Humans have exceptionally large brains; taking body size into account the human brain is seven times larger than those of other mammals. It has long been assumed that the benefit of this large brain is high intelligence A Brazilian, Suzana Herculano-Houzel, has recently challenged this. There are a number of small brained animals who are very intelligent, such as the capuchin monkey, and also large brained animals who are low on smarts. Herculano-Houzel wondered if it was a mistake to compare brains in terms of size (allowing for brain:body weight ratio). She thought brains from different groups of animals might be organised differently and this might matter more. To investigate this she calculated the number of neurons in each species’ brains – no one had done this before.  Up to this point they had just estimated the number of neurons in a brain. She devised a fractionator method to do this and found human brains contain 86 billion neurons.

Comparing the neuron counts of different animals what she found was that human brains had more neurons per brain volume than say rodents – but not more than other primates. Herculano-Houzel says people have been overemphasising the importance of body weight in comparisons of brain size. For example, gorillas weigh about the same as humans but their brains are about 1/3 the size. It’s the body size of the gorilla that is the outlier. In primates it seems that neuron size doesn’t increase with body size so the brain: weight ratio doesn’t apply. So gorillas brains are just as powerful as human brains.

The conclusion to be drawn is that the number of neurons in a brain and how they are connected matters more than size. You can read more about this research here.