Rhesus macaque

Name and taxonomy

The naturalist Eberhard Zimmermann first described the species in 1780. The name rhesus recalls a Thracian king in the Iliad, but Jean-Baptiste Audebert, who chose it, said it had no meaning. Most people simply call the animal the rhesus monkey. The genus Macaca holds the other macaques, such as the bonnet macaque of southern India. The family is Cercopithecidae, the Old World monkeys, in the order Primates.

Zoologists recognise six to nine subspecies in two groups, one from India and one from China. The Indian form carries the name Macaca mulatta mulatta. Several Chinese forms were described between 1867 and 1917. Genetic differences between populations are small, so the subspecies are not firmly settled.

Evolution and relatives

Humans and rhesus macaques share about 93% of their DNA and a common ancestor about 25 million years ago. The macaque genome was sequenced in 2007, the second non-human primate genome after the chimpanzee. Scientists compare the three genomes to work out which gene changes belong to the human line. The species has 21 pairs of chromosomes.

Fossil teeth and jaw fragments from caves near Zhoukoudian show that the species lived in northern China in the Pleistocene. Fossils from near Pyongyang in North Korea belong to the same species. Its closest relatives are the other macaques, and in southern India its range now touches that of the bonnet macaque.

Appearance

The coat is brown or grey, with a grizzled look on the back and a paler belly. The face is bare and pink, and in adults the face and the rump turn red. The hair on the head is short. The body has a wide rib cage and, on average, 50 vertebrae. The arms are a little shorter than the legs. Cheek pouches let the monkey stuff food away to eat later.

Head and body length is 45 to 64 centimetres, and the tail adds 19 to 32 cm. A typical male measures about 53 cm and weighs about 7.7 kilograms, and a typical female is about 47 cm and 5.3 kg. A large male can reach 12 kg. A newborn weighs 400 to 500 grams. On the ground the monkey walks on all fours, on the toes of the hind feet and the flat of the hands.

Habitat and range

The native range runs from eastern Afghanistan and Pakistan through India, Nepal, Bhutan and Bangladesh to Myanmar, Thailand, Vietnam and southern China. No other non-human primate covers so much ground. The species lives in grassland, dry scrub, woodland and forest, and in the Himalayas up to about 2,500 metres. It copes with desert heat and with winters well below freezing. Populations in southern China and Tibet fell sharply in the twentieth century.

Rhesus macaques move readily from the countryside into towns, where they live on handouts and rubbish. Troops in human landscapes grow larger than troops in forest. Small groups live in the crowded cities of northern India, and a troop that is moved out tends to seek another settlement. The monkey is a strong swimmer and crosses rivers with ease.

Introduced colonies live in Florida, on Morgan Island in South Carolina and on Cayo Santiago off Puerto Rico. The Florida colony began in 1938, when a boat-tour operator released monkeys near Silver Springs. He did not know that they could swim, so they left his island at once. Florida held an estimated 550 to 600 macaques in 2020. GBIF holds records from 30 countries, many of them from zoos and laboratories.

Diet and feeding

Fruit is the main food, and it also supplies much of the water in the monsoon. The monkey also eats seeds, roots, buds, bark and cereal crops. One estimate counts 99 plant species from 46 families in its diet. Animal food includes termites, grasshoppers, ants, beetles, spiders, crabs, honeycomb and birds' eggs. City troops eat human food and pick through rubbish.

In the mountains of northern Pakistan, the monkeys eat clover in summer. When snow covers the ground, they switch to pine needles and oak leaves, lose weight, and survive. Far from rivers they lick dew from leaves and drink rain from holes in trees. Cheek pouches hold food during a quick raid on a field or a bin. The animal forages through the day and drinks at streams.

Behaviour and social life

The species is active by day and moves both in trees and on the ground. A troop holds 20 to 200 monkeys, and females can outnumber males by four to one. Females stay in the troop where they were born, and each inherits her rank from her mother. Oddly, a mother's youngest daughter outranks her older sisters. Males leave the natal troop at four to five years, and an adult male earns his rank through age and experience.

The troop has a centre and an edge. The two or three oldest males sit at the centre with the top females and their young, and they decide where the troop goes. Lower-ranking males lead groups on the edge, and the youngest males at the rim raise the alarm. Rhesus society is despotic: high-ranking animals show little patience with those below them. Even so, females of one troop rarely fight, and fights between troops are rare.

Troops are not territorial, and the ranges of neighbours overlap. When two troops meet, the weaker one moves off. A troop that grows to 80 to 100 animals may split, when a set of females leaves to found a new one. Grooming fills much of the day, and the monkeys have been seen grooming langurs and even sambar deer. In one filmed case in Kanpur, a monkey revived a companion knocked out by a power line.

