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.

[7] Another common name for M. fascicularis, often used in laboratory settings, is the cynomolgus monkey which derives from Greek Kynamolgoi meaning "dog milkers".

It is likely that crab-eating macaques were introduced to Timor and Flores (both on the east side of the Wallace line), by humans around 4,000–5,000 years ago.

[1] Rhesus and crab-eating macaques hybridize within a contact zone where their ranges overlap, which has been proposed to lie between 15 and 20 degrees north and includes Thailand, Myanmar, Laos, Vietnam.

This primate is a rare example of a terrestrial mammal that violates the Wallace line, being found out across the Lesser Sunda Islands.

It also easily adjusts to human settlements and is considered sacred at some Hindu temples and on some small islands,[32] but as a pest around farms and villages.

[27] 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.

[27] Between 1988 and 1994, a total of 520 crab-eating macaques including 58 males and 462 females were released on Tinjil Island for the purpose of starting a natural habitat breeding facility.

[35][36] This may be a sustainable way of supplying monkeys for research, but it is in a legal gray area for trading regulations, using captive bred codes (F, C) rather than wild-caught (W).

[2] Interactions have been reported between crab-eating and southern pig-tailed macaques, Colobinae species, proboscis monkey, gibbons and orangutans.

[2] Dusky leaf monkeys, crab-eating macaques and white-thighed surilis form tolerant foraging associations, with juveniles playing together.

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.

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 spitefully toward one another.

[54] Infants are born with black fur which will begin to turn to a grey or reddish-brown shade (depending on the subspecies) after about three months of age.

[44] Crab-eating macaques are omnivorous frugivores and eat fruits, leaves, flowers, shoots, roots, invertebrates, and small animals in variable quantities.

[17] Crab-eating macaques can become synanthrope, living off human resources when feeding in cropfields on young dry rice, cassava leaves, rubber fruit, taro plants, coconuts, mangos, and other crops, often causing significant losses to local farmers.

[64] describing crab-eating macaques in Thailand using ax shaped stones to crack rock oysters, detached gastropods, bivalves, and swimming crabs.

[70] Token-robbing and token/reward-bartering are cognitively challenging tasks for the Uluwatu macaques that revealed unprecedented economic decision-making processes, i.e., value based token selection and payoff maximization.

This spontaneous, population specific, prevalent, cross-generational, learned and socially influenced practice may be the first example of a culturally maintained token economy in free-ranging animals.

[71] The crab-eating macaque has been categorized as Endangered on the IUCN Red List; it is threatened by habitat loss due to rapid land use changes in the landscapes of Southeast Asia and the surging demand by the medical industry during the COVID-19 pandemic.

M. f. umbrosa 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.

China banned animal trade in January 2020 due to concerns of COVID-19, yet this cannot account for the significant decrease in crab-eating macaque exports in 2019, the drivers of this decline are still unclear.

[79] In November 2022, following a five-year investigation by the DoJ and US Fish and Wildlife, the DoJ indicted Cambodian government officials and Cambodian owner and staff of Vanny Bio Research Corporation LtD, a macaque breeding center in Cambodia, for their alleged involvement in laundering wild-caught monkeys as captive bred.

[38] Unfortunately, the crab-eating macaques involved in the Cambodian smuggling ring imported by Charles River are in limbo – they are ineligible for research but they cannot go back to the wild either.

[1] These interactions include the skyrocketing demand for crab-eating macaques by the medical industry during the COVID-19 pandemic,[1] and the rapid development of the landscape in Southeast Asia.

[87][page needed] Humans and crab-eating macaques have shared environments since prehistoric times, and both tend to frequent forest and river edge habitats.

[75] Crab-eating macaques are used primarily by the biotechnology and pharmaceutical industry in the evaluation of pharmacokinetics, pharmacodynamics, efficacy, and safety of new biologics and drugs,[90] they are also used in infectious disease, TB, HIV/AIDS, and neuroscience studies.

[91] The use of crab-eating macaques and other nonhuman primates in experimentation is controversial with critics charging that the experiments are cruel, unnecessary and lead to dubious findings.

[96][97][98][99][100] In June 2023, BBC exposed a global online network of sadists who shared videos of baby long-tailed macaques being tortured by caretakers in Indonesia.

There were many torture methods, from teasing the primates with baby bottles to killing them in blenders, sawing them in half, or cutting off their tails and limbs.