Ungulate foraging strategies: energy maximizing or time minimizing?

1. Many classical models of ungulate foraging are premised on energy maximi zation, yet limited empirical evidence and untested currency assumptions ma ke the choice of currency a non-trivial issue. The primary constraints on f orage intake of ungulates are forage quality and availability. Using a mode l that incorporates these two constraints, we predicted the optimal biomass of forage patches for ungulate grazers using an energy maximizing vs, a ti me minimizing strategy. 2. We tested these predictions on wood bison (Bison bison athabascae Rhoads ) grazing naturally occurring sedge (Carex atherodes Spreng). The digestive constraint was determined by a series of ad libitum feeding trials using s edge at different stages of growth. Sedge digestibility declined with bioma ss. Ad libitum intake of sedge by bison declined with sedge digestibility a nd thus decreased with sedge biomass. On the other hand, short-term sedge i ntake rates of wood bison increased with biomass. 3. Incorporation of these constraints resulted in the prediction that daily energy gain of bison should be maximized by grazing patches with a biomass of 10 g m(-2), whereas a bison could minimize daily foraging time needed t o fulfil its energy requirement by cropping patches with a biomass of 279 g m(-2). 4. To test these quantitative predictions, we used a staggered mowing regim e to convert even-aged stands of sedge to a mosaic of patches varying in qu ality and quantity. Observations of bison grazing these mosaics indicated t hat patches of biomass below 120 g m(-2) were avoided, while patches of bio mass 156 and 219 g m(-2) were highly preferred, with the greatest preferenc e for the latter. 5. These results indicate that bison were behaving as time minimizers rathe r than energy maximizers. Daily cropping times of free-ranging bison from t he literature corroborate our results.