In primates, the group size correlates with brain size. In other mammals and birds this is not the case.

Machu Picchu, Peru
Machu Picchu, Peru

There, however, it is apparent that the monogamous species (in pairs) have a larger brain than the polygamous species. Dunbar’s interpretation of this finding is,

that the original incentive for the evolution of larger brains can be found in the development of bonding in couples, which usually goes hand in hand with the fact that both parents care for the offspring (…)

We can imagine that in the event that when pair bonding became an established practice, it led to larger brains and the cognitive ability to deal with complex relationships (relations). Primates have succeeded in exploiting these cognitive abilities by generalising them so that they would be available to all members of social groups. (pp. 249s)

So with the larger brain, which was developed through pair bonding, more complex social systems could be mastered with friends i.e. with non-reproductive partners.


The larger the brain, more precisely: the neocortex (historically the youngest part of the cerebral cortex) in a primate species,

Machu Picchu, Peru
Machu Picchu, Peru

the larger the group in which this primate species can live. That implies that the size of the brain depends on the number of social contacts that individuals of this species manage. Dunbar continues:

Furthermore, additional analyses have shown that a number of behavioural patterns that are particularly associated with the social complexity of primates are also correlated with the relative size of the neocortex.

These include the size of the grooming clique (grooming: mutual body and fur care), the use of alternative mating strategies in males, the use of coalitions and alliances, manoeuvring for tactical deception, and the quality of social play. (pp. 247s)


Hypothesis 10

The social brain

Parade, Cusco, Peru, 27. 8. 1989
Parade, Cusco, Peru, 27. 8. 1989

With the brain size of monkeys, the potential size of the social group increases, as individuals with the larger brains can establish and maintain social relationships with a larger number of group members (Robin Dunbar).

The advantage of the larger social group and the advantage of greater contact ability lead to a selection of individuals and groups with the larger brains. Selection promotes brain growth, as confirmed by hominid fossils.


The brain is an expensive organ because it consumes a lot of energy that the living being has to provide. Why did it paid off in evolution to invest in large brains?

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III.     The brain

Hypothesis 9

The “expensive” brain

Parade, Cusco, Peru, 27. 8. 1989
Parade, Cusco, Peru, 27. 8. 1989

The brain is a very expensive organ for a living organism because it consumes a lot of energy. Nevertheless, in mammals, primates, monkeys, great apes and finally hominids all the way up to humans, a steady increase in relative brain size can be observed.


In the beginning, two statistics related to energy consumption should make the costs of the brain clear: 1. The brain is responsible for approximately 20% of the total energy consumption of the body. 2. More than 50% of the energy that the human foetus absorbs is used to build up its brain.

Now three authors who have weighed in on the issue of the costs of the brain in the context of human evolution will have their say. Robin Dunbar, from whom I used the first percentage, wrote:

Brain tissue is unusually expensive to grow and maintain. It needs about ten times more energy than one would expect, based on its weight, and it is the most expensive tissue after that of the heart and liver.

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