CRAB-EATING MACAQUES

Crab-eating macaques (Macaca fascicularis) are also called long-tailed macaques, cynomolgus or Java monkeys. Most don’t live by the sea, preferring trees instead . Those that do have access to the sea feed on crabs, other crustaceans and other animals exposed by low tide. They are usually unsuccessful in efforts to chase crabs but are able to catch them when the they wait at the crab’s hole and grab when it emerges to take a look. These monkeys have been observed getting pinched by a crab and frantically flapping their hands in the air. There are also inland A crab-eating macaque at the Sacred Monkey Forest Sanctuary in Ubud, a Balinese town in Indonesia.

Crab-eating macaques are not endangered. The International Union for Conservation of Nature (IUCN) Red List classifies them as a species of “Least Concern”. In CITES (Convention on International Trade in Endangered Species of Wild) they are in Appendix II, which lists species not necessarily threatened with extinction now but that may become so unless trade is closely controlled. The average lifespan for females in captivity is 25 years.[Source: Christopher Bonadio, Animal Diversity Web (ADW) /=]

Crab-eating macaques have been involved in aggressive interactions with people. They have been known to feed in cultivated fields on things like young dry rice, cassava leaves, rubber fruit and taro plants. They also take food from graveyards, garbage cans, and garbage pits. The fact that these macaques destroy crops has prevented some governments from making serious conservation efforts. Crab-eating macaques are one of the five most used primate species in medical research. Many have been exported to the United States and Great Britian and elsewhere.

The population densities of crab-eating macaque varies from 10 to 400 per square kilometer. There are nine national parks, nine reserves, and two sanctuaries in which some crab-eating macaques reside. Regardless of the type of habitat, there must be at least 500 square kilometers of habitat necessary to support a viable population of 5,000 crab-eating macaques. This is the minimum size for a reserve for this species. /=\

Crab-Eating Macaque Habitat and Where They Are Found

Crab-eating macaques are found in southeast Asia from Myanmar (Burma) to the Philippines and southward through southern Thailand, Cambodia, southern Vietnam, Malaysia, and Indonesia, and as far east as the Timor Islands. They are regarded as the most arboreal (tree living) of all macaque species. You can find them in tropical areas and areas with an extended dry season in primary rainforests, scrub forests, disturbed and secondary forests, and riverine and coastal forests of nipa palm. One study of crab-eating macaque behavior reported that they never came to the ground except within five meters of the edge of a river near their tree. They also live in coastal areas, near brackish water and in mangroves and swamps, agricultural areas and areas near rivers. [Source: Christopher Bonadio, Animal Diversity Web (ADW) /=]

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Crab-eating macaque range

Crab-eating macaques are "ecologically diverse." Among the habitats in which they have had the most success are disturbed habitats and on the periphery of forests. In Sumatra, they have achieved their highest population densities in mixed mangrove swamps, secondary hill forests, and riverine forests. Some were also observed in freshwater swamps, scrub grassland, lowland primary forests, and rubber groves. /=\

In Thailand, crab-eating macaques occur in evergreen forests, bamboo forests, and in deciduous forests. In Malaysia, they are abundant in coastal lowland forests. There they have been observed drinking much water and eating crabs. Of the various habitats occupied by crab-eating macaques, the swamp forests seem to have the highest density of them. In recent years, habitat alteration has expanded the range of some populations. In Malaysia, cleared land and plantation areas have been colonized by these macaques, and they appear to do better in disturbed habitats than they do in some pristine forests. /=\

Crab-eating macaques receive some protection in temple ruins in Thailand and food in temples in Bali. Some people in Bali consider them to be sacred. In Malaysia, crab-eating macaques are legally protected, and they are fed and protected in urban forests and parks. In the Philippines, there is much interest in protecting this species. In Indonesia, the species is well-protected, but some of the reserves are being considered for oil drilling, logging and plantation agriculture. In Thailand, and Borneo crab-eating macaques are hunted and captured for the pet and scientific animal trade. The export of this species is regulated by a quota system. /=\

Crab-Eating Macaque Characteristics and Feeding

Crab-eating macaques range in weight from three to seven kilograms (6.6 to 15.4 pounds). They range in length from 40 to 47 centimeters (15.75 to 18.5 inches). The greyish- brown or reddish colored tail is 50 to 60 centimeters long. Sexual Dimorphism (differences between males and females) is present: Males are about a third larger than females. The average weight for males is 4.8 to seven kilograms and three to four kilograms for females, [Source: Christopher Bonadio, Animal Diversity Web (ADW) /=]

The body fur of crab-eating macaques tends to be grey-brown to reddish brown. These colors are always paler on the front of their body compared to the back. Their face is brownish-grey with cheek whiskers. The eyes are directed forward for binocular vision. The nose is flat and the nostrils are narrow and close together (catarrhine condition). Crab-eating macaques have shovel-shaped incisors, conspicuous canines, and bilophodont molars. The tooth formula is I 2/2, C 1/1, PM 2/2, and m 3/3.

