Monkeys: Facts about the largest group of primates

A family of proboscis monkeys in a tree in Borneo.

(Image credit: USO via Getty Images)

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Monkeys are a large and diverse mammal group that includes most primates. Humans, chimpanzees (Pan troglodytes) and other apes share an ancestor with monkeys but belong to a separate group of primates that diverged from monkeys millions of years ago. Monkeys are typically smaller than apes and usually have tails, which apes lack. The monkey group doesn't include lemurs, which are another branch of the primate family tree and live in Madagascar.

There are a variety of monkeys, spread across the world, with very different lifestyles. They come in many shapes, sizes and colors, but they are all intelligent and social creatures.

Monkeys vary significantly in appearance. For example, proboscis monkeys (Nasalis larvatus) from the island of Borneo in Southeast Asia are known for males' big noses. A 2018 study published in the journal Science Advances found that they use their noses to attract female mates, which prefer longer noses. Their large noses also enhance their vocalizations. Proboscis monkeys have red faces and potbellies. Their looks contrast sharply with those of some other species, such as black and brown spider monkeys, which are long and slender with small noses.

Mandrills (Mandrillus sphinx), from West Central Africa, are the world's largest monkeys. Males of this species grow up to 43.3 inches (110 centimeters) long and can weigh more than 72 pounds (33 kilograms), according to the San Diego Zoo Wildlife Alliance. However, they are similar in size and shape to their close relatives drills (Mandrillus leucophaeus), which weigh about 71 pounds (32 kg), according to the Wisconsin National Primate Research Center.

Some species have adapted to living in harsh environments, such as desert-like savannas or snowy mountains. For example, Japanese macaques (Macaca fuscata), or snow monkeys, have thick fur to survive the most northern parts of their range in Japan, which can be covered in snow for up to a third of the year, according to the Wisconsin National Primate Research Center. Some Japanese macaques bathe in human-made hot springs in the winter to keep warm, Live Science previously reported.

Humans have introduced monkeys to areas outside of their native range. For example, there are monkeys in Florida even though the U.S. has no native monkey species. A group of green monkeys (Chlorocebus sabaeus) lives near the Fort Lauderdale airport because their ancestors escaped from a nearby private zoo in 1948. The zoo likely imported the monkeys from Sierra Leone, in West Africa, and they've adapted to life in the Floridian mangrove forest, Live Science previously reported. Scientists do not fully understand the impact of this "alien" population on native wildlife, but other monkey populations have become invasive species, meaning they cause environmental or economic harm.

Rhesus macaques (Macaca mulatta) are highly adaptable, and their native range — which stretches throughout Asia and includes countries such as India — is thought to be the largest of any nonhuman primate, according to the CABI Invasive Species Compendium, an online encyclopedic resource on invasive species. Rhesus macaques are now invasive in Florida, South Carolina and Puerto Rico. Because of their flexible diet and ability to live in a range of habitats — from tropical forests to cold, mountainous regions — they have the potential to become invasive on every continent except Antarctica.

Monkey diets can shift with changing seasons. For example, proboscis monkeys mostly eat fruit when it is ripe, from January to May, and eat more leaves from June to December, according to the Wisconsin National Primate Research Center. Some species have more specialized diets. Species of colobus monkeys mainly eat leaves and have complex stomachs so they can digest toxic foliage that other monkeys can't, according to the African Wildlife Foundation, a nonprofit that focuses on conserving wildlife in Africa.

Monkeys are social creatures that usually live together; a group of monkeys is called a troop. Monkeys have a variety of social systems. One of the most common troop structures is called a one-male group, in which a male leads a group of females and other males, usually staying close to a particular location. The male has access to all of the females for mating, but his reign is constantly threatened by other males from outside the group that seek to overthrow him. Males outside the one-male group often live in all-male groups until they can lead a group of their own, according to Nature Education. Patas monkeys (Erythrocebus patas) from Africa and species of howler monkeys from Central and South America are examples of monkeys that live in one-male and all-male groups.

Monkeys form strong social bonds and maintain them through activities such as grooming. The many species of titi monkeys and some marmosets from South America operate in a pair-bonded system, in which a monogamous male-and-female breeding pair form the basis of a group and defend their territory from other troops. Some other structures are less exclusive. Species of macaques, capuchins and baboons live in multimale, multifemale groups where males and females are polygamous and mate with multiple group members.

Monkeys make a variety of noises, from hoots to screams, often vocalizing back and forth like humans talking to each other. Howler monkeys have one of the loudest calls of any terrestrial animal and can be heard from about 3 miles (4.8 kilometers) away in the right conditions, according to the Smithsonian's National Zoo in Washington, D.C. Male howler monkeys from different groups call to each other in the mornings and when moving to a new feed site to stake their claim on feeding trees. The howling helps troops know where other troops are so they don't need to constantly patrol their territories or fight each other for resources.

Monkeys breed in different scenarios depending on how their societies are structured. Many monkeys breed seasonally. For example, Guianan squirrel monkeys (Saimiri sciureus), from South America, mate between September and November and give birth between February and April, with a gestation period of 160 to 170 days, according to the University of Michigan's Animal Diversity Web. In contrast, baboons breed continuously throughout the year and give birth after 178 days of gestation.

The report included other Asian primates, such as pig-tailed snub-nose langurs (Simias concolor) from Indonesia, a species that is threatened by commercial logging, human encroachment into their habitat and hunting. In Africa, monkey species such as Roloway monkeys (Cercopithecus roloway) and white-thighed colobus monkeys (Colobus vellerosus) face extinction because humans are destroying and degrading their habitat and hunting them for bushmeat.

In South America, pied tamarins (Saguinus bicolor) are killed by power lines and human pets, such as dogs and cats; they are also captured from the wild to become pets themselves. The species is also losing its habitat to urban expansion and agriculture, the latter of which is the main threat to Olalla brothers' titi monkeys (Plecturocebus olallae), a critically endangered monkey species from Bolivia.

It is legal to have monkeys as pets in some U.S. states but they are often restricted or banned. For example, Montana and Georgia prohibit all pet monkeys, according to FindLaw, a website that provides legal information. The law also varies depending on the monkey species and the circumstances in which it is kept. For example, Ohio allows spider monkeys as service animals but not as pets. There are an estimated 15,000 pet monkeys in the U.S. A 2021 study of the primate pet trade published in the journal PLOS One found that marmosets were the most common monkey for sale.

But monkeys are not suitable pets. They naturally live in complex environments with very particular requirements. For example, without special diets and outdoor access to ultraviolet light, marmosets get health issues such as bone disease, according to the Marmoset Care website hosted by the University of Stirling in Scotland. They are also expensive, smelly and prone to becoming bored and stressed. Monkeys are naturally aggressive animals that can attack and bite their owners.

Most pet monkeys are kept alone and not in their natural social setting, such as a large group. Alexander Piel, an anthropologist at University College London, and Fiona Stewart, a lecturer in wildlife and primate conservation at Liverpool John Moores University in England, argued in favor of a ban on keeping monkeys as pets in the U.K., partly because of the negative impact of keeping monkeys on their own.

Patrick Pester is the trending news writer at Live Science. His work has appeared on other science websites, such as BBC Science Focus and Scientific American. Patrick retrained as a journalist after spending his early career working in zoos and wildlife conservation. He was awarded the Master's Excellence Scholarship to study at Cardiff University where he completed a master's degree in international journalism. He also has a second master's degree in biodiversity, evolution and conservation in action from Middlesex University London. When he isn't writing news, Patrick investigates the sale of human remains.

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