What Is the Smartest Primate? How Apes and Monkeys Rank

Humans are the smartest primate on broad measures, though a chimpanzee can beat us on a single task. Among the other primates, the great apes lead. On a self-control task, chimpanzees scored 100%, orangutans 99.1% and gorillas 94.4%, from only 5 to 11 animals per species. The capuchin monkey, at 95.9% (22 animals), matched them. No ape has an IQ score, because IQ tests are normed on people.

The new finding is how thin the evidence is under the famous names. Only one chimpanzee has a cortex neuron count, and the whole-brain numbers for chimpanzees and gorillas are predictions, not counts. Below: the table we built from the published tests, a ranking of apes and monkeys, and the two headline chimpanzee studies, read at the source.

What is the smartest primate?

People are, on the broadest comparisons we found. In the Herrmann battery, 2.5-year-old children scored about 74% on social tasks against 33% to 36% for apes. The human cortex holds about 2.2 times the neurons of the only chimpanzee cortex counted (16.3 divided by 7.4, our arithmetic). Single tasks can go the other way. The harder question is which non-human primate comes next. Here is how eight primates scored on two tests given the same way to many species, with the human figures for reference.

• Chimpanzee: self-control 100% (5 animals); memory test 84.4% (40 animals); cortex neurons about 7.4 billion (one animal)
• Orangutan: self-control 99.1% (11 animals); no memory-test score and no neuron count in our data
• Capuchin monkey: self-control 95.9% (22 animals); memory test 63.5% (66 animals); cortex neurons about 1.1 billion
• Bonobo: self-control 95.0% (6 animals); memory test 82.4% (20 animals)
• Gorilla: self-control 94.4% (9 animals); memory test 77.5% (9 animals)
• Rhesus macaque: self-control 80.0% (6 animals); memory test 61.5% (28 animals)
• Olive baboon: self-control 76.2% (8 animals); memory test 52.6% (6 animals)
• Common marmoset: self-control 31.9% (11 animals); memory test 43.0% (46 animals)
• Human: no score on either task in these studies; about 86.1 billion neurons in the brain and about 16.3 billion in the cortex (Azevedo and colleagues, 2009)

The self-control task is the cylinder test from MacLean and colleagues (PNAS, 2014): an animal must reach a food reward by a detour instead of grabbing straight at it. The memory test is a delayed-response task from the ManyPrimates project (421 primates, 41 species; 2021 preprint, doi 10.31234/osf.io/5etnf). The numbers come from our smartest animals study, which checked each one against the papers' tables.

Read the table with care. A species score rests on 5 to 11 animals in the self-control task. A percentage correct on one task is also not a general intelligence score. It shows how animals compare on that task, not how clever they are overall.

What is the smartest ape?

So what is the smartest ape? Chimpanzees have the most evidence, and orangutans match them on one test, so the smartest ape depends on the task. No study has ranked all four great apes on one full battery.

The closest thing is Herrmann and colleagues (Science, 2007). They gave the same battery of 16 tasks to 106 chimpanzees, 32 orangutans and 105 children aged 2.5 years. On the physical tasks (space, quantities and causality), chimpanzees and children scored about 68% and orangutans about 59%. On social tasks, the children scored about 74%, and both apes scored 33% to 36%.

So the ranking flips with the task. Orangutans trail chimpanzees on the physical battery, yet they scored 99.1% on the self-control test. Bonobos and gorillas were not in the Herrmann battery. All the apes were captive animals living in sanctuaries or care centres, the chimpanzees aged 3 to 21 and the orangutans aged 3 to 10. Our guides to chimpanzee IQ and gorilla IQ go through each species.

Which is the smartest monkey species?

On the self-control test, the capuchin monkey is the smartest type of monkey in our data. Capuchins (Cebus apella in the data) scored 95.9% across 22 animals, in the same range as the great apes at 94.4% to 100%. Rhesus macaques scored 80.0% (6 animals) and common marmosets 31.9% (11 animals).

MacLean and colleagues drew the same picture when they rebuilt the family tree of the skill: they found a convergence between apes and capuchin monkeys. They also report that, within primates, a wider diet predicted better self-control, while group size did not. So group size did not explain the differences between species.

Brain counts do not settle it. The capuchin cortex holds about 1.1 billion neurons and the long-tailed macaque's about 0.8 billion (Herculano-Houzel, 2009; Gabi and colleagues, 2010). Yet capuchins beat the macaques tested on both self-control tasks in our data: the cylinder task (rhesus macaque 80.0%) and the A-not-B task (long-tailed macaque 66.7%, against 86.2% for capuchins). The smartest monkey species is a claim about one task, not a rank.

What is the average IQ of an ape?

There is none. IQ tests are built and normed on people, so no ape has an IQ score. The average IQ of an ape is not a number that anyone has measured.

Studies compare apes with children on tasks and report percentages correct, as Herrmann and colleagues did. An IQ is different. It places a person inside a reference group, on a scale with a mean of 100 and a standard deviation of 15. Our IQ percentile calculator works only for scores on that scale.

Can a chimpanzee beat a human at a memory test?

One young chimpanzee did, on one task, until a follow-up study changed the picture. Inoue and Matsuzawa (Current Biology, 2007) tested six chimpanzees, three mother and infant pairs, on a numeral memory task. Five numerals from 1 to 9 flashed on a touch screen and then turned into white squares. The animal had to touch the squares in the numeral order. Chance for five numerals is 1 in 120.

The chimpanzee Ayumu and the best mother, Ai, were compared with 9 university students at hold times of 650, 430 and 210 milliseconds. The students did worse as the time shrank. Ayumu stayed about as accurate, and the paper says he outperformed all the human subjects in speed and accuracy.

Then came the caveat. Silberberg and Kearns (Animal Cognition, 2009) pointed out that Ayumu had many practice sessions and the students had none. Ayumu's accuracy at 210 milliseconds was about 79%, as they note, and two humans who were given practice matched him. The comparison is a good test of practice, not of species. You can try a memory task yourself in our memory game.

How does the human brain compare with the chimpanzee brain?

The human brain is bigger and holds more cortical neurons, but the gap in neuron counts is smaller than the gap in weight. The human brain averages 1,508 g (adult men, Herculano-Houzel, 2009) and the chimpanzee brain in our data 344 g. The human cortex has about 16.3 billion neurons. The only chimpanzee count we found is 3.7 billion for one cortical hemisphere of one animal (Collins and colleagues, PNAS, 2016), so about 7.4 billion for both, our doubling.

Whole-brain counts exist for people (86.1 billion). We found none for chimpanzees or gorillas. The 28 billion and 33 billion neurons predicted for them are estimates from scaling rules, as Herculano-Houzel (PNAS, 2012) says. She argues that the human brain is a scaled-up primate brain, remarkable mainly for its very large number of neurons. For another comparison, see human brain vs dog brain, and for the wider picture, the smartest thing in the world.

Brain size is not the same as ability inside our own species either: see what a correlation of 0.24 means. And to see where your own reasoning sits on a scale built for people, take the IQ Metrics IQ test.

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