Crab-eating macaques are omnivores (eats a variety of things, including plants and animals). They eat leaves, fruit, flowers and fungi. Animal foods include insects and aquatic crustaceans. They exploit many different food types, reflecting the diversity of habitats they live in. The average length of feeding bouts is 18.3 minutes, with an average of 20 bouts per day. Clay may be eaten for the potassium found in it, although the potassium levels in the clay are low. About 96 percent of the feeding time per day is spent eating fruit. Some limited observations suggest that crab-eating macaques select fruit based on ripeness, which is based on color.

Crab-Eating Macaque Behavior

tool use by a Crab-eating macaques

Crab-eating macaques are arboreal (live mainly in trees), diurnal (active mainly during the daytime), motile (move around as opposed to being stationary), sedentary (remain in the same area), territorial (defend an area within the home range), social (associates with others of its species; forms social groups), and have dominance hierarchies (ranking systems or pecking orders among members of a long-term social group, where dominance status affects access to resources or mates). Their average territory size is 1.25 square kilometers, with a day range average of 1,900 square meters. [Source: Christopher Bonadio, Animal Diversity Web (ADW) /=]

Christopher Bonadio wrote in Animal Diversity Web: Crab-eating macaques live in multi-male groups consisting of about thirty members. At sexual maturity, males leave their natal group, and join either bachelor groups or new social groups. Since males leave the natal group, they are subject to more predation, disease, and injury than are females. Once a male finds another group in which to reside, he may replace some of the existing high-ranking males. Male replacement itself is a process in which a foreign male adult successfully takes over a resident male's harem position. These events are highly aggressive activities, and the participating adults are usually injured.

Despite aggression between males, this species is characterized by a genral lack of aggression between groups. Intergroup interactions are avoided/ When these groups meet, there tend to be high levels of aggression, involving chasing the non-resident group out of the area. Loud vocalizations and branch-bouncing are characteristic parts of these interactions. Males and females may use open mouth threats. This involves showing the enemy the incisors and canines, and pulling the ears and nose back. The alpha male is usually the one who leads the aggression by doing 90 percent of the branch-bouncing. Within groups, a strict linear dominance hierarchy develops among males. This hierarchy is determined by such factors as age, size, and fighting skills. /=\

Since females do not disperse, they are the stable core of a group. Females have a tendency to have close bonds with their maternal relatives throughout their lives. There is, however, a dominance hierarchy among females in a group. The acquisition of rank involves active intervention by maternal kin, and the differential treatment by unrelated members of the group. Grooming among females is a common activity. It is especially common for low-ranking females to groom higher-ranking females. This allows the lower-ranking females to receive less harassment, more support in aggresive interactions, and access to limited resources. /=\

Crab-eating macaques are unique among other non-human primates because of their ability to show learned or cultural behavior. This cultural behavior was observed in the preparation of food by crab-eating macaques. On one occasion, an adult female dipped a piece of fruit into a river and then she consumed it. It was proposed that perhaps the female was cleaning sand off the fruit. Scientists investigated this further on other individuals who showed this behavior. Some of the macaques washed sandy fruit in the river, but some of them also washed fruit the scientists had cleaned prior to distributing them. There were also crab-eating macaques that simply ate the cleaned fruits without washing them. The controversy of what cultural behavior means is still being researched. /=\

Male Monkeys Pay for Sex

According to the paper, "Payment for Sex in a Macaque Mating Market," published in the December 2008 issue of Animal Behavior male crab-eating macaques in Indonesia "paid" for sexual access to females — and that the going rate for such access dwindled as the number of available females went up. [Source: Krista Mahr, Time, January 7, 2008]

AFP reported: “Selling sex is said to be humankind's oldest profession but it may have deep evolutionary roots, according to a study into our primate cousins which found that male macaques pay for intercourse by using grooming as a currency. Michael Gumert of Nanyang Technological University in Singapore made the discovery in a 20-month investigation into 50 long-tailed macaques in Kalimantan Tengah, Indonesia, New Scientist reported.

On average, females had sex 1.5 times per hour. But this rate jumped to 3.5 times per hour immediately after the female had been groomed by a male -- and her partner of choice was likely to be the hunky monkey that did the grooming. Market forces also acted on the value of the transaction. If there were several females in the area, the cost of buying sex would drop dramatically -- a male could "buy" a female for just eight minutes of nit-picking. But if there were no females around, he would have to groom for up to 16 minutes before sex was offered.

Gumert told the New York Times that about 89 percent of the male-grooming-female episodes observed “were directed at towards sexually active females. When males groomed these females it appears they expected sex or at least genital inspection. By contrast when mothers groomed their young it appeared they did so to clean and sooth them.

Gumert told the New York Times, “What led me to think of grooming as a form of payment was seeing how it changed across different market conditions. When there were fewer females around, the male would groom longer, and hwn there were lots of females, the grooming times whet down. Males also grooomed females of high-rank considerably longer than ones of lower rank.

The work supports the theory that biological market forces can explain social behaviour, the British weekly says. "There is a very well-known mix of economic and mating markets in the human species itself," said Ronald Noe of France's University of Strasbourg. "There are many examples of rich old men getting young attractive ladies."

