We describe short-term changes in foraging behavior by wild Yakushima macaques (Macaca fuscata yakui), which inhabit a warm-temperate broad-leaved forest on Yakushima Island (30 degrees N, 131 degrees E), Japan. Rapid changes of dietary composition, activity budget, and range use by the monkeys occurred from May to dune, apparently associated with changes in the availability of the fruit of Myrica rubra Before the fruit ripened, monkeys spent less time moving and more time feeding on many species of leaves, which accounted for 40% of feeding time. However when M. rubra began to ripen, they fed intensively on the fruit, which accounted for three-fourths of feeding time, though the activity budget remained unaffected As fruit of M. rubra decreased, the monkeys fed more on the fruit of other species and on insects, and spent more time moving at higher speeds. There marked shifts in foraging pattern occurred within only two months. In terms of moving cost and dietary qualify, Yakushima macaques shifted their foraging pattern according to the availability of M. rubra from a ''low-cost low-yield'' strategy to a ''low-cost high-yield'' strategy, and then to a more costly strategy. The ability to make such rapid shifts in foraging pattern may allow the macaques to effectively use the highly variable food supply within their small range.
