The intelligence of monkeys is the result of millions of years of evolution in a complex social environment. They are intelligent not because they have a large brain in itself, but because their brain is adapted to solve specific tasks: surviving in a group, foraging for food, and manipulating their fellow primates.

Social Brain: The Main Driver

The most convincing theory explains primate intelligence by social complexity. Living in a group requires remembering dozens of relationships, tracking alliances, cheating, and detecting deceit. The more complex the society, the larger the brain.

• Social Brain Hypothesis. The size of the neocortex, that is, the new cortex, in primates correlates with the size of the group they live in. Monkeys living in large groups are smarter than those living in small ones.
• Politics and Manipulation. Monkeys form coalitions, seek revenge, reconcile, and deceive. For example, baboons are capable of tracking for years who is friends with whom and who owes whom.
• Understanding Other Minds. Some monkeys understand that others can make mistakes or not know something. This is called theory of mind and is considered the basis of complex social behavior.

Tools and Ecology

The second factor is the need to obtain hard-to-reach food.

• Use of Tools. Chimpanzees use stones to crack nuts, sticks to catch termites, and leaves as sponges for water. This requires planning and training.
• Extractive Foraging. Fruits, nuts, insects hidden in inaccessible places require ingenuity. Monkeys whose diet includes such resources demonstrate more complex behavior.

Anatomy and Brain

The monkey brain is disproportionately large compared to the body. In chimpanzees, it weighs about four hundred grams — less than a human's, but the structure is very similar.

• Prefrontal Cortex. Responsible for planning, self-control, and decision-making. The prefrontal cortex in primates is much more developed than in other mammals.
• Mirror Neurons. Help learn through imitation and understand the actions of others. This is the key to the transmission of knowledge within the group.

Examples of Intelligence in Action

• Chimpanzees. Solve memory tasks better than university students in tests of short-term memory for numbers.
• Capuchins. Understand the value of money and can negotiate, refusing to work for low pay.
• Marmosets. Learn from each other how to open containers, indicating the transmission of knowledge through culture.

Specific Monkeys Proving Their Intelligence

Gorilla Koko

Koko is probably the most famous talking monkey in history. She knew more than a thousand sign language symbols and understood about two thousand English words. Koko became famous not only for her vocabulary but also for her ability to express emotions. When her beloved kitten was killed by a car, she showed signs of sadness, gloom, and crying. She lived for forty-six years and became a symbol of how deeply monkeys can understand human language.

Chimpanzee Ai

Ai was a genius at Kyoto University. She became the first chimpanzee to successfully symbolize numbers. By the age of five, she had mastered numerical symbols from one to six, and by seven and a half — all twenty-six letters of the English alphabet and began to recognize lexigrams for words such as apple and banana. She also distinguished more than a hundred Chinese characters and eleven colors. In her free time, Ai loved to draw, and one of her paintings was printed on a scarf given to primatologist Jane Goodall.

Bonobo Kanzi

Kanzi is a dwarf chimpanzee who understood English by ear and communicated using a keyboard with lexigrams, that is, symbols representing words. He knew about three thousand spoken words. His uniqueness lies in the fact that he became interested in symbols independently, observing the training of his foster mother, rather than being specially trained. Kanzi also made stone tools and played video games, including Minecraft.

Chimpanzee Washoe

Washoe was the first animal to learn American Sign Language. She mastered more than one hundred thirty signs, which corresponds to the vocabulary level of a four-year-old child. Washoe is also known for teaching some signs to her foster son and other chimpanzees, demonstrating the transfer of skills within the group.

Chimpanzee Ayumu

Ayumu is the son of Ai, inheriting an extraordinary memory. In a test of short-term visual memory, he outperformed university students, remembering the arrangement of numbers on the screen for fractions of a second. This proved that monkeys can excel in some cognitive tasks.

What Monkeys Cannot Do

Despite their impressive abilities, there are limits.

• Syntax of Language. They can learn word symbols, but they do not build complex sentences with grammar.
• Culture Accumulation. Human culture accumulates from generation to generation. There is knowledge transmission in monkeys, but it does not accumulate as quickly.

Conclusion

Monkeys are intelligent because their evolution has been along the path of complicating social relationships and solving ecological tasks. Their intelligence is not a subhuman mind, but another, highly specialized form of thinking, ideally adapted for life in a complex monkey society. The examples of Koko, Ai, Kanzi, Washoe, and Ayumu show that this intelligence is not abstract but measurable: language, memory, tool making, and solving complex tasks.
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