Though ape and monkey are often used interchangeably in the English language, they are not the same from a scientific point of view.
Apes and monkeys are primates that have evolved different physical and mental characteristics throughout time to respond to different needs and environments. For example, most monkeys have an easily visible tail, but no apes do, and while monkeys are physically built for a life in the trees, apes tend to be built for a life lived in the trees and on the ground.
Historically, there has been debate in and out of science over what qualifies as an "ape" and what qualifies as a "monkey." The two words have different language origins, but they have often been used interchangeably in translations and popular culture. Even past versions of academic resources, like the 1910 edition of Encyclopaedia Britannica, have indicated that "ape" and "monkey" are synonymous.
To understand the scientific differences between an ape and a monkey, you have to look at the way they are scientifically classified. If you go back millions and millions of years into the past, Great Apes (which humans fall under) and Lesser Apes, Old World (Africa- and Asia-based) monkeys, and New World (Americas-based) monkeys all share common ancestors. However, Old World monkeys, apes, and New World monkeys' development, and therefore their scientific classification, branches off separately after a certain point in the evolutionary tree.
New World monkeys belong to a group, or parvorder, known as Platyrrhini (meaning flat-nosed), while Old World monkeys and apes belong to a parvorder known as Catarrhini (meaning hook-nosed). Old World monkeys are further differentiated from Great Apes and Lesser Apes by superfamilies: Cercopithecoidea for Old World monkeys and Hominoidea for Great Apes and Lesser Apes.
Sometimes understanding this ordering can be difficult because the visual shorthand for differentiating between apes and monkeys—that monkeys have tails, while apes do not—can seem overly simplistic.
Many have pointed to the Barbary macaque as an example of a tailless monkey, which is somewhat true, but it's also more complicated than that. The Barbary macaque is a hook-nosed Old World monkey, which means that it is more closely related to Great Apes and Lesser Apes than it is to flat-nosed New World monkeys; it's hard to tell whether it's an ape or monkey. For this reason, the Barbary macaque is sometimes erroneously called the Barbary ape, even though it's a monkey.
Naming conventions, some of which were developed before the theory of evolution was widespread, have changed over time in response to new evidence and have sometimes caused further confusion as the general public's understanding has lagged behind the scientific community's.
Due to the complex nature of classification, some scientists and educators have supported the near-interchangeable usage of "ape" and "monkey" that is already common. Others, though, have expressed frustration over the supposed lack or inadequacy of scientific education that is at the core of this common misunderstanding.
edit Habitat and Health
Old World monkeys are found in Africa and Asia, while New World monkeys are in Africa, Asia, and Central and South America. Great Apes are only found in or near tropical rain forests in Africa and, in the case of orangutans, Southern Asia. Humans are the only primates that live all over the planet in a wide variety of climates.
New World monkeys are small and live most of their lives out in trees, with many using a tail as a sort of "fifth limb." Old World monkeys and apes are usually larger than New World monkeys. While no apes have tails, only vestigial ones like humans have, the appearance and usefulness of a tail in an Old World monkey depends on its environment. Most Old World monkeys have tails, but they are not necessarily used as an extra limb, like New World monkeys use theirs; some Old World monkeys have barely any tail at all, as is the case with the aforementioned Barbary macaque.
Primates are omnivorous, meaning they eat both plants and animals. Apes and monkeys mostly eat fruits, leaves, and other plant material, but they will also eat insects, eggs, and sometimes meat. Chimpanzees, in particular, are predators of smaller monkeys, like the red colobus.
A recent decades-long study performed by researchers at the University of Wisconsin-Madison found Rhesus macaque monkeys that consumed fewer calories led longer, healthier lives. This suggests that primates benefit from low-calorie diets. Researchers caution, however, that the lifespan of Rhesus macaques is quite different from some other primates, including humans, so further study is required.
Lifespans vary greatly between apes, monkeys, and their various subspecies. Provided an environment is not too threatening, many are able to live between 20 and 60 years of age, with apes usually being able to live longer than monkeys. Most species live longer in captivity, but then quality of life comes into question.
Environment plays a very important role in the lifespan of apes and monkeys, especially for Great Apes. While all have a variety of natural predators, such as crocodiles, humans have often encroached on nonhuman Great Apes' rainforest environments, creating situations where shrinking habitats cause space and food scarcity. Some nonhuman primates are endangered or critically endangered because of humans, meaning entire species could go extinct in the near future.
edit In Culture
Nonhuman primates have long been found in popular culture, science, religion, and even on the dinner table. They feature in popular films, literature, and games, like King Kong, 2001: A Space Odyssey, and Curious George. They are also currently used in medical research and have even been sent into space, with Iran claiming it sent a monkey into space as recently as in 2013. In Hinduism, Buddhism, and other beliefs and folklore, monkeys and apes play important roles as gods, tricksters, or luck-bringers. And in some parts of the world, like China, monkey brains have been served as a delicacy.
edit Similarities to Humans
As apes, monkey, and humans are all primates, there are many similarities between all three. However, humans have more in common with their Great Ape (Hominidae family) cousins than they do with monkeys. Great Apes include humans, bonobos, common chimpanzee, Western and Eastern gorillas, and Bornean and Sumatran orangutans.
Around 96-99% of human DNA is the same in humans as it is in their close cousins, so it is not surprising that there is some overlap physical traits and behaviors. The physical similarities, such as larger and heavier bodies, bigger brains, upright posture, and various degrees of bipedalism, are numerous and often obvious; less obvious similarities include things like a vestigial — undeveloped or unusable — tail and appendix.
Which behaviors are similar and which are different is sometimes a hotly debated topic within the sciences, particularly in anthropology and evolutionary psychology. Some draw links between human wars and chimpanzee (and other apes') aggression, while others have suggested humans have more in common with peacefully promiscuous bonobos.
The following video from BBC Wildlife documents chimpanzee tool use.
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