Rhesus Macaque, Macaca mulatta
Geographic Distribution and Habitat
Rhesus macaques, also referred to as rhesus monkeys, are Old World monkeys from Asia that range in geographic distribution from Afghanistan to the Pacific coast of China, including India, Bhutan, Laos, Nepal, Bangladesh, Thailand, Vietnam, and Pakistan. They boast the largest native range of any other nonhuman primate species. Rhesus macaques are extremely adaptable monkeys that can be found in elevations ranging from sea level to as high as 13,123 ft (4,000 m). They dwell in tropical and temperate habitats including deciduous, semidesert, mixed deciduous, bamboo, temperate, and tropical forests, as well as mangroves and swampland.
Introduced colonies of rhesus macaques are also found in the United States, specifically in Florida and South Carolina, as well as the U.S. territory Puerto Rico. As a way to gain a larger tourist population, six rhesus macaques were introduced to Florida’s Silver Springs State Park in the 1930s; another six were introduced in 1948. Being the excellent swimmers that they are, they quickly made their escape from Silver River Island by swimming into neighboring forests, and eventually into human developments. Morgan Island, a Sea Island in South Carolina, is also home to a sizable colony of introduced rhesus macaques. They were imported in the 1970s for the purposes of biomedical research in local laboratories. In addition, the tiny Caribbean island of Cayo Santiago, Puerto Rico, houses approximately 1,000 rhesus macaques that have been studied in a free-range colony since they were introduced in the 1930s.
Size, Weight, and Lifespan
Rhesus macaques are medium-sized monkeys. Gender dimorphism is present in this species; males and females display differences in body size. Males measure on average 1.7 ft (53 cm) and weigh, on average, 17 lb (7.70 kg), while females have an average height of 1.5 ft (47 cm) and an average weight of 11.8 lb (5.34 kg). Their medium-length tails average between 8.2 and 9 in (21–23 cm).
The lifespan of a rhesus macaque is between 25 and 30 years. They live between 20 and 40 years in captivity; their lifespan is smaller in the wild due to fights, predators, and humans.
A matriline is a line of descent from a female ancestor to a descendant (of either sex) in which the individuals in all intervening generations are mothers – in other words, a “mother line.”
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Rhesus macaques charismatic monkeys that range in color from pale brown to auburn with intertwined streaks of gray hair. Their underbellies are a faded brown color. Their expressive faces are hairless and reddish-pink in tone, a color that matches that of their bottoms. Their pink ears appear large and pointed.
Underneath their protruding eyebrows, their eyes are almond-shape and yellowish-brown in color. Their nose bone is narrow and rather flat, and ends with slanted nostrils. Their mouth protrudes out and their lips are thin.
Babies don’t have as much hair as adults; their face and the top of their head is bare, pink, and wrinkly. Their ears appear larger and more pointed than those of their adult counterparts. The hair on the inside of their arms and legs and on their chest is very light, exposing large patches of pink skin. Their hair thickens as they age.
Their large cheek pouches are used to store food while foraging through their surroundings.
Rhesus macaques are omnivorous. They consume a large number of plants, roots, seeds, bark, and fruit. They also eat insects, eggs, and chicks.
Groups that live closer to human communities glean a considerable amount of their diet from human activities. They raid crops, snack on leftover food from garbage cans, and even steal directly from people they encounter. Up to 93% of their diet can come directly from humans, whether as handouts or from foraging in their crops and farms.
Human-provided food typically includes bananas, bread, peanuts, fruits, and vegetables. Because their palate is so extensive, rhesus macaques tend not to have any issues finding sustenance. This is one of the keys to their success.
Rhesus macaques that live in forested areas forage for their food. Everyone in the group that discovers any food is expected to vocalize and call out, so others can partake of the feast. However, generally speaking, females call out to signal the location of a good food source more frequently than males. Furthermore, females of a large matriline tend to signal when they discover a food source more frequently than those with a smaller matriline. Unless they are high-ranking, individuals who do not call out to signal the location of a good food source are more than likely to get a beating when others find out.
Humans are the only other primates with a broader geographic distribution than the rhesus macaque.