Monkeys Get Their Rock Off with Rocks

stone tools used by crab-eating macaques in Laem Son National Park in Thailand

In a study published online in August 2022 in the journal Ethology, scientists how monkeys in Indonesia engage in "self-directed tool-assisted masturbation" — getting their rocks off so to speak — by using actual rocks, supporting what's known as the sex toy hypothesis. Patrick Pester wrote in Live Science: Researchers studying long-tailed macaques (Macaca fascicularis) found that the monkeys repetitively tapped and rubbed their genitals with stones to pleasure themselves.[Source: Patrick Pester, Live Science, August 19, 2022]

The team found that males and females of different age groups all used stones to play with themselves, but there was some variation among the groups: Female monkeys were pickier about the stones they used, while young males engaged in the activity the most. Researchers watching the monkeys usually didn't have to wait long to observe the behavior. "You do see this genital stone tapping and rubbing quite regularly," lead author Camilla Cenni, a doctoral candidate at the University of Lethbridge in Alberta, Canada, told Live Science. "They are not, of course, constantly doing it, but if you stop and see them and they start playing with stones, they are likely going to do it."

Some macaque populations regularly manipulate stones as part of their behavioral repertoire, seemingly as a form of play. They carry stones around, rub them on surfaces and bash them together. This stone manipulation is likely cultural, because its only seen in certain populations, Cenni said. The "self-directed tool-assisted masturbation" described in the new study likely stems from this wider stone use. However, it's been documented in only one population of macaques in Bali, Indonesia. "When we talk about tool use in animals, we normally think about survival-dependent instances," Cenni said. For example, chimpanzees (Pan troglodytes) use stones to crack nuts so they can eat them. "There is an increasing number of studies that are suggesting that using objects as tools doesn't have to be a matter of survival. This is clearly an example."

The new research builds on a study led by Cenni and published in the journal Physiology & Behavior in 2020. The study first proposed the sex toy hypothesis in male macaques, while the new research looked at males and females and their potential motivation. Young males spent significantly more time engaging in the activity than mature males did. Mature males, in fact, were the least partial to stone masturbation, possibly because they had access to mature females. However, there was a lot of variation among individual macaques of both sexes. "Within those groups, you have monkeys that do it way more than others," Cenni said.

The monkeys were urban-dwelling macaques living in and around the Sacred Monkey Forest Sanctuary in the town of Ubud. They are free-roaming and fed by people. The researchers suggested that the feeding might relax pressures on the monkeys to forage, leading them to engage more in the stone behaviors. In other words, they have more time on their hands than other monkeys, and they choose to spend it with the stones.

Crab-Eating Macaque Mating, Reproduction and Offspring

Crab-eating macaques are polygynous (males having more than one female as a mate at one time) and polygynandrous (promiscuous), with both males and females having multiple partners. They engage in seasonal breeding — usually from December to February, with births peaking from May to July, corresponding with the rainy season. Females have an average interbirth interval of 390 days., with the average number of offspring being one. [Source: Christopher Bonadio, Animal Diversity Web (ADW) /=]

Christopher Bonadio wrote in Animal Diversity Web: In Sumatra, each social group of these monkeys contains an average of 5.7 males and 9.9 adult females. Although females outnumber males, indicating that males have more mates available to them than do females, both sexes may mate with multiple partners. However, the alpha male, or highest ranking male, of the group has the greatest access to mates of any males, and probably sires the most offspring. /=\

Higher-ranking females become reproductively mature before lower-ranking females. The offspring of these higher-ranking females have a greater chance of survival than do the offspring of lower-ranking females. Both of these phenomena are related to the greater food availability and decreased amount of aggression experienced by the higher ranking females. /=\

Female crab-eating macaques show a conspicuous cyclicity of sexual behavior during their menstrual cycle. As they approach ovulation, females experience a swelling of the skin in the perineal region. However, there has been no direct correlation between the frequency of copulation and the degree of swelling of genital tissues. This concealment of ovulation could exist in order to persuade consorting males to stay with females longer. /=\ As a side effect of the prolonged estrus period and concealment of ovulation, it is difficult for humans to estimate the length of gestation. However, it seems likely that the gestation averages approximately 162 days.

Young are nursed until they are approximately 420 days of age. Both the length of nursing and the interbirth interval may be affected by the rank of the mother. On average females reach sexual or reproductive maturity at age four years while males do so at age 4.2 years. Females provide the bulk of the care for offspring. The young receive nourishment, encouragement, grooming, protection, and teaching from the mother. Young females are likely to remain in the troop into which they were born, whereas males are likely to emigrate at or near sexual maturity. /=\

Image Sources: Wikimedia Commons

Text Sources: Animal Diversity Web animaldiversity.org ; National Geographic, Live Science, Natural History magazine, David Attenborough books, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Discover magazine, The New Yorker, Time, Reuters, Associated Press, AFP, Lonely Planet Guides, Wikipedia, The Guardian, Top Secret Animal Attack Files website and various books and other publications.

Last updated December 2024