The biological definition for primates is that it is the order of mammals that includes tree shrews, lemurs, monkeys, gibbons, apes and humans. Primates are largely arboreal (i.e. they live up in the trees) and have limbs modified for climbing, leaping or swinging. They have large brains in relation to their body weight and other mammals and generally have highly defined sight, often having stereoscopic vision. I plan to suggest reasons as to how and why the large brains have developed in primates. Primates can be distinguished unmistakeably by their round brainy skulls, high foreheads, forward looking eyes, mobile arms and skilful hands.

What difference does brain size make?

Brain size by itself does not explain why we and other primates can be classed as intelligent. A horse’s brain is more than six times larger than that of the much smaller monkey. If the two animals were of the same size, the monkey’s brain would be 20 times larger than that of the horse’s. Scientists do not consider horses to be exceptionally intelligent animals. It can be seen, therefore, that brain size in relation to body mass can be an indication to intelligence. Brain size on its own cannot.

Different parts of the brain?

Some scientists, such as Damon Clark from Princeton University, believe that the important differences concerning brain size are the sizes of the different brain components. The cerebellum (a pair of bun-like moulds), located at the base of the brain, is concerned with coordinating muscle movement. It accounts for approximately 13% of the total brain’s volume in primates. This is a lot less in non-primates.

Cerebellum – This part of the brain controls balance, posture and coordinates muscle movement.

Corpus Callosum – This acts as a connection between the left and right cerebral hemispheres.

Cerebrum Frontal Lobe – This is used for reasoning, emotions and judgement.

Medulla Oblongata – This controls automatic functions such as breathing.

Cerebrum Occipital Lobe – This receives input from the eyes and controls what the visual cortex sees.

Cerebrum Parietal Lobe – This controls sensory inputs.

Cerebrum Temporal Lobe – This controls hearing and memory.

Pituitary Gland – This secretes hormones.

Pons – This connects the cerebrum with the cerebellum.

The size of the cerebrum varies widely across species. A cerebrum can only be found in mammals but is most developed among humans. This is the most recently evolved part of the brain and controls high level processes such as logic, creative thought, language and the interpretation of sensory information. The size varies widely across different species. Primate evolution has been characterised by the expansion of the cerebrum. The cerebrum takes up 80% of the human brain size as opposed to a mere 20% in shrews. Shrews are not considered to be very intelligent creatures. The primate cerebrum is considerably larger and more wrinkled than that found in other mammals. The wrinkles increase the surface area allowing a greater density of nerve cells to be present, thus making the cerebrum more efficient in primates.

Why then are primate brains large?

Many studies have been carried out concerning primates and non-primates of the same body weights. These studies have shown that on average the primate brain is 2.3 times larger than that of non-primates with the same body mass. Evidence for this can be seen numerous times through various experiments. Jill Locantore is a scientist working for the National Academy of Sciences, and has carried out experiments on the subject of brain size and agrees with these findings.

It has been found that species of primates that eat mainly fruit tend to have larger brains that those that eat mainly leaves. This is due to the ease in obtaining the food. Fruit eating monkeys (such as Ateles geoffroyi) have considerably larger brains than leaf eating moneys (such as Alouatta spp) even though they are very closely related. Leaves are easy to find and are commonly available throughout the year. Contrary to this, fruit is much less abundant, available at certain times of the year and can be found only in specific places. Harvesting of fruit requires greater amounts of processing power. It requires the animal to remember where fruit bearing trees are situated and when they will be in season. The evolution of larger brains has helped the fruit eating monkeys deal with both the change in diet and their food foraging methods.

Foraging strategies can evolve which require thought and thus larger brains. Many primates have evolved to make use of tools to help them extract food from otherwise inaccessible locations. The use of tools is a strong selection pressure for evolution as animals with the ability to obtain food easily are more likely to survive. Frontal areas of the cerebrum, such as the Occipital Lobe (visually stimulated part of the brain), are used to interpret visual information to plan and execute motor movements. The Occipital Lobe receives visual signals from the eyes.

