Rhesus Macaque, Macaca mulatta
RHESUS MACAQUE
Macaca mulatta
Geographic Distribution and Habitat
Rhesus macaques, scientifically known as macaca mulatta and commonly known as rhesus monkeys, are native to mainland Asia. Their native habitats span across Afghanistan, Pakistan, India, Bhutan, Myanmar, Nepal, Bangladesh, Thailand, Laos, Vietnam, and China. While native to these regions, rhesus monkeys have found themselves on completely different continents. Rhesus macaques are incredibly adaptable to different environments with the largest native range of any nonhuman primate, giving them the namesake as the most global of the nonhuman primates. Their environments range from arid grasslands to woodlands, and all the way to mountainous regions at altitudes as high as 13,123 feet (4,000 meters). Since they are both arboreal and terrestrial they can be found both in the trees and on the ground.
Outside of Asia, rhesus macaques have been introduced to areas in the US. In the 1930s six monkeys were introduced to an island on the Silver River in Florida. With their excellent swimming skills, the rhesus macaques escaped and managed to create a stable colony along the river. Further south, approximately 1,500 macaque monkeys thrive on a small island one kilometer off the coast of Puerto Rico on Cayo Santiago. These individuals were brought over from India and have an ever-growing population that has been observed since they arrived in 1938. Aside from Florida and Puerto Rico, rhesus monkeys have been relocated to various research facilities across the country.
There is some discussion on the subspecies of rhesus macaques with a few sources that mention the existence of Chinese and Indian subspecies. The Chinese-derived subspecies are M. m. vestita, M. m. lasiota, M. m. sanctijohannis, and M. m. brevicauda. Indian-derived subspecies are M. m. mulatta and M. m. villosa They are not all from India but are informally labeled “Indian-derived”. While some sources make note of these subspecies, they are not well documented elsewhere and are largely ignored in most sources about the rhesus macaque.
Size, Weight, and Lifespan
Rhesus macaques are considered medium-sized monkeys at 18-22 inches (45-55 cm) in total length, with tails that add 8.27-9 inches (21-23 cm), and 8.82-22.05 pounds (4-10 kgs) in weight. Sexual dimorphism is present in this species, with males up to 44% larger than their female counterparts.
Wild and free-ranging macaque monkeys have a lifespan of 25-30 years. In captivity, they can live up to 40 years old.
Appearance
Rhesus macaques can range from dark brown to light golden in their coloring with eyes of various shades of brown. There are some monkeys with blonde hair and blue eyes existing in colonies alongside their more brunette, brown-eyed companions. Their underbellies are typically lighter tan and their faces are a light pink skin color.
As infants, there is less fur so more of their pink skin can be seen. As they get older their fur thickens and creates an even and slightly lustrous coat all over their bodies. Rhesus macaques notably lack fur in five areas, their face, their sex organs, their bottoms, their ears, and their hands. The skin of their faces and ears are light pink while their sex skin and bottoms are more of a bright red. Their hands have opposable thumbs and are usually dark brown to black.
Macaque monkeys have large pouches for food storage during foraging. With their non-grasping (nonprehensile) tails used for balance, they are primarily quadrupedal but occasionally use their hind legs to stand bipedally.
Diet
Macaques, just like in their environmental flexibility, are also very flexible in their eating habits. They are omnivores and will eat fruits, seeds, leaves, bark, grasses, roots, fungi, insects, and fish.
Macaques will consume what is dominant in the environment that they are in. Those who live in human-dominated environments will glean much of their diet from human crops, waste, and simply stealing from humans directly.
Since food is a limited resource, there is a lot of competition among group mates. Food is typically foraged and then taken from the area it was found in to avoid aggressive interactions.
Behavior and Lifestyle
Rhesus macaques are not only adaptable in their habitat and diet, but their adaptability is also evident in their daily lives. Rhesus monkeys are curious, highly intelligent individuals and typically spend most of their day foraging for food, interacting with and grooming group mates, or resting. Younger monkeys also spend a large chunk of their time playing. With frequent interactions among individuals, macaque monkeys are highly sensitive to group hierarchies and dynamics.
