A 13-month ecological study was conducted at Bossou, Guinea, West Africa, to elucidate how a community of wild chimpanzees (Pan troglodytes verus) deals with the scarcity of main foods. During the study period, fruit availability fluctuated radically. The chimpanzees were confirmed to depend heavily on three "keystone resources" which were available when their main foods (fruit pulp) were scarce. These were fruits of Musanga cecropioides, oil-palm (Elaeis guineensis) nuts, and oil-palm pith. These are abundant in the chimpanzees' home range and their nutritional contents compensate for a decrease in nutritional intake from fruit pulp. The presence of these excellent backup foods may explain the high reproductive performance of Bossou chimpanzees. Here, chimpanzees consumed two of the three keystone foods using two types of tool behavior: nut-cracking for oil-palm nuts and pestle-pounding for oil-palm pith. These tool-using behaviors accounted for 31.9% of the total feeding time spent in June (the month in which the highest frequency occurred) and 10.4% in total for the year. It is suggested that the Bossou chimpanzees depend strongly on tools for their subsistence. This implies a possible function for tool technology in the evolution of our human ancestors.