Thai National Parks

Species of Thailand

Crab-eating macaque

Macaca fascicularis, Thomas Stamford Raffles, 1821

(In Thai: ลิงแสม)

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, which literally means dog-skin or dog-hide monkey; this name is commonly used in laboratory settings.


The 10 subspecies of M. fasciularis are:

  • Crab-eating macaque, M. f. fascicularis, synonym Macaca irus
  • Burmese long-tailed macaque, M. f. aurea
  • Nicobar long-tailed macaque, M. f. umbrosa
  • Dark-crowned long-tailed macaque, M. f. atriceps
  • Con Song long-tailed macaque, M. f. condorensis
  • Simeulue long-tailed macaque, M. f. fusca
  • Lasia long-tailed macaque, M. f. lasiae
  • Maratua long-tailed macaque, M. f. tua
  • Kemujan long-tailed macaque, M. f. karimondjawae
  • Philippine long-tailed macaque, M. f. philippensis

Physical characteristics

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 (7-13 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 (16.4 feet). 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.

Group living

Macaques live in social groups that contain three to 20 females, their offspring, and one or many males. The groups usually have fewer males than females. In social groups of macaques, a clear dominance hierarchy is seen among females. These ranks remain stable throughout the female’s lifetime and also can be sustained through generations of matrilines. Females have their highest birth rates around 10 years of age and completely stop bearing young by age 24.

The social groups of macaques are female-bonded, meaning the males will disperse at the time of puberty. Thus, group relatedness on average appears to be lower than compared to matrilines. More difference in relatedness occurs when comparing high-ranking lineages to lower ranking lineages, with higher-ranking individuals being more closely related to one another. Additionally, groups of dispersing males born into the same social groups display a range of relatedness, at times appearing to be brothers, while at other times appearing to be unrelated.

In addition to the matrilineal dominance hierarchy, male dominance rankings also exist. Alpha males have a higher frequency of mating compared to their lower-ranking conspecifics. The increased success is due partially to his increased access to females and also due to female preference of an alpha male during periods of maximum fertility. Though females have a preference for alpha males, they do display promiscuous behavior. Through this behavior, females risk helping to rear a nonalpha offspring, yet benefit in two specific ways, both in regard to aggressive behavior. First, a decreased value is placed on one single copulation. Moreover, the risk of infanticide is decreased due to the uncertainty of paternity.

Increasing group size leads to increased competition and energy spent trying to forage for resources, and in particular, food. Further, social tensions build and the prevalence of tension-reducing interactions like social grooming fall with larger groups. Thus, group living appears to be maintained solely due to the safety against predation.


Group living in all species is dependent on tolerance of other group members. In crab-eating macaques, successful social group living maintains postconflict resolution must occur. Usually, less dominant individuals lose to a higher-ranking individual when conflict arises. After the conflict has taken place, lower-ranking individuals tend to fear the winner of the conflict to a greater degree. In one study, this was seen by the ability to drink water together. Postconflict observations showed a staggered time between when the dominant individual begins to drink and the subordinate. Long-term studies reveal the gap in drinking time closes as the conflict moves further into the past.

Grooming and support in conflict among primates is considered to be an act of reciprocal altruism. In crab-eating macaques, an experiment was performed in which individuals were given the opportunity to groom one another under three conditions: after being groomed by the other, after grooming the other, and without prior grooming. After a grooming took place, the individual that received the grooming was much more likely to support its groomer than one that had not previously groomed that individual. These results support the reciprocal altruism theory of grooming in long-tailed macaques.

Crab-eating macaques demonstrate two of the three forms of suggested postconflict behavior. In both captive and wild studies, the monkeys demonstrated reconciliation, or an affiliative interaction between former opponents, and redirection, or acting aggressively towards a third individual. Consolation was not seen in any study performed.

Postconflict anxiety has been reported in crab-eating macaques that have acted as the aggressor. After a conflict within a group, the aggressor appears to scratch itself at a higher rate than before the conflict. Though the scratching behavior cannot definitely be termed as an anxious behavior, evidence suggests this is the case. An aggressor’s scratching decreases significantly after reconciliation. This suggests reconciliation rather than a property of the conflict is the cause of reduction in scratching behavior. Though these results seem counterintuitive, the anxiety of the aggressor appears to have a basis in the risks of ruining cooperative relationships with the opponent.

Kin altruism and spite

In a study, a group of crab-eating macaques was given ownership of a food object. Unsurprisingly, adult females favored their own offspring by passively, yet preferentially, allowing them to feed on the objects they held. Interestingly, when juveniles were in possession of an object, mothers robbed them and acted aggressively at an increased rate towards their own offspring compared to other juveniles. These observations suggest close proximity influences behavior in ownership, as a mother’s kin are closer to her on average. When given a nonfood object and two owners, one being a kin and one not, the rival will choose the older individual to attack regardless of kinship. Though the hypothesis remains that mother-juvenile relationships may facilitate social learning of ownership, the combined results clearly point to aggression towards the least-threatening individual.

