抄録
We present data on the density of Japanese macaques in the western lowland forest of Yakushima, one of the most intensively studied long-term research sites for this species. Defining a group as a set of individuals moving together at a given time, we estimated a group density of 3.78 groups/km², based on point censuses and group follows conducted in August 2023. Defining a troop as a social unit with stable membership, we estimated a troop density of 2.59–4.49 troops/km² and a population density of 85–170 individuals/km², based on home range sizes and group compositions of four intensively studied troops in 2023–2024. Comparison with previous studies reporting comparable parameters indicates that population density in this area has increased 1.4–2.8 fold over the past half-century. However, whether density has increased since the 1990s remains unclear, owing to differences in census methods across study periods and possible spatial variation within our study site. Group density was 1.1–1.7 times higher than troop density, suggesting that each troop frequently divides into multiple groups.