Recent studies carried out by the University of Palma have found and named a certain set of neurons in the cerebellum “mirror cells”. These cells, as their name suggests, recognise specific actions and participate in the performing of the same action, i.e. imitating. Giacomo Rizzolatti, a scientist at the university, observed the firing pattern of these cells in monkeys. He found that when a monkey observes another monkey performing some action, these cells fire. These cells play a role in imitative behaviour (learning through observation). This is where the phrase “monkey see, monkey do” comes from. Observational learning is extremely important in teaching infants new techniques. An example of this occurred at the Gombe National Park situated in Tanzania (East Africa). Infant chimpanzees imitated their mother as she broke off small branches and poked them into holes in the search of termites.

The “mirror” cells are also important to the ability of trying to take the mental perspective of others. This ability allows primates to speculate about what other primates are thinking. Through various tests carried out by the University of Palma, it has been shown that primates that have suffered damage to the frontal cerebrum are less empathetic, sympathetic and miss social cues leading to inappropriate actions and judgements being made.

The brain changes according to its environment. Tests have shown that string instrument players have larger cortical areas (areas in the cerebral cortex) devoted to their left hand than their right hand. This has been reported in the American Science magazine, October 1995.

Brain Costs – How did primates afford large brains?

Brain tissue requires large amounts of oxygen and glucose and requires constant cooling via the circulatory system. A lack of these will cause damage to the brain. Therefore, brains are expensive to maintain. When we are resting, our brain uses between 20% and 25% of our total energy. The building blocks of brains consist of fatty acids (called arachidonic acid) and DHA (docosahexanoic acid). These fatty acids are essential for brain growth and without such, growth would cease. The primate’s diet would have had to have changed in order for them to have obtained the required nutrients for brain growth. Many scientists believe that primates overcame the issue of increased energy for brain growth by losing roughly the same amount of intestinal tissue. Intestinal tissue is used to extract nutrients from poor diets, such as leaves.

Evidence for this theory can be seen by looking at Loren Cordain’s papers from the University of Colorado. Humans have considerably less intestinal tissue than apes. Apes require more of this tissue to extract the valuable nutrients from their poorer diet. Our ancestors probably fed on richer foods in terms of calories and the essential brain fatty acids leading to an increase in brain size over time. Even with the richer food sources, evolution was required to give larger brains more time to develop. This has led to offspring being carried for longer and slower offspring growth. This slow down gave our ancestors more time to accumulate the brain building blocks. Presently, brain size in human offspring cannot become any larger due to pregnancy. If offspring brain size became any larger, the females of the species would not be able to bring their young into the world.

Humans are the slowest maturing primate and the most carnivorous. Much research has been carried out into what our ancestors would have had to have eaten in order for the brain to have evolved to a size larger than that of the ape. A scientist called Cordain carries out research in East Africa at the Gombe National Park. Cordain has stated that the human diet would have had to have consisted of freshwater fish, shellfish, liver, brain, muscle from large game along with the original wild nuts, roots, tubers and vegetation.

The large ape spends between eight and ten hours a day foraging and eating. Our human ancestors may not have had to spend as much time eating in order to obtain the equivalent nutritional value as their food was of a higher value. This would have given our ancestors more time to explore and learn new skills. Obtaining food through hunting requires many brain skills.

Could it be social reasons?

With the exception of prosimians who live solitarily, all other primates are social animals who interact with one another. All monkeys and apes live within their own socially defined groups interacting with one another. This complex sociality brings about its own social selection pressure which favours the evolution of social problem solving skills and social adaptations. Social interactions are controlled by the prefrontal and temporal cortices in the brain. These brain areas control the cognitive mechanisms underlying knowledge. They detect category specific related patterns. Therefore, the degree of development in these areas in different species could be used to reflect their different levels of sociality.

It has been found that primate species that live in large social groups tend to have bigger brains. Large social groups tend to bring about more complex problems and interactions. An example of such a group of primates is gibbons. The animals live in small family groups between two and six individuals. Gibbons have the smallest brains of all apes. On the other side, chimpanzees form social groups of seventy or more. These animals carry out a lot of social interaction and have a large brain.