A study was conducted in which food was given to 11 females. They were then given a choice to share the food with kin or nonkin. The kin altruism hypothesis suggests the mothers would preferentially give food to their own offspring. Yet eight of the 11 females did not discriminate between kin and nonkin. The remaining three did, in fact, give more food to their kin. The results suggest it was not kin selection, but instead spite that fueled feeding kin preferentially. This is due to the observation that food was given to kin for a significantly longer period of time than needed. The benefit to the mother is decreased due to less food availability for herself and the cost remains great for nonkin due to not receiving food. If these results are correct, crab-eating macaques are unique in the animal kingdom, as they appear not only to behave according to the kin selection theory, but also act spiteful toward one another.


After a gestation period of 162–193 days, the female gives birth to one infant. The infant's weight at birth is about . 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 coat may indicate to others the status of the infant, and other group members treat infants with care and rush to their defense when distressed. Immigrant males 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. A young juvenile stays mainly with its mother and relatives. 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 are more successful than those that leave alone. Young females, though, stay with the group and become incorporated into the matriline into which they were born.

Male crab-eating macaques 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.


Crab-eating macaques typically do not consume crabs; rather, they are opportunistic omnivores, eating a variety of animals and plants. Although fruits and seeds make up 60 - 90% of their diet, they also eat leaves, flowers, roots, and bark. They sometimes prey on vertebrates (including bird chicks, nesting female birds, lizards, frogs, and fish), invertebrates, and bird eggs. In Indonesia, the species has become a proficient swimmer and diver for crabs and other crustaceans in mangrove swamps.

This species exhibits particularly low tolerance for swallowing seeds. Despite their inability to digest seeds, many primates of similar size swallow large seeds, up to 25 mm (0.98 in), and simply defecate them whole. The crab-eating macaque, though, spits seeds out if they are larger than 3–4 mm (0.12-0.16 in). This decision to spit seeds is thought to be adaptive; it avoids filling the monkey’s stomach with wasteful bulky seeds that cannot be used for energy.

Although the species is ecologically well-adapted and poses no threat to population stability of prey species in its native range, in areas where the crab-eating macaque is not native, it can pose a substantial threat to biodiversity. Some believe the crab-eating macaque is responsible for the extinction of forest birds by threatening critical breeding areas as well as eating the eggs and chicks of endangered forest birds.

The crab-eating macaque can become a synanthrope, living off human resources. They are known to feed in cultivated fields on 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.

Tool use

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.

Another instance of tool use is washing and rubbing foods such as sweet potatoes, cassava roots, and papaya leaves before consumption. Crab-eating macaques either soak these foods in water or rub them through their hands as if to clean them. They also peel the sweet potatoes, using their incisors and canine teeth. Adolescents appear to acquire these behaviors by observational learning of older individuals.

Distribution and habitat

The crab-eating macaque lives in a wide variety of habitats, including primary lowland rainforests, disturbed and secondary rainforests, shrubland, 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 are pests around farms and villages. Typically, they prefer disturbed habitats and forest periphery. The native range of this species includes most of mainland Southeast Asia, from extreme southeastern Bangladesh south through Malaysia, 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. This primate is a rare example of a terrestrial mammal that violates the Wallace line.

Introduced range

M. fascicularis is an introduced alien species in several locations, including Hong Kong, Taiwan, Irian Jaya, 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". M. fascicularis is not a threat to biodiversity in its native range.

The immunovaccine porcine zona pellucida (PZP), which causes infertility in females, is currently being tested in Hong Kong to investigate its use as potential population control.

Relationship with humans

Crab-eating macaques extensively overlap with humans across their range in Southeast Asia. Consequently, they live together in many locations. Some of these areas are associated with religious sites and local customs, such as the temples of Bali in Indonesia, Thailand, and Cambodia, while other areas are characterized by conflict as a result of habitat loss and competition over food and space. Humans and crab-eating macaques have shared environments since prehistoric times, and tend to both frequent forest and river edge habitats. Crab-eating macaques are occasionally used as a food source for some indigenous forest-dwelling peoples. In Mauritius, they are captured and sold to the pharmaceutical industry, and in Angaur and Palau, they are sold as pets. Macaques feed on sugarcane and other crops, affecting agriculture and livelihoods, and can be aggressive towards humans. Macaques may carry potentially fatal human diseases, including herpes B virus.

In scientific research

M. fascicularis is also 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 Reston ebolavirus 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 (Herpesvirus 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 species used as space test flight animals. Plasmodium knowlesi, which causes malaria in M. fascicularis, can also infect humans. A few cases have been documented in humans, 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 Southeast 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, and too little information is available on some subspecies to assess their conditions. The M. f. umbrosa subspecies is likely of important biological significance and 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 Philippine 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.