Communication and senses

Rhesus macaques talk with the face, the voice, the body and the hands. A lower-ranking monkey bares its teeth in silence to a superior, or grimaces and screams to turn aside an attack. It may also raise the tail and present its rump. A dominant animal threatens with a silent open-mouth stare and a stiff, raised tail.

Coos and grunts pass between troop mates on the move and before grooming. A warble or a chirp announces a rare, rich food. A single shrill bark warns of danger. Screeches, growls and pant-threats mark a fight. An infant geckers to call its mother. Like other monkeys, the species probably relies more on sight than on smell.

Reproduction and life cycle

Troops that face cold winters mate in autumn, so that births come in spring. Where the seasons are mild, mating is spread through the year. The female cycle lasts about 29 days, and she is receptive for 8 to 11 days of it. She presents her hindquarters to a male, and the skin around the tail reddens. Both sexes mate with many partners, and females prefer strangers from outside the troop.

Pregnancy lasts about 165 days, and almost every birth is a single infant. The infant weighs 400 to 500 grams and clings to its mother's belly, then rides on her back. It suckles for about a year. A female can give birth every year when food is good. Females mature at 2.5 to 3 years and usually give birth first at about four, and males mature at 4.5 to 7 years. Females reach menopause at about 25 years.

Mothers do nearly all the care, and a male does not know which infants are his. In captivity a lone male will raise an unrelated infant if no female is present. The species can live about 30 years in the wild, and one captive animal reached 40.

Predators and threats

Large cats, eagles and snakes probably take rhesus macaques, but predation is little studied. Sharks in the Ganges estuary catch monkeys that swim across at low tide. On Cayo Santiago there are no predators at all. Infants at the edge of a troop risk infanticide by males from outside.

The species as a whole faces no threat, but forest loss and farming push troops into villages. Himachal Pradesh in India allowed legal culling in 2010. Released pets of other macaque species breed into wild troops and dilute the gene pool. The population in Thailand is under local pressure.

Population and conservation

The IUCN lists the rhesus macaque as Least Concern, and it exists in large numbers. It tolerates almost any habitat, including city streets. Its range in southeast India has grown by about 3,500 square kilometres, partly because troops from towns were moved there. That spread threatens the bonnet macaque, which is native to the south. Despite the mountain of research on its behaviour, few studies count wild populations.

In practice, management now means keeping monkeys and people apart. Guards with dogs, slingshots and firecrackers drive troops from fields. Farmers plant buffer crops that monkeys do not like along the forest edge. Cities plant fruit trees in parks on the outskirts to keep troops out of homes. Translocation and contraception offer non-lethal alternatives to culling.

Relationship with humans

The rhesus macaque is the classic laboratory monkey. It gave its name to the Rh blood factor, found by Karl Landsteiner and Alexander Wiener. Research on the species produced vaccines for rabies, smallpox and polio, and drugs against HIV. Harry Harlow used rhesus infants in his maternal deprivation experiments of the 1950s. Albert II became the first primate in space in 1949, and Able came back alive from a flight in 1959. Tetra, the first cloned primate, was born in 1999, and ANDi, the first transgenic primate, in 2001.

Around farms and towns the monkey is a pest. Crop raiding costs a Nepalese farming household about 4.2% of its yearly income. In Himachal Pradesh macaques cause a large share of all crop losses, and in cities they enter houses and bite people. In 2018 a monkey carried off a baby from a house in Odisha, and the child died. Macaques carry herpes B virus, which rarely infects people but can kill when it does. Hindus regard the monkey as sacred, so many people feed it and few harm it.

In culture and myth

In Hindu tradition the monkey recalls Hanuman, the vanara hero who serves Rama in the Ramayana. In the temple towns of northern India, feeding the monkeys earns merit, and the rhesus is the usual temple monkey. At Swayambhunath in Kathmandu, the so-called Monkey Temple, local stories say the monkeys came because the hill is holy. A Tibetan origin story tells how a monkey and a rock ogress became the parents of the first people. Folk tales also cast the monkey as a trickster who steals sweets.

In the West the name entered daily speech through the Rh factor, so rhesus appears on blood-group cards worldwide. The Florida colony has its own folklore: a false story links it to the Tarzan films shot at Silver Springs. One escapee, the Mystery Monkey of Tampa Bay, dodged capture for years and inspired a song. The Florida monkeys keep local supporters who oppose their removal.