The crab-eating macaque (Macaca fascicularis), also known as the long-tailed macaque, is a cercopithecine primate native to Southeast Asia. It is referred to as the cynomolgus monkey in laboratories. It has a long history alongside humans; they have been alternately seen as agricultural pests, sacred animals in some temples, and more recently, the subject of medical experiments. The crab-eating macaque lives in matrilineal social groups with a female dominance hierarchy, and male members leave the group when they reach puberty. They are opportunistic omnivores and have been documented using tools to obtain food in Thailand and Myanmar. The crab-eating macaque is a known invasive species and a threat to biodiversity in several locations, including Hong Kong and western New Guinea. The significant overlap in macaque and human living space has resulted in greater habitat loss, synanthropic living, and inter- and intraspecies conflicts over resources.
Macaca comes from the Portuguese word macaco, which was derived from makaku, a Fiot (West African language) word (kaku means monkey in Fiot). The specific epithet fascicularis is Latin for a small band or stripe. Sir Thomas Raffles, who gave the animal its scientific name in 1821, did not specify what he meant by the use of this word.
In Indonesia and Malaysia, M. fascicularis and other macaque species are known generically as kera, possibly because of their high-pitched cries.
The crab-eating macaque has several common names. It is often referred to as the long-tailed macaque due to its tail, which is often longer than its body. The name crab-eating macaque refers to its being often seen foraging beaches for crabs. Another common name for M. fascicularis is the cynomolgus monkey, from the name of a race of humans with long hair and handsome beards who used dogs for hunting according to Aristophanes of Byzantium, who seemingly derived the etymology of the word cynomolgus from the Greek κύων, cyon: dog (Gen. cyno-s) and the verb ἀμέλγειν, amelgein: to milk (Adj. amolg-os), by claiming that they milked female dogs. This name is commonly used in laboratory settings.
The 10 subspecies of M. fascicularis are:
The body length of the adult, which varies among subspecies, is 38–55 cm (15–22 in) with relatively short arms and legs. Males are considerably larger than females, weighing 5–9 kg (11–20 lb) compared to the 3–6 kg (6.6–13.2 lb) of females. The tail is longer than the body, typically 40–65 cm (16–26 in), which is used for balance when they jump distances up to 5 m (16 ft). The upper parts of the body are dark brown with light golden brown tips. The under parts are light grey with a dark grey/brown tail. Crab-eating macaques have backwards-directed crown hairs which sometimes form short crests on the midline. Their skin is black on their feet and ears, whereas the skin on the muzzle is a light grayish pink color. The eyelids often have prominent white markings and sometimes there are white spots on the ears. Males have a characteristic mustache and cheek whiskers, while females have only cheek whiskers. Crab-eating macaques have a cheek pouch which they use to store food while foraging. Females show no perineal swelling.