Since most of their day consists of foraging, rhesus macaques spend hours combing through grasses and trees with fellow group mates. They are primarily terrestrial and partially arboreal, meaning they spend most of their time on the ground and sometimes up in the trees. Once they’ve located a food source, they typically bring their food to a second location and use their fingers to pry bark apart, nails to pick out seeds, and their grasping hands to hold onto fruits. Since dominance hierarchies are heightened in the presence of limited resources, consuming food away from the group eases the pressure on the individual who might need to fight someone off for their resource. As you go up the social ladder, monkeys are more bold in their dining locations.
Parallel foraging activities reinforce community bonds and encourage group cohesion by promoting frequent interactions. This can quickly cause aggressive interactions which may at first appear to destroy relationships that one may assume would cause groups to fall apart, but in fact does just the opposite. Since rhesus monkeys interact so frequently and are so social, group infighting is routine and is necessary to maintain the social order they thrive in.
Rhesus macaques are considered the most despotic and nepotistic of the nonhuman primates. They have an incredibly strict social hierarchy fueled by aggressive and affiliative interactions. As mentioned above, infighting is one of the biggest threats to the rhesus macaque’s survival. They are extremely violent with each other and are ruthless in their attacks. Rhesus monkeys are strategic in their blows, aiming for the eyes, the whole face, limbs, and genitalia in an effort to permanently hurt or ultimately kill the opponent.
One of their most frequent peaceful activities is allogrooming. Monkeys of all ranks methodically groom a partner individual in search of parasitic bugs or to clear away debris from the fur. This activity promotes strong social bonds within the group and solidifies social hierarchies. A large chunk of their day is spent tending to their group mates.
Only younger rhesus macaques play. In areas with bodies of water, rhesus monkeys dive, jump, and swim with each other. While both sexes enjoy splashing around, there are oftentimes many more young males on the playground than females. As they get older, the play starts to evolve into bullying and more aggressive encounters. Male juveniles form little gangs to harass other animals or group mates.
Rest is very important to rhesus macaques. They are diurnal, or active in the day, but take frequent naps. They are extremely social while resting and clump into small groups to rest. In the summer when the weather is hot, the monkeys find shade and spend more time lying around. Once the weather starts to cool down in the colder months, the rhesus monkeys spend more hours of the day active.
Rhesus macaque’s natural threats vary by environment with some of the biggest offenders being unsuitable cold climates, starvation, natural predators, and infighting. With monkeys that live farther north, unusually cold winters can cause starvation and hypothermia. In warmer climates, natural predators such as snakes, hawks, eagles, and large cats target juvenile monkeys. In order to evade their predators, rhesus macaques have alarm calls that alert others in the area of a potential threat. They will retreat to tree tops to gain higher ground and locate the danger.
Rhesus macaques have generally medium brown fur with brown eyes but there are some blonde blue-eyed monkeys too!
Rhesus macaques are really good swimmers! Before the relocation of macaques to islands in Silver Springs Florida, Cayo Santiago Puerto Rico, and Morgan Island South Carolina, scientists assumed that they were bad swimmers like their chimpanzee relatives. How wrong they were! While they aren’t so good as to escape islands further than .62 miles (a kilometer) away from the nearest shore, they can still swim down the rivers of Silver Springs!
Smiling is not friendly to a macaque, in fact it is a fear response. Seeing a monkey smile unfortunately means that they are afraid of something.
A yawn can be just a yawn but it can also signal an uncomfortable and tense social situation where the yawner will show their fangs casually as a warning.
The name “rhesus” is the name of a minor character from the Iliad, “Rhesus of Thrace”, but the person who named rhesus monkeys, Jean-Baptiste Audebert, said the name has no meaning.
Rhesus macaques live in gregarious multi-male and multi-female groups with an average of 32 individuals, but group sizes can reach as large as 250. Group size depends heavily on the environmental constraints of their habitat and groups are constantly fluctuating. Males leave their natal, or original, group once they reach sexual maturity while females are philopatric, meaning they stay in the group they are born into. In a typical group, there are as many as 3 times the number of females to males.
