Use of Pine Nuts by Grizzly and Black Bears in the Yellowstone Area

Abstract

Whitebark pine (Pinus albicaulis), an important tree of high altitudes in the northern Rocky Mountains and Sierra Nevada, produces nuts eaten by bears. Grizzly bear (Ursus arctos) and black bear (U. americanus) use of pine nuts was studied in Yellowstone National Park and adjacent areas during 1978 and 1979. Spring use appeared to be correlated with cone produc? tion in the preceding year, while fall use was correlated with the current crop. Most of the nuts eaten by bears came from cones cached by red squirrels (Tamiasciurus hudsonicus). Pine nuts were a nutritious food which was often present in early spring and late fall when alternate foods were scarce or low in digestible energy and when nutritional requirements of bears were high. No evidence was found that bears ate the nuts of limber pine (P. flexilis). Int. Conf. Bear Res. and Manage. 5:166-173 The large seeds (pine nuts) of whitebark pine are commonly eaten in the spring (March-May) and fall (September-November) by grizzly and black bears in Yellowstone National Park and ad? jacent areas (Craighead and Craighead 1972, Blanchard 1978, Mealey 1980) and western Mon? tana (Tisch 1961; J.Sumner and J.J.Craighead, unpubl. rep., Montana Coop. Wildl. Res. Unit, Univ. Montana, Missoula, 1973). Similar nuts from limber pine are eaten by grizzly bears on the east Rocky Mountain Front of northwestern Montana (Schallenberger and Jonkel, annual rep., Border Grizzly Project, Univ. Montana, Missoula, 1980). The nuts of the European stone pine (P. cembra) are an important food for brown bears (U. arctos) throughout the taiga zone in the Soviet Union (Pavlov and Zhdanov 1972, Ustinov 1972, Yazan 1972). Both the pro? duction of whitebark pine cones (Forcella 1977, Blanchard 1978, Mealey 1980) and the quantity of nuts consumed by bears vary annually (Mealey 1975, Blanchard 1978). Pine nuts are also an important food for red squirrels in whitebark forests. In fall, squirrels remove cones from trees and cache them in middens. Bears as well as other mammalian and avian seed predators compete with squirrels for whitebark nuts (Forcella 1977, Tomback 1977). Confusion about the ripening process of whitebark pine cones has resulted in errors in the literature on the availability of pine nuts as a bear food. Whitebark cones are indehiscent and do not disintegrate (Tomback 1981). Vertebrate for? aging probably leaves few, if any, seed-bearing cones on trees by late fall; the cones remaining abscise sometime thereafter (Tomback 1981). Because cones do not abscise or release their seed in fall, bears may obtain pine nuts in 2 ways. Black bears may climb whitebark pine trees and break off cone-bearing branches to feed on cones (Tisch 1961, Mealey 1975, Forcella 1977); or both black bears and grizzly bears may raid squirrel caches to feed on pine nuts (Tisch 1961, Craighead and Craighead 1972, Blanchard 1978). The purpose of this study was to determine (1) the major source of pine nuts for bears, (2) why cone scales do not appear in bear scat containing pine nuts, and (3) what factors influ? ence bear use of pine nuts. Funding for this study was provided by the Na? tional Park Service and the U.S. Fish and Wildlife Service. I am grateful for the cooperation of per? sonnel from Yellowstone National Park and U.S. Forest Service district offices within the study area. I thank R.R. Knight, T.W.Weaver, H.D. Picton, W.R.Gould, and M. Meagher for their helpful reviews of the manuscript, all the mem? bers of the Interagency Grizzly Bear Study (IGBS) who helped me with field work and data reduction, and D.Sizemore for conducting the feeding trials on the Vancouver grizzlies.