The Latin name of the species is Macaca mulatta. Macaca means macaque and mulatta means dark or black.
Because rhesus macaques are so successful at adapting to changing environments and even to the presence of humans, they are referred to as “weed macaques” in some primatology circles—a reference to the fact that weeds can rapidly grow and spread anywhere.
It is estimated that half the population of rhesus macaques in India live among humans in villages and urban areas.
In the mid-1930s, a scientist named Clarence Ray Carpenter decided to bring 500 rhesus macaques from Asia to Cayo Santiago, a small island off the coast of Puerto Rico. Initially, a lot of them fought and died, but between 1940 and 1942, the population grew so rapidly that food had to be imported for them to survive. This population has been used to provide subjects to research labs in the US.
Rhesus macaques are very commonly used in research and many were imported from India until the 1970s, when the country banned primate export. China and Nepal have since been the main suppliers.
Another group of rhesus macaques was introduced by Colonel Tooey, a glass-bottom boat manager, in Silver Springs State Park in Florida in the 1930s. By releasing six monkeys in the park, he hoped to increase tourism in the area and generate more revenue for his business. Little did he imagine that the monkeys he initially confined to an island in the Silver River would swim and populate the surrounding forests. The population grew rapidly from six monkeys in the 1930s to 400 in the 1980s. Many were trapped and sold for bio-medical research. The current population is around 200, but some migrated to the Ocklawaha River and the size of that population is unknown.
Behavior and Lifestyle
Rhesus macaques are highly adaptable to coexisting alongside humans, which is most commonly seen in India, where the monkeys are considered to be sacred creatures and are left undisturbed.
While they are predominantly quadrupedal (that is, they walk on all fours), they can be both arboreal (tree-dwelling) and terrestrial (ground-dwelling), depending on the habitat in which they are located.
They are active both during the day and at night, and are considered to be boisterous creatures. Feeding, resting, traveling, grooming, and playing encompass the majority of day-to-day activities. Climate and weather affect the types of daily activities that the rhesus macaque partakes in. The hotter the season, the lazier and more inactive they are, as opposed to their more active temperate months.
Rhesus macaques are gregarious and live in groups with several adult males and many more females of all ages and their offspring. These females and their young belong to different matrilines, which can include up to six generations. Group size varies depending on whether they live in urban or forested habitats. A study conducted in Bangladesh, for instance, revealed that urban groups include between 22 and 90 individuals, whereas forest groups include between 10 and 78 individuals, for an average of 30 individuals in most places.
These are bold, extremely curious, and adventurous monkeys. Their society is based on a strong hierarchical system in which high-ranking individuals behave like despots and subordinates are at their mercy.
Males disperse away from their native group at adulthood and have to work hard at establishing friendships throughout their lives. All females in a group, however, are related to one another (grandmother, mother, sisters, daughters, and cousins) and form a strong and supportive social network. They only allow a few selected adult males to stay in the group in exchange for protection against other macaques and predators.
When members of different groups encounter one another, they react with fear and aggression. If the groups have never met before, rhesus macaques are willing to kill in order to establish dominance. They know exactly where to strike—biting the face, extremities, or genitalia of an opponent is sure to deliver a fatal blow or, at the very least, cause permanent damage.
If the groups know each other, all the dominant group has to do to maintain its status is threaten or chase members of the other groups. Male mortality increases during mating season, when they sometimes inflict serious wounds that become infected.
Hierarchies remain rigid for a long time and changes only occur when females decide so. In the wild, if groups become too large, for instance, a subordinate matriline may move out to form its own group and, at the same time, elevate its rank.
In captivity, such opportunity is not available. There is no place to go; the only way for one or more subordinate matrilines to change their status is to engage in a bloodbath against the dominant matriline, until it is overthrown. There is no planning involved. The subordinates just take advantage of an opportunity as it arises. Males usually keep to themselves unless they have no choice—like male infants or juveniles of a matriline at war. Overthrown alpha females become the lowest-ranking females in the group and have quite a bit learn to adapt to their new situation.