The formation of social groups to compete for food, mates and status plays a very important role in brain size and evolution. This correlation can also be applied to the humans. It is estimated that the largest group of people where everyone knows each other well is about 150 people. This is about the same size as a modern army unit. From this, it can be seen that the primate brain has managed to evolve to meet the demands of complex social interaction.

Much research has been undertaken into brain size and the size of the animal’s territory. Two scientists, Clutton-Brock and Harvey, found that the brain size of cercopithecines (a type of old world monkey) correlates closely with the size of their territory. They also discovered that monogamous species (where the male mates with a specific female and vice versa) have smaller brains than polygynous species (where multiple mating occurs). They found that the size of the animals’ territory varies with the number of individuals in their social group. As expected, monogamous species have smaller social groups than polygynous ones. This is because monogamous species do not require such a large social group as they mate only together.

The number of extra-cortical indices (the greater the number of extra-cortical indices, the larger the brain – ECI) have been studied in old and new world monkeys by Clutton-Brock and Harvey. They have discovered that polygynous groups had a higher count of ECI’s than that found in monogynous groups. This indicates that that social environment does make a difference to the animal’s brain size. They have also discovered that in old world monkeys, terrestrial (monkeys that live on the ground) groups had a higher ECI count than that of arboreal groups (monkeys that live in the trees). This suggests that the animal’s social environment does make a large difference to the primate’s brain size.

Sociality can be classified into three main categories. These are transmission of novel behaviours, deception and alliance information. Capuchin monkeys, a type of new world monkey, have learnt to break open hard nuts by smashing them against a rock shaped like an anvil. This useful behaviour has been transmitted to others in the primate’s social group through watching and learning. Deception and alliance information require that the primates have a knowledge of rank relations throughout the group.

Deception can occur in primates. It does not happen that regularly but when we see it, we take a lot of notice of it. An example of this has been clearly described by primatologist Frans de Waal (University of Emory). He observed an incident between two dominant male chimpanzees. One of the chimpanzees sustained a minor injury to his hand during a fight and was seen limping afterwards, but only when the other chimpanzee was in sight. When the other chimpanzee was out of sight he walked normally. The injured chimpanzee could be seen to be deceiving the other chimpanzee in the hopes that he would be left alone. This indicates that chimpanzees have an idea about the mental states of others. However, evidence is limited to support this.

Alliance formation requires an animal to analyse a considerable amount of information using a lot of processing power. The animal must be able to compare the costs and benefits of actions involving predictions of other animals actions before a situation arises. Primate’s strengthen their relationship with their allies before a situation arises. If a primate is under attack, it will look for help from its allies. This security requires the use of a large prefrontal and temporal cortex.

Combinations of these social skills give the individuals an advantage in social environments. This was suggested more than a century ago by Darwin. He suggested that the evolution of intelligence is linked with living in social groups.

This section indicates the ratchet effect occurring. The more socially active an animal becomes, the more complicated the required tasks become. This ratchet effect could be used to partially explain why primates brains are large. Complicated tasks require large amounts of processing and it is the brain that carries out this processing. Primates with larger brains would be more likely to survive as they will know better survival techniques, so are therefore more likely to reproduce.

Humans are primates so why are our brains not getting any bigger?

Evolutionarily stable strategies (ESS) are species specific strategies which have evolved to a point where they cannot be bettered by any feasible alternative and therefore cannot be invaded by a new mutant gene.

I believe human brains have reached a stage where they have become stable. We currently do not use 100% of our brains so room for improvement is still possible. Due to this, I think that our brains will stay a similar size for many years to come.

However, this is my personal opinion and will not be able to be proved either way for many years to come.


After looking at the different theories presented above, there is a considerable amount of evidence suggesting that the evolution of intelligence correlates with the size of primates brains.

If we accept Darwin’s theory that knowledge and intelligence is evolved through living in social groups, more research is required to ascertain whether or not social groups of all cultures and standards have evolved intelligence.

In conclusion, it takes a combination of all of the factors mentioned previously for primates to obtain larger brains. As long as the brains building blocks are available in sufficient quantity and a reason for evolution exists, whether it be social or essential for foraging, then the primate brain will continue to evolve.