Since females stay in the group and males move to new groups once they reach sexual maturity (and sometimes afterward), the social system is organized around matrilines. Female hierarchy is first determined by the matriline they are born into and second by their age, with younger females outranking their older sisters. For males, since they leave their natal group and arrive in a new group, their hierarchy is determined by the amount of time they’ve spent in their new group. Surprisingly, aggressive rank changes are not common in male rhesus monkeys. They wait patiently in a group while their rank goes up, having aggressive and affiliative interactions to solidify their place in the hierarchy. For females, the hierarchy is very stable except for when a lower-ranking matriline tries to overtake the top-ranking matriline. In this case, the entire social system is sent into a chaotic bloodbath and the alpha female falls to the very bottom of the hierarchy. Males typically stay out of the scuffle if they can help it, but are occasionally drawn in as juveniles.
Rhesus monkeys operate on a win-lose system with the dominant monkey always coming out on top unless it is an unstable period within the hierarchy. An interaction is initiated by encountering another individual. The dominant individual “wins” if the other monkey runs away in fear or communicates an acknowledgment of their submission. Aggressive interactions can be anywhere from taking someone’s seat on a log to blatant physical attacks, where a dominant individual attacks a subordinate individual for some transgression. Transgressions include passing too close by, eating food that “belongs” to the dominant monkey, or anything considered by the dominant macaque as not being submissive enough.
A very common interaction is called displacement. If a subordinate individual sits on a log, they will move out of the way of a dominant individual who is walking towards them to take their spot. Small interactions between rhesus macaques can very quickly become lethal and if the subordinate does not move, the subordinate risks angering the dominant and risks their own life. There is no such thing as ‘minding your business’ as a macaque because they must always be vigilant of a dominant in their vicinity.
Since rhesus macaque interactions are based on a win-lose system, rhesus monkeys in captivity have a harder time maintaining the strict social hierarchy the species is known for. While much of their interactions occur by physically being near groupmates, there are still other forms of communication that can help solidify the hierarchical system within a group.
Rhesus macaques also interact with other animals in their habitat—the most notable being humans. In areas with large human and monkey populations living side by side, monkeys have been documented stealing, bullying, and generally interacting with humans. To many humans in these environments, rhesus macaques are considered pests and go to great lengths to keep them out of homes, restaurants, and grocery stores. Aside from humans, they are known to “bully” other animals as a form of exploration. They can poke lizards, pick at bugs, or pull the tails of rats to understand what they are.
Rhesus macaques have very sophisticated communication skills which are cultivated due to their very strict social hierarchy. While many rhesus monkey interactions occur physically, communication can also occur using vocal signals and body language. They can be used to convey threats, communicate submission or dominance, and can also be used to call attention to a potential threat.
Communication in the form of body language involves facial expressions, gaze and gaze following, postural signals, and object manipulation to convey a message.
For humans, facial expressions are important when conveying information. Rhesus macaques have a few more than half of the facial muscles we have, but use them in an extensive number of facial expressions that help them convey emotional states. For example, monkeys that are looking for a friend relax their eyebrows and pucker their lips, called lip smacking, and maintain eye contact with the individual they are making peace with. Conversely, when their eyebrows are raised, their teeth bared (called a grimace), and ears flapping, they are conveying fear and submission. Holding eye contact is considered very aggressive and a challenge to the individual’s dominance, unless in an affiliative context.
Facial expressions are typically paired with some form of vocalization as well. While monkeys cannot “speak” like humans, their vocalizations are distinct and full of meaning. They “coo”, “girney”, or “grunt” for affiliative meanings. These are usually a little higher pitched, sing-songy, and are not overwhelmingly loud. In aggressive interactions they “bark”, and “pant-threat”. These are characterized by quick rough types of sounds meant to convey warning and anger. They also “squeak” in submission, and “chirp” with alarm. If the situation is particularly distressing, rhesus monkeys scream or screech. This is a very loud and high-pitched sound, usually performed by younger submissive individuals.
Body language is also a method of communication. Dominant and confident individuals stand on all fours with their chest out and tail up, engaging in eye contact, and displacing individuals around them. A submissive monkey sneaks around. Their tail is relaxed, they scuttle and rush around more dominant individuals, and their gaze is diverted. All monkeys of the hierarchy also shake branches in an effort to make a ruckus and look big.
Body language can also invite grooming sessions. If a higher-ranked individual approaches a lower-rank individual and sits near them (not displacing), this is an invitation for grooming. Monkeys also show their rears as a way to show submission called “displaying”.
