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.
Types of monkeys
Monkeys are broadly split into two groups: Old World monkeys and New World monkeys. Old World monkeys live in Asia and Africa and have downward-pointing nostrils, according to Nature Education (opens in new tab). New World monkeys live in North and South America and have outward-pointing nostrils.
Each group has special skills. For example, some New World monkeys, such as spider monkeys from South America, have prehensile tails that they can use to grasp and hold tree branches and other objects, according to The University of Edinburgh (opens in new tab) in Scotland, while many Old World monkeys have pouches in their cheeks where they can store food.
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 (opens in new tab) 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 (opens in new tab). 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 (opens in new tab).
The smallest monkeys in the world are pygmy marmosets (Cebuella pygmaea) from the Amazon region of South America, according to the Wisconsin National Primate Research Center (opens in new tab). These tiny monkeys weigh an average of 4.2 ounces (119 grams) and grow to only 5.4 inches (13.6 cm) long. When they are babies, pygmy marmosets are smaller than human fingers, earning them the nickname "finger monkeys."(opens in new tab)
Where do monkeys live?
Monkeys live on all continents except Australia and Antarctica. They often make their homes in trees in warm and wet tropical rainforests, including the Amazon rainforest in South America and the Congo Basin in Central Africa.
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 (opens in new tab). 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 (opens in new tab), 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.(opens in new tab)
What do monkeys eat?
Monkeys are famous for eating bananas, but their diet varies considerably across species, depending on where they live and which foods are available. For example, saki monkeys may eat up to 50 fruit species in a single day, BBC News reported (opens in new tab) in 2013. Most monkeys are omnivores; they eat plant-based foods, such as fruits and nuts, as well as some meat, such as lizards and bird eggs.
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 (opens in new tab). 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 (opens in new tab), 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 (opens in new tab). 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.(opens in new tab)
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 (opens in new tab) 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.
A 2019 study published in the journal Frontiers in Psychology (opens in new tab) found that Old World monkeys are capable of combining two items in a language sequence but do not use the open-ended language humans use, where we can string together a theoretically infinite number of words — so monkey dialogue is much more limited.
However, monkeys also use body language to communicate. Their body language can have very different meanings from similar body language in people. For example, when monkeys bare their teeth, it may look like a human smile, but it's actually a sign of fear or aggression, Live Science previously reported. Other signs of monkey aggression include yawning and head bobbing.
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 (opens in new tab). In contrast, baboons breed continuously throughout the year and give birth after 178 days of gestation.
The number of offspring a monkey has also varies. Baboons give birth to a single young, and twins are rare, according to the University of Notre Dame's Amboseli Baboon Research Project (opens in new tab). However, pygmy marmosets normally give birth to twins and can give birth to triplets, according to the San Diego Zoo (opens in new tab).
Baby monkeys are often cared for primarily by their mothers and ride around on their backs or cling to their fronts. In pair-bonded social systems, the males usually help raise offspring. Many marmosets are polyandrous, meaning one female breeds with multiple males and then some or all group members help with offspring care, according to Nature Education.
Monkey life span varies tremendously. A baboon may live 20 to 40 years, while a pygmy marmoset lives 10 to 12 years. Individual monkeys can also defy the age expectations of their species. For example, rhesus macaques usually live up to 30 years, but the world's oldest captive rhesus macaque was 43 when she died in a zoo in Japan in 2021, Japanese newspaper The Mainichi (opens in new tab) reported at the time.
Related: Neuroscientists discover 'engine of consciousness' hiding in monkeys' brains (opens in new tab)(opens in new tab)
What is the rarest monkey?
The rarest monkeys on Earth are likely Cat Ba hooded black leaf monkeys (Trachypithecus poliocephalus), also called golden-headed langurs. Fewer than 50 mature individuals remain across their range on Cat Ba Island in northern Vietnam, and their habitat is threatened by fires, tourism and human development, according to the International Union for Conservation of Nature (opens in new tab) (IUCN).
Between 2018 and 2020, Cat Ba hooded black leaf monkeys were one of the world's 25 most endangered primates, according to the "Primates in Peril (opens in new tab)" report by the IUCN Species Survival Commission (SSC) Primate Specialist Group, the International Primatological Society, Global Wildlife Conservation and the Bristol Zoological Society.
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.(opens in new tab)
Can you have monkeys as pets?
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 (opens in new tab), 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 (opens in new tab) 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 (opens in new tab) 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.
"We know from early experimental work that isolating these animals from their group causes profound social problems," Piel and Stewart wrote in The Conversation (opens in new tab) in 2019. "In most cases, normal social behaviour can't be recovered in pets that are later released. Human owners of especially young pet monkeys cripple the animals for life by removing them from their natural surroundings."
Related: Starving monkey 'gangs' battle in Thailand as coronavirus keeps tourists away (opens in new tab)
Learn more about the different monkey species and other primates in "Primates of the World: An Illustrated Guide (opens in new tab)" (Princeton University Press, 2013). The book details almost 300 primate species and includes illustrations.
To learn more about what separates monkeys from humans and other apes, see this page from the Jane Goodall Institute (opens in new tab).
For more detailed information on monkey conservation and the protection of other primate species, visit the IUCN SSC Primate Specialist Group (opens in new tab) website or browse the conservation assessments of individual monkey species on the IUCN Red List of Threatened Species (opens in new tab).
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This article was originally written by Live Science contributor Alina Bradford and has since been updated.