Rhesus macaques live in multi-male, multi-female troops that contain between 10 and 80 members, with 4 times as many females as males, regardless of the habitat they live in. Females remain in their natal groups and dominate others based on hierarchical level within the group. In contrast, promiscuous males migrate to other groups in search of mating opportunities throughout their lifetime. This emigration begins just before puberty.
Physical proximity is dangerous for anyone—except the alpha male and the alpha female. Rhesus macaques have zero tolerance. A low-ranking individual may sit next to a family member, but if they are not related, that individual runs the risk of being attacked or is required to provide extensive grooming services for the privilege (although grooming and allo-grooming are most common between members of the same matriline). The strongest bonds are between mothers and infants or juvenile daughters, and between sisters.
When a fight occurs, unlike with most other primate species, rhesus macaques do not necessarily reconcile. In fact, they are known to hold lifelong grudges. If a reconciliation occurs between two previous opponents, it can only be initiated by the aggressor because the hierarchy is too rigid to allow low-ranking individuals to approach dominant ones.
Rhesus macaques are constantly competing—not so much for resources, but for dominance. Each individual in the group is aware of his or her own status and the status of everyone else around them. Some are more dominant. Individuals that are dominant over many others can get whatever they want and walk with their tails held high.
The alpha male is dominant over everyone in the group, both males and females, who in turn are dominant over different group members. But it is important to note that males are not necessarily dominant over females. During a fight, females are backed up by their family members and can defeat a male. The alpha female benefits from the protection of the alpha male. She is dominant over all the females and all the adult males in the group. She has so much power that she can alter the fate of any male by not acknowledging him as the winner of a fight; she can even decimate the alpha male by forming an alliance with another male.
Females within a group are organized in hierarchical matrilines, so members of one matriline can be dominant over members of another matriline. Daughters remain subordinates to their mothers for life. Sister hierarchy goes in reverse order so the youngest has the highest rank and the elder has the lowest. While male dominance is volatile, female hierarchies are very stable and usually last a lifetime.
Low-ranking individuals find out from a very young age that to survive, it is best not to attract the attention of dominant members of the group; they learn to be quiet and sneaky about everything from food to relationships.
Communication among rhesus macaques is primarily achieved through vocalizations and visual cues. The social fabric of the rhesus macaque society is complex and so is their communication. They use facial expressions that involve a variety of mouth positions (i.e., mouth open, grimace, lip-smacking, chin up, yawn, teeth-chatter, or puckered lips), ear positions (forward, back, or flap), and eyebrow positions (raised, lowered, or flashing). The head and tail position of macaques, as well as their eye gaze, give out important information to individuals they are interacting with. A silent threat is characterized by a stare with a lowered head, ears pointing forward, and an open mouth. A friendly approach, on the other hand, is characterized by a tail wag, ears pulled back and lip-smacking.
Rhesus macaques also use different vocalizations to communicate about everything, from where food is located to the types of predators roaming around. When vocalizing, they use fewer facial expressions than when they are silent. But ear flapping, lunging, crouching, piloerection, chasing, fighting, and moving back and forth are often accompanied by vocalizations.
Since their lives are intertwined with each other, all members of a group need to pay attention to the signals that others send. Subordinate individuals have to keep track of the dominant individuals at all time and use the appropriate signals to avoid conflict. When subordinates are face to face with the alpha male, for instance, the right thing to do is to flash their teeth to signal submission. Bare teeth are not a sign of aggression; rather, they indicate fear. If the alpha male is coming from another direction, low-ranking macaques present their behinds to him as a sign of respect.
When a fight is about to break out, the aggressor practices “show-looking”—i.e., he or she looks around, tail held high, standing on all fours, inviting willing participants to join in. He or she may also get them excited by screaming. Those around who want nothing to do with the conflict avoid eye contact and walk away. Lower-ranking males may help a higher-ranking aggressor in the hope of raising their own status. The loser may, in turn, chase another individual and let him have it. The most efficient way to dissuade anyone from attacking them in the future is for macaques to redirect the aggression against a family of the aggressor.