Younger juveniles mount one another as a way to display dominance, but once they become adults, that habit becomes reserved for females in sexual encounters.
Rhesus macaque reaction times are fast and they are able to communicate using their bodies very quickly. Lunges can happen at the drop of a hat and fear grimaces can diffuse the situation immediately. Keeping track of how fast things go cold can be very difficult and sometimes the monkeys themselves find themselves completely blindsided.
Rhesus macaques are polygynandrous meaning both males and females have multiple mating partners. Puberty begins at 3 for females and 5 for males. Once sexual maturity is reached, females will start initiating sexual encounters with males in their group. Younger males will leave their natal group in search of a new pool of females in another group. While most, if not all, males make the journey to new groups, some don’t make it due to the environmental pressures as well as the murderous dominance hierarchy once they arrive.
Females would ideally give birth every birthing season starting from 3-5 years of age until reproduction slows down at around age 17, and ultimately until menopause at 25, but environmental constraints make this difficult. Large environmental occurrences like hurricanes, earthquakes, and other disasters halt reproduction until the group can get back on its feet.
Females are sexually selective, meaning that they have preferences as to which males they decide to mate with. They tend to prefer middle-aged monkeys who are newer to the group and have darker red faces. With as many as 4 mating partners in a single fertile period, mating season typically occurs in the fall with birthing periods in the spring. This changes depending on the intensity of the seasons. For rhesus macaques that live in areas without distinct temperature, pressure, and environmental changes by the season, their mating and birthing periods become unsynchronized and irregular.
After a gestation of roughly 166 days, babies are born weighing just over a pound (about half a kilogram) and are introduced to solid foods 2 weeks into life. After the birth, new mothers form groups or coalitions with other females and are removed from the males of the group. This is to ensure their child’s safety from infanticide by a male. During this time, females groom one another while tending to their own newborns. At 6 weeks, the baby is able to move independently and starts to explore the surrounding area. Weaning begins at 4 months and is complete right around the first birthday. Juveniles tend to stick around their mothers until a sibling is born and, in the meantime, develop important social skills and play with groupmates.
Mothers and their children have a very strong yet tough love bond. On one hand, mothers constantly groom and care for their children, keeping watch of them at all times. On the other hand, they never share food with their children and will sometimes take food away from them just to enjoy it for themselves. They also slap, hit, or bite their own kids and other young monkeys if they get too close to their food or are misbehaving.
Fathers are relatively absent in the child’s life and typically do not interact with their child. Possibly due to the sheer number of mates, there is paternity uncertainty—they are not exactly sure who is theirs and do not want to unnecessarily invest in offspring that don’t carry their DNA. That said, many fathers do form general relationships with juvenile males. While paternity is uncertain, juvenile males tend to interact more with paternal kin than non-kin.
The ecological role of the rhesus macaque is not very well documented. Rhesus macaques are animals of prey and have a distinct place in the animal food chain. They can be frugivorous which means they also spread the seeds of the fruits they consume.
The rhesus macaque is classified as Least Concern by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Their classification reflects the rhesus macaque’s ability to survive and doesn’t reflect the weight of the human impact on their natural habitats. Rhesus macaque habitats are being destroyed at alarming rates by deforestation and urbanization. With the destruction of their homes, they are pushed into unhealthy relationships with the humans that urbanize their land. They are considered pests in many places because of their tendency to pillage and raid crops and gardens. This causes many violent interactions between humans and rhesus monkeys that can lead to injuries and in the worst case, even death. While the ability of the monkeys to adapt is remarkable and they can live in close proximity with humans, they are healthier living in forested lush environments.
In addition to the destruction of their homes, rhesus monkeys are the number one studied nonhuman primates in the world and are taken from their natural environments for research. 65% of nonhuman primate research is comprised of rhesus macaques and they have helped in the development of polio, smallpox, and rabies vaccinations. Due to their high intelligence and close evolutionary proximity to humans, rhesus monkeys have been cited in thousands of research papers from vaccination developments to understanding the female reproductive system.
The rhesus macaque is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. Protection status varies widely throughout the species range. Rhesus macaques reside in a large number of protected areas throughout their range.
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Written by Nami Kaneko, January 2025