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.

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Animals have evolved various sensory systems to identify appropriate foods and avoid harmful substances. Taste, a chemical sense, plays a central role in food selection. This review explores the functional evolution of taste receptors in primates in the context of dietary diversification. Special focus is placed on bitter taste receptors (TAS2Rs), which detect potentially harmful compounds. Primates exhibit a wide range of dietary preferences—from insectivory and frugivory to folivory and bamboo specialization—making them suitable models for studying the link between taste perception and feeding ecology. The evolution of bitter taste receptors in association with diet has been studied from two major aspects: the number of receptor genes in the genome and the function of individual receptors. Comparative genomic analyses have revealed that TAS2R gene repertoire size correlates with dietary strategies, with omnivorous species typically retaining more TAS2Rs than specialists (e.g., folivorous species). In-depth functional analyses of individual bitter taste receptors, such as TAS2R16, have uncovered conservation and diversification of receptive ranges to plant-derived compounds in a variety of primate lineages. These studies have also identified potentially adaptive changes in receptor sensitivity in specialists, such as folivorous or bamboo-eating primates. Furthermore, studies of other taste receptors, including TAS2R38 and the sweet/umami taste receptor TAS1Rs, suggest evolutionary changes in their receptor functions associated with dietary transitions to folivory. As high-quality primate genomes become more accessible, integrated studies combining behavior, genomics, and receptor function will be key to elucidating how primates diversified their dietary strategies through sensory adaptation. This integrated approach not only enhances our understanding of primate evolution but also sheds light on the evolutionary origins of human taste perception.

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We observed an enclosure group of Yakushima macaques (Macaca fuscata yakuii) to examine the embracing behaviors which is known as the social custom of wild Yakushima macaques. As a result of observing the group composed of 3 adult males, 4 adult females, 2 adolescent males, 4 juvenile males, 1 juvenile female, 1 infant male, and 1 infant female at Okinawa Zoo and Museum in March, 2023-2024, Yakushima-type embracing characterized by ventro-lateral embraces accompanied by kneading another’s fur by opening and closing the palm was observed. Unexpectedly, the following infrequently observed phenomena and behaviors in wild/provisioned groups of Japanese macaques were also observed despite the short observation period (38 h 28 min for a total of 6 days): female dominance over male, sexual prematurity, mutual grooming, grooming chain involving four individuals simultaneously, threat via self-wrist biting, tree-shaking by adult female, pirouettes, self-suckling, grasping male genitals, and sucking male genitals. Some of these could be explained by high nutritional status (sexual prematurity) and high social stress (threat via self-wrist biting and self-suckling) under captive conditions or by recent social change (female dominance over males and grasping male genitals). Given the ease of individual identification by artificial marking, the enclosure group of Yakushima macaques in this zoo can be suitable not only for study focused on infrequently observed behavior such as those reported here, but also for training student to collect data on frequently observed behaviors, such as social grooming and aggression.

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