Crab-Eating Macaque Macaca fascicularis




The scientific name of the Crab-Eating Macaque is Macaca fascicularis. Macaca comes from the Portuguese word macaco, which was picked up from makaku, a Fiot (West African language) word (kaku means 'monkey' in Fiot). 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, although it is presumed it had something to do with his observation of the animal's colour. This animal has several common names. It is often referred to as the long-tailed macaque because its tail is usually about the same length as its body and because its long tail distinguishes it from most other macaques. The species is also commonly known as the crab-eating macaque because it is often seen foraging beaches for crabs. Another common name for M. fascicularis is the cynomolgus monkey, which literally means "dog-milker" monkey; this is the name most commonly used in laboratory settings. In Indonesia, M. fascicularis and other macaque species are known generically as kera, possibly because of the high-pitched alarm calls they give when in danger ("krra! krra!").

Physical characteristics

Depending on subspecies, the body length of the adult monkey is 38-55 cm (1522 in) with comparably short arms and legs. The tail is longer than the body, typically 4065 cm (1626 in). Males are considerably larger than females, weighing 5-9 kilograms (11-20 lb) compared to the 36 kg (7-13 lb) of females.

Social structure

M. fascicularis is a very social animal that lives in groups of 560 or more animals. These are multiple-male groups, normally containing two to five males and two to three times as many females. The number of juveniles is usually comparable to the number of females. Group size often depends on the level of predation and availability of food. Groups are female-centred, as the females are philopatric (i.e. remain in one place across generations), while the males move in and out of these female-based groups. Males generally first emigrate from their natal group at the age of four to six years. They will remain in a group up to four or five years and thus will emigrate several times throughout their lives. These monkeys are highly despotic and have a strict dominance hierarchy. Adult males rank higher than females. Female ranks are more stable than males, as males are defeated from time to time and then lose rank. High-ranked males are generally the most successful at reproduction, and high-ranked females generally fare best at raising surviving offspring. The females are organized into matrilines, which are the female-based families consisting of the related females and their descendents. Matrilines are ranked, and some families have greater social power than others; this difference in rank is maintained over several generations. Matrilineal overthrows rarely occur, and when they do, they have severe consequences to the reproductive success of the defeated matriline in the following year.


After a gestation period of 167193 days, the female gives birth to one infant. The infant's weight at birth is approximately 350 grams (12 oz). Infants are born with black fur which will begin to turn to a yellow-green, grey-green, or reddish-brown shade (depending on the subspecies) after about three months of age. This natal coat is suggested to indicate to others the status of the infant, and other group members treat infants with care and rush to their defence when distressed. Immigrant males will sometimes kill infants not their own, and high-ranking females sometimes kidnap the infants of lower-ranking females. These kidnappings usually result in the death of the infants, as the other female is usually not lactating. Young juveniles stay mainly with the mother and relatives, and as male juveniles get older, they become more peripheral to the group. Here they play together, forming crucial bonds that may help them when they leave their natal group. Males that emigrate with a partner seem to be more successful than those that move off alone. Young females, on the other hand, stay in the centre of the group and become incorporated into the matriline into which they were born.

Male Crab-Eating Macaques will groom females to increase the chance of mating. A female is more likely to engage in sexual activity with a male that has recently groomed her than with one that has not.

The Crab-Eating Macaque can become a synanthrope, living off human resources. They are known to feed in cultivated fields on such items as young dry rice, cassava leaves, rubber fruit, taro plants, coconuts, mangos and other crops, often causing significant losses to local farmers. In villages, towns, and cities, they frequently take food from garbage cans and refuse piles. The species can become unafraid of humans in these conditions, which can lead to Macaques directly taking food from people, both passively and aggressively.

In Thailand and Myanmar, Crab Eating Macaques use stone tools to open nuts, oysters and other bivalves, and various types of sea snails (nerites, muricids, trochids, etc.) along the Andaman sea coast and offshore islands.

Distribution and habitat

The Crab-Eating Macaque is found in a wide variety of habitats, including primary lowland rainforests, disturbed and secondary rainforests, and riverine and coastal forests of nipa palm and mangrove. They also easily adjust to human settlements; they are considered sacred at some Hindu temples and on some small islands, but a pest around farms and villages. Typically, they prefer disturbed habitats and forest periphery. The native range of this species includes most of mainland Southeast Asia, and the Maritime Southeast Asia islands of Sumatra, Java, and Borneo, offshore islands, the islands of the Philippines, and the Nicobar Islands in the Bay of Bengal. Thailand and Myanmar

M. fascicularis is an introduced alien species in several locations, including Hong Kong, Western New Guinea, Anggaur Island in Palau, and Mauritius. Where it is not a native species, particularly on island ecosystems whose species often evolved in isolation from large predators, M. fascicularis is a documented threat to many native species. This has led the World Conservation Union (IUCN) to list.  M. fascicularis as one of the "100 worst invasive alien species". Insofar as it is present as an invasive alien species on several islands, it has been labelled a "weed" species and is yet another significant ecological threat to those ecosystems and the species within them. However, M. fascicularis is not a biodiversity threat in its native range, where other species have adapted to their presence through evolutionary time.

Relationship with humans

M. fascicularis is  used extensively in medical experiments, in particular those connected with neuroscience and disease. Due to their close physiology, they can share infections with humans. Some cases of concern have been an isolated event of Ebola-Reston virus found in a captive-bred population shipped to the US from the Philippines, which was later found to be a strain of Ebola that has no known pathological consequences in humans, unlike the African strains. Furthermore, they are a known carrier of monkey B virus (Herpes virus simiae), a virus which has produced disease in some lab workers working mainly with Rhesus Macaques (M. mulatta). Nafovanny, the largest facility for the captive breeding of nonhuman primates in the world, houses 30,000 Macaques. The Crab-Eating Macaque is one of the types of monkeys that have been used as space test flight animals. It has been discovered recently that Plasmodium knowlesi, which causes malaria in M. fascicularis, can also infect humans. A few cases have been documented in human, but for how long humans have been getting infections of this malarial strain is unknown. It is, therefore, not possible to assess if this is a newly emerging health threat, or if just newly discovered due to improved malarial detection techniques. Given the long history of humans and macaques living together in SE Asia, it is likely the latter.

Conservation status

The Crab-Eating Macaque has the third-largest range of any primate species, behind only humans and Rhesus Macaques. The IUCN Red List categorizes the species as Least Concern, and CITES lists them as Appendix II ("not necessarily threatened with extinction", in which trade must be controlled to avoid use incompatible with their survival). A recent review of their populations suggests a need for better monitoring of populations due to increased wild trade and rising levels of human-macaque conflict, which are reducing overall population levels despite the species being widely distributed.

Each subspecies faces differing levels of threats, or simply lacks sufficient information to assess their situation. The M. f. umbrosa subspecies is argued to be of important biological significance. It has been recommended as a candidate for protection in the Nicobar Islands, where its small, native population has been seriously fragmented, and is listed as vulnerable on the IUCN Red List. The Philippines Long-Tailed Macaque (M.f. philippensis) is listed as near threatened, and M.f. condorensis is vulnerable All other subspecies are listed as data deficient and need further study, although recent work is showing           M.f aurea and M.f. karimondjawae need increased protection. One concern for conservation is, in areas where M. fascicularis is not native, their populations need to be monitored and managed to reduce their impact on native flora and fauna.