Adult male rhesus macaques can measure more than 76cm tall and weigh 15kg; adult females are smaller; averaging 47cm in length, and weighing around 5kg. They have a tail that measures between 20 and 23cm long. They have pink-coloured hairless faces, but the rest of their bodies are covered with a brownish-grey coloured fur. Their hands and feet are prehensible with hairless palms with oppossed thumbs. Their upper canine teeth are long and straight, while the lower canines curve inward. They have callous, hairless buttocks. They can live up to 25 years. (Álvarez-Romero & Medellín, 2005)

Macaca mulatta individuals have a high frequency for carrying and transmitting the Herpes B virus. Wild population carrying numbers are estimated in the upper 70% while captive populations vary broadly but can be estimated at near 100% (Lutwick & Deaner, 2006). In Australia, M. mulatta has been declared a pest animal and must remain in a licensed zoo, wildlife park, or research facility (Lardner, 2007).

They prefer to stay in a single place, but if perturbed, they can migrate until they find a better or safer place. When a population reaches a size that strain available food sources, small groups, and even individuals, can secede from the original population.

Rhesus macacques are used as laboratory animals primarily for biomedical and behavioural research.M. mulatta are the most studied nonhuman primate both in the field and the laboratory. Medical developments using the rhesus macaque include the development of the rabies, smallpox, and poilo vaccines (Cawthon, 2005).

Rhesus macaques are adapted to a variety of habitats from tropical coastal lowlands to snowy mountain valleys 2,500m above sea level, from dense tropical forest, to temperate pine grooves, to semi-desert conditions. They are opportunistic omnivores, although they prefer fruit. They are excellent swimmers and enjoy living next to water bodies. They prefer living on trees, but descend to floor level to forage in search of food. They are intelligent, social and gregarious and adapt easily to life among humans and domestic animals, if tolerated. However, they are territorial and aggressive and attack in groups whenever they feel threatened. (Álvarez-Romero & Medellín, 2005)

Their reproduction is sexual. Males reach sexual maturity between 2 to 3 years old, while females need to be between 2 to 4 years old. Males are promiscuous and can fertilize many females in a short period of time. Females need to be in their estrous cycle to be fertile and receptive to males, but they can be on estrous multiple times in a year.The estrous cycle lasts from 26 to 28 days. After a gestation period of seven months, females give birth to a single pup, usually every two years. Females reach menopause at about 25 years of age. (Álvarez-Romero & Medellín, 2005)

Although rhesus macaques prefer fruit, they are opportunistic omnivores and will eat what they can grab, including seeds, leaves, branches, tree bark, small animals (vertebrate and invertebrate), eggs, etc. (Álvarez-Romero & Medellín, 2005)

Rhesus macaques are very popular as laboratory animals and are used for biomedical and behavioural research in the internatonal trade.

In Puerto Rico, wild rhesus macaques are considered pests on various levels: they frequently invade fruit farms and eat or damage crops; they can carry diseases that can be passed to humans, and their size, strength, teeth and agressiveness poses a potential for attacks on humans and domestic animals. They are also voracious omnivores and may have an impact on populations of native plants and small animals.

In Puerto Rico, whenever wild macaquess are reported to the authorities, they are captured, taken to a government holding facility, and either exported or eliminated. A project to find an efficient way to eradicate wild populations began in September, 2007.