The genome of the crab-eating macaque has been sequenced.

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Scientific classification

Macaca fascicularis

Common names

  • German:
    • Javaneraffe
    • Langschwanzmakak
    • Krabbenesser
  • English:
    • Crab-eating macaque
    • Cynomolgus monkey
    • Long-tailed macaque
  • Spanish: Macaca cangrejera
  • French:
    • Macaque crabier
    • Macaque de buffon
  • Italian:
    • Macaco cinomolgo
    • Macaco di Buffon
    • Macaco di Giava
  • Dutch:
    • Java-aap
    • Krabbenetende makaak
  • Swedish:
    • Krabbmakak
    • Långsvansad makak
  • Thai: ลิงแสม


  • Macaca fascicularis atriceps, Cecil Boden Kloss, 1919

    Common name: Dark-crowned long-tailed macaque

    Range: Koh Kram Island, near Cape Liant, Inner Gulf of Siam, Chon Buri, Thailand, Asia

  • Macaca fascicularis aurea, Étienne Geoffroy Saint-Hilaire, 1831

    Common name: Burmese long-tailed macaque, Myanmar long-tailed macaque

    Range: Domel Island, Mergui Archipelago, Tanintharyi, Myanmar, Asia

  • Macaca fascicularis condorensis, Cecil Boden Kloss, 1926

    Common name: Con Song long-tailed macaque

  • Macaca fascicularis fascicularis, Thomas Stamford Raffles, 1821

    Common name: Crab-eating macaque

    Range: Natuna Islands, Sirhassen Island, Kepulauan Riau, Indonesia

  • Macaca fascicularis fusca, Gerrit Smith Miller, Jr, 1903

    Common name: Simeulue long-tailed macaque

    Range: Simalur Island, Sumatra, Aceh, Indonesia

  • Macaca fascicularis karimondjawae, Henri Jacob Victor Sody, 1949

    Common name: Kemujan long-tailed macaque

  • Macaca fascicularis lasiae, Marcus Ward Lyon, Jr., 1916

    Common name: Lasia long-tailed macaque

  • Macaca fascicularis philippinensis, Étienne Geoffroy Saint-Hilaire, 1843

    Common name: Philippine long-tailed macaque

    Range: Alag River, Camp Number 2, Mindoro, Mindoro Oriental Province, Philippines

  • Macaca fascicularis tua, Remington Kellogg, 1944

    Common name: Maratua long-tailed macaque

    Range: Pulo Muara Tua, Borneo, Kalimantan Timur, Indonesia

  • Macaca fascicularis umbrosa, Gerrit Smith Miller, Jr, 1902

    Common name: Nicobar long-tailed macaque

    Range: Little Nicobar, Nicobar Islands, Andaman and Nicobar Is, Asia


  • Macaca irus, Georges-Frédéric Cuvier (1818)
  • Simia aygula, Carolus Linnaeus (1758)

Conservation status

Least Concern (IUCN3.1)

Least Concern (IUCN3.1)


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Crab-eating macaque
Macaca fascicularis - Mu Ko Tarutao NP
Macaca fascicularis

Range Map

Distribution map of Crab-eating macaque, Macaca fascicularis in Thailand
  • Ang Thong National Marine Park
  • Banphot Phisai District, Nakhon Sawan
  • Chaloem Rattanakosin National Park
  • Erawan National Park
  • Huai Kha Khaeng Wildlife Sanctuary
  • Kaeng Krachan District, Phetchaburi
  • Kaeng Krachan National Park
  • Khao Ang Rue Nai Wildlife Sanctuary
  • Khao Chakan District, Sa Kaeo
  • Khao Khiao - Khao Chomphu Wildlife Sanctuary
  • Khao Sam Roi Yot National Park
  • Khao Sok National Park
  • Khlong Saeng Wildlife Sanctuary
  • Ko Chang District, Trat
  • Ko Lanta National Park
  • Ko Samui District, Surat Thani
  • Kui Buri National Park
  • Lang Suan District, Chumphon
  • Mueang Krabi District, Krabi
  • Mueang Lopburi District, Lopburi
  • Mueang Phichit District, Phichit
  • Mueang Prachuap Khiri Khan District, Prachuap Khiri Khan
  • Mueang Songkhla District, Songkhla
  • Namtok Sam Lan National Park
  • Pak Chong District, Nakhon Ratchasima
  • Pak Tho District, Ratchaburi
  • Pang Sida National Park
  • Phi Phi Islands
  • Sai Yok District, Kanchanaburi
  • Sai Yok National Park
  • Similan Islands
  • Suan Phueng District, Ratchaburi
  • Takua Thung District, Phang Nga
  • Tarutao National Marine Park
  • Tha Chana District, Surat Thani
  • Thale Ban National Park
  • Wichian Buri District, Phetchabun
Range map of Macaca fascicularis in Thailand