Mothers use grunts and gurneys to talk to their infants. Some females even wag their tails while engaged in these vocalizations. Young macaques use “gecker” calls (best described as “ik ik ik” screams with a body jerk) when in distress to attract their mothers’ attention. These abrupt high-amplitude vocalizations are easily located and hard to ignore. They are mostly heard when an infant is engaged in grooming or play with her mother, while following the mother, or when the mother moves away and leaves the child behind. Males start using them as early as 1 or 2 months of age, but both males and females use them regularly from the time they are 4 months old. Females’ gecker bouts are much higher in pitch and last much longer than those of males. Mothers respond more often to male offspring; they always respond to young infants 4 months old or younger, but totally ignore macaques older than 12 months.
If lower-ranking macaques want to approach and groom a dominant macaque, they indicate their intention through lip-smacking and little grunts. To request grooming services from another individual, a macaque also lip-smacks and lies in front of the other monkey to present the part of the body that needs it the most. The opposite is true too. Before approaching a low-ranking female, the alpha male must lip-smack and pucker (i.e., purse his lips and retract his scalp or bare his teeth) to let her know he is not a threat and wants some love. Males can also communicate their love interest by performing a ritual dance—they take a few steps toward her, turn around, then come back a little closer and repeat the moves a few times.
Males reach puberty at 4 or 5 years old, but their reproductive success depends on their skills and luck at finding willing females. All males have great sexual appetites; this is why, as soon as they become successful with females, the older fellows in the group kick them out. These young studs have no alternative but to find a new group. Some do not make it, starving in the transition or getting eaten by predators. If they manage to find a new group, it is best for them to keep a low profile and use any opportunity to work their way up the ranks overtime—provided females like him. The alpha male mates with the alpha female, her family members, and many other females in the group.
Females become fertile at 3 or 4 years old. At that age, the skin on their face and genitalia becomes redder. They decide the type of relationship they want. Sometimes they choose a passing fling with a seemingly random outsider; other times, they opt for a long-term arrangement with a male from the group in exchange for protection. They communicate their intentions by following a male around, sitting next to him, and presenting their behind to him. They are insistent and sometimes even slap males who do not respond to their advances. They go into estrus during the rainy season when food is most abundant, and give birth to a single offspring after six months of gestation. Interestingly, there are usually about the same number of male and female infant births, but mortality is high, especially for males.
Once they have given birth, females distance themselves from males and stay together for about six months in order to protect their offspring from infanticide by outside males. After that period, they are ready to mate again. Female rhesus macaques can reproduce every year, but their fertility declines with age. To ensure they are ready for the next mating season, mothers start discouraging babies from suckling as early as three weeks old. Otherwise, they appear quite devoted to their own offspring. They touch them, talk to them and spend hours grooming them. They allow young females to visit but never to babysit. They constantly watch out for their offspring and do not hesitate to jump in a fight to protect them from other macaques or predators. But they never ever share their food with them (or anyone else for that matter). In fact, they have no qualms about slapping infants that show too much interest in their meal and have been know to actually steal their baby’s food right out of their mouth. As for their girlfriends’ babies, they remorselessly pull, drag, hit, and even bite them; some infants occasionally die from this.
Infants start eating solid food within a few months of birth and are weaned by the time they reach their first birthday. They learn about different food sources from their mothers and other individuals in the group who call out when food is found.
Young rhesus macaques seem to understand quickly the workings of the hierarchy they were born into and start acting dominant toward the sons and daughters of females that rank lower than their own mothers. They also learn social skills like grooming, but not until they are 6 or 8 months old. Juvenile females groom older females and young macaques and start grooming males at puberty. Males make friends with other males the same age or older and only groom them. In fact, they don’t groom any females until they move out of their native group.
While the ecological role of the rhesus macaque has not fully been described by researchers, it has been suggested that they help to disperse fruit seeds, and may possibly affect predator population since they are a prey species.
