MOOSE BROWSING ON FELTLEAF WILLOW: OPTIMAL FORAGING IN RELATION TO PLANT MORPHOLOGY AND CHEMISTRY

Moose bite-size selection on feltleaf willow during the winter shows remarkably low variance, despite the large range of bite diameters available on current annual growth twigs. We hypothesized that this apparent selectivity is based on the compromise moose make in their foraging behavior, between the constraints of digestion and rate of food intake. We developed a model of optimal bite diameter based on morphometric and chemical characteristics of current annual growth twigs of feltleaf willow, An important browse species throughout interior Alaska. The model closely predicted the observed bite-size selection of moose, and further showed how differences in twig morphology, spatial density, and forage chemistry affect optimal bite diameter and energy gain per time unit. Twig digestibility, twig volume, clipping rate, and rumen turnover time are all important factors that govern the choice of bite diameter. Numerical analyses showed that, within the plausible range of variation in the field of these parameters, optimal bite diameter is more sensitive to twig morphology and clipping rate rather than twig chemistry. However, twig chemical characteristics are most important in determining daily energy and nitrogen gain