Rhesus monkeys (Macaca mulatta) are macaques belonging to the primate family Cercopithecidae. These medium-sized monkeys are colored from golden-brown to gray-brown. Rhesus monkeys spend most their time on the ground, although they take to trees readily, and have great agility in climbing and leaping. Typical body weights range from 11 to 26.5 lb (5-12 kg) for adult male rhesus monkeys, and from 9 to 24 lb (4-11 kg) for adult females. The facial skin of rhesus monkeys is light tan, while the skin of the rump becomes pink to reddish in adult females during estrus, when mating takes place.
Rhesus monkeys have the widest geographic distribution of any species of non-human primate, occurring naturally in Afghanistan, Pakistan, India, Nepal, Bhutan, Myanmar, Laos, Thailand, Vietnam, and China. In India, rhesus monkeys live in desert habitats of Rajasthan, the agricultural plains of the Gangetic Basin, the tropical forests of southeastern Asia, the temperate pine forests of the Himalaya mountains, and the rugged mountains of north central China. Rhesus monkeys are the most adaptable of all non-human primates, with the broadest range of habitat, and the most cosmopolitan food habits. These monkeys are generally herbivorous, eating a wide variety of natural and cultivated plants, but they also forage occasionally for insects. In agricultural areas, rhesus monkeys frequently raid both field crops such as rice, wheat, pulses (a leguminous, bean-like plant), and sugar cane, and garden vegetables and fruits, such as bananas, papayas, mangos, tomatoes, squash, and melons. In forest areas, rhesus monkeys feed on more than 100 different species of trees, vines and shrubs, on fruits, buds, young leaves, and even bark and roots, of species such as sheesham, ficus, and neem.
Rhesus monkeys are intensely social animals, living in groups of 10–60 individuals or more. An average group of 30 monkeys would have 4–5 adult males, 8–10 adult females, 6–8 infants (less than one year of age), and 8–10 juveniles (one to three or four years of age).
Both male and female rhesus monkeys have social hierarchies of dominance, established by aggressive behavior and social tradition. Once established, dominance is usually maintained by social gestures and communication. Young adult males often leave the groups in which they were born, wander independently, and attempt to enter other social groups. Females usually stay in their natal groups, forming consistent lineages and social traditions within the group.
Mating occurs throughout the year, but is most prevalent from September to December, and most young are born from March to June, after a gestation period averaging 164 days. Young monkeys are cared for intently by the mother for a year. Typically 60-80% of the adult females in a social group give birth to one young every year. Infants are weaned by about one year of age, and enter the juvenile period, in which they still retain an association with their mother, but also spend more time independently and with other juveniles. At this time, they are delightfully rambunctious in play with games of running, climbing, chasing, jumping, wrestling, and swimming.
Sexual maturity is normally reached at about three and a half to four years of age for females, and between four and five years of age for males. Rhesus monkeys live up to 25 years, some even reach 30 years.
Forty years ago, the rhesus monkey population in India alone was about two million. Rhesus monkeys are used extensively in biomedical research, pharmaceutical testing, and vaccine production. Rhesus monkey populations declined drastically to under 200,000 in India, according to a three-year field census by the Zoological Survey of India, completed in the mid-1970s. In 1978, the government of India banned the export of rhesus monkeys, increased conservation programs, and improved food production in India. The rhesus monkey population in India has now increased to between 800,000 and one million.
Rhesus monkeys have been a mainstay of biomedical research in many areas of human physiology, immunology, and health, and they have also been used widely in psychological studies, especially of behavioral development, learning, and social adjustments. The human blood factor, Rh, is named for the rhesus monkey, because our understanding of blood antigens was most clearly demonstrated in studies of these monkeys. Rhesus monkeys were used for the discovery, development, and testing of the polio vaccine. The use of rhesus monkeys in laboratory programs is opposed by some animal rights groups, but many scientists feel that the use of these animals is essential and justified given their uniquely valuable contributions for medical knowledge, so long as that use is humane. Virtually all rhesus monkeys used in biomedical or behavioral research in the United States are bred in colonies under close veterinary supervision, excellent conditions, and humane care.
In India, Nepal, and China, rhesus monkeys enjoy a deep cultural and religious affection, especially by people of Hindu and Buddhist faiths. Rhesus monkeys feature prominently in the Hindu epic story the Ramayana, in which rhesus monkeys enabled Rama (the incarnation of the god Vishnu, the embodiment of good) to defeat Ravana (the Devil King). Ravana had abducted Sita, Rama's wife, and taken her away to the island of Ceylon (Sri Lanka). Hanuman, the monkey god, and his troop of monkeys enabled Rama to find Sita and rescue her from the evil Ravana. Hanuman and his troop were actually langur monkeys, but rhesus monkeys also enjoy a sacred status in traditional Hinduism.
Rhesus monkeys have had a significant impact on human societies, particularly in the areas of science, culture, and ecology.
Lindburg, D.G. "The Rhesus Monkey in North India: An Ecological and Behavioral Study." In Primate Behavior: Developments in Field and Laboratory Research. by L. A. Rosenblum. New York: Academic Press, 1971.
Hearn, J.P. "Conservation of Primate Species Studied in Biomedical Research." American Journal of Primatology 34, no. 1 (1994): 1-108.
Southwick, C.H., and M.F. Siddiqi. "Population Status of Primates in Asia, with Emphasis on Rhesus Macaques in India." American Journal of Primatology 34 (1994): 51-59.
Charles H. Southwick