Constraints on herbivory by grizzly bears

Although well known as carnivores and not capable of digesting plant fiber, grizzly bears (Ursus arctos horribilis) consume over 200 species of plants and are entirely vegetarian in some ecosystems. Even in ecosystems with ab undant meat resources, green vegetation can be an important seasonal food r esource. Therefore, we examined the morphological, physiological, and envir onmental constraints that determine the nutritional value of herbaceous veg etation to grizzly bears. Shortterm, board foraging trials were used with c aptive grizzly bears to determine constraints on intake rate including bite size, bite rate, bear size, plant species, plant height, and plant distrib ution. Feeding trials were conducted to determine the effect of protein lev el (12-35%) and digestible dry matter intake on weight gain. Finally, maxim um daily intake, daily foraging time, and weight change were measured for c aptive bears foraging on highly abundant and nutritious forbs and grasses d uring 12-day trials. Intake during short-term board trials overestimated th e intake of freely foraging bears from two-to seven-fold depending on bear size. Because of their relatively larger bite sizes, smaller absolute energ y requirements, and relatively larger intake capacity, smaller bears (< 120 kg) made greater weight gains than very large bears on herbaceous vegetati on. Smaller bears with ad libitum access to palatable, nutritious forbs gai ned weight at rates equal to wild bears. However, depending upon plant char acteristics, bite sizes and available daily foraging time increasingly prev ented large bears (> 120 kg) from gaining weight on herbaceous vegetation. Both captive and wild bears select forbs over grasses at similar growth sta ges because forbs are generally higher in protein and more digestible than grasses. Therefore, the nutritional well-being of wild grizzly bears could be improved in areas where bears are largely herbivorous and, thus, relativ ely small by purposefully managing for nutritious forbs.