The rhesus macaque is listed as Least Concern on the International Union for Conservation of Nature Red List of Threatened Species (IUCN, 2015). Despite the many disturbances caused by human activities—like deforestation and land conversion—that have brought other species to the brink of extinction, rhesus macaques have adapted well and even thrive. More and more are being displaced and find themselves in or close to urban areas. Those that live near temples can be revered and fed by devotees, but in general, they are not welcome by humans because they destroy gardens, pillage fruit trees, and raid crops. Regular conflicts consequently occur, which often result in the beating and killing of monkeys and severe scratches or bites to humans. Some measures to protect the human population include translocation of rhesus macaque groups to non-urban areas. This is only a temporary solution as groups reappear in villages and cities regularly. Other management techniques include the vasectomy of dominant males, the distribution of contraceptive medicine to adult females as well as the use of olfactory, taste, and noise deterrents.
While they do have natural predators such as dogs, weasels, leopards, tigers, sharks, crocodiles, and snakes, rhesus macaques are generally unthreatened by highly disturbed environments. The most significant threat they face is being abducted from their homes for laboratory or biomedical research purposes. Due to their anatomical and physiological closeness to humans, they are the nonhuman primate of choice on which to conduct research on human and animal health-related topics. The development of smallpox, rabies, and polio vaccinations, the discovery of Rhesus factor in blood, the creation of HIV/AIDS-managing drugs, and understanding of the female reproduction cycle are just some of the few ways in which rhesus macaques have been used for research in laboratories.
Because their population grows so rapidly, these monkeys can be an environmental threat themselves. On Morgan Island in South Carolina, where the population is used for medical research, tidal creeks around the island have elevated levels of E-coli. They destroyed the mangroves on two islands in the Florida Keys and severely reduced the population of shore birds in Puerto Rico. They are also a potential threat to humans through bites and scratches, as they can carry asymptomatically the Herpes B virus.
Rhesus macaques are listed on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II. Many rhesus macaques inhabit protected areas throughout the wide range of countries in which they roam and protection measures vary in these locations. Because their population grows rapidly and is extremely adaptable to various habitats, most efforts concerning the species focus on population management and avoidance of conflicts with humans.
Although difficult to enforce regulations that protect these primates, there are several conservation acts around the world that attempt to preserve the rhesus macaque species. Some of these include:
- Schedule I as a protected animal in the new Wildlife Conservation and Security Act, 2012
- Schedule III in the Bangladesh Wildlife Preservation Amendment Act, 1974
- Schedule I, Part I in the Indian Wildlife Protection Act, amended up to 2002
- Category II of the Chinese Wildlife Protection Act, 1989
- Nepalese National Parks and Wildlife Conservation Act, 1973
- Acoustics and behavioral contexts of “gecker” vocalizations in young rhesus macaques (Macaca mulatta) – Erik R. Patel and Michael J. Owren
- Social rank and cortisol among female rhesus macaques (Macaca mulatta) – Dong-Dong Qin, Joshua Dominic Rizak, Xiao-Li Feng, Xun-Xun Chu, Shang-Chuan Yang, Chun-Lu Li, Long-Bao LV, Yuan-Ye Ma, Xin-Tian Hu.
- Macachiavellian Intelligence, How Rhesus Macaques and Humans Have Conquered the World – Dario Maestripieri
- Behavior and Social Dynamics of Rhesus Macaques on Cayo Santiago – Diario Maestripieri and Christy L. Hoffman.
- A comparative Study of Reconciliation in Rhesus macaques and Token macaques – C. Demaria and B. Thierry.
- Costs of deception: Cheaters are punished in Rhesus monkeys (Macaca mulatta) – Marc D. Hauser.
- Single and Multichannel Signal Composition: Facial Expressions and Vocalizations of Rhesus macaques (Macaca mulatta) – Sarah R. Partan
- Distribution of Rhesus macaques (Macaca mulatta) in Bangladesh: Inter-Inter-Population Variation in Group Size and Composition – Md. Kamrul Hasan, M. Abdul Aziz, S.M. Rabiul Alam, Yoshi Kawamoto, Lisa Jones-Engel, Randall C. Kyes, Sharmin Akhtar, Sajeda Begum and M. Mostafa Feeroz.
- History and Status of Introduced Rhesus macaques (Macaca mulatta) in Silver Spring State Park, Florida – C. J. Anderson, S. A. Johnson, M. E. Hostetler and M. G. Summers.
Written by Sylvie Abrams, February 2019