The crab-eating macaque (Macaca fascicularis), also known as the long-tailed macaque or cynomolgus macaque, is a cercopithecine primate native to Southeast Asia. As a synanthropic species, the crab-eating macaque thrives near human settlements and in secondary forest. Crab-eating macaques have developed attributes and roles assigned to them by humans, ranging from cultural perceptions as being smart and adaptive, to being sacred animals, being regarded as vermin and pests, and becoming resources in modern biomedical research. They have been described as a species on the edge, living on the edge of forests, rivers, and seas, at the edge of human settlements, and perhaps on the edge of rapid extinction.
Crab-eating macaques are omnivorous and frugivorous. They live in matrilineal groups ranging from 10 to 85 individuals, with groups exhibiting female philopatry and males emigrating from natal group at puberty. Crab-eating macaques are the only old-world monkey known to use stone tools in their daily foraging, and they engage in a robbing and bartering behavior in some tourist locations.