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Why are orangutans classified as great apes rather than monkeys?

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An illustration of a female Bornean orangutan (Pongo pygmaeus) with long reddish-brown fur, hanging from tree branches with one arm extended upward and one leg hooked over a branch. A scale bar at the bottom shows lengths of 30 centimeters and 12 inches for size reference. — orangutan Female orangutans (*Pongo*) have the longest breeding interval of any mammal, giving birth on average once every eight years.

Orangutans are classified as great apes rather than monkeys because they belong to the superfamily Hominoidea, which is distinct from monkeys in several key biological and evolutionary ways.12

The primary physical differences include:
* Absence of a tail: Unlike almost all monkeys, orangutans and other apes do not have external tails.12
* Anatomy: Apes have broad chests and shoulder joints that allow for greater flexibility, whereas monkeys are typically narrow-chested and built for running across branches. Additionally, apes possess an appendix, a feature monkeys lack.12
* Cognitive Ability: Great apes exhibit higher intelligence than monkeys, including the ability to recognize themselves in mirrors, reason abstractly, and use tools.12

Within the family Hominidae, orangutans are grouped with other great apes—such as gorillas, chimpanzees, and humans—due to their large size, complex brains, and close evolutionary relationships. While monkeys are numerous and diverse, there are only a few species of apes, all of which share these defining characteristics.231