulnerable prey selected a diet that fit a new energy-maximization strategy and incurred an energetic cost for reduced feeding time.  Diets selected by the smaller and more vulnerable prey appeared to be intermediate, suggesting a trade-off between energy maximization and time minimization.  With the observed changes in feeding time when faced with predation risk, all three ungulates incurred a 5-38% reduction in daily surplus energy intake – a substantial effect of risk on summer foraging that could impact subsequent winter survival and reproduction.  This study represents a first attempt to employ a mechanistic linear programming foraging model to evaluate alternate strategies and energetic costs of large ungulates foraging in the presence of predators.