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A model of a human brain from three different angles

We’ve helped clarify centuries of controversy surrounding brain size evolution - and resolved some puzzling complexities in the relationship between brain size and body mass.

Researchers from the University of Reading and the Department of Anthropology here at Durham analysed an enormous dataset of brain and body sizes from around 1,500 species.  

Bigger brains relative to body size are linked to intelligence, sociality, and behavioural complexity – with humans having evolved exceptionally large brains.  

For over a century, scientists have assumed that the brain size and body mass relationship was linear, meaning that brain size gets proportionally bigger the larger an animal is.   

This new research reveals that the relationship is a curve, meaning very large animals have smaller brains than expected.  


The study, published in the journal Nature Ecology & Evolution, identified the ‘rule-breakers’ between brain and body size across all mammals.  

Species that fall outside of the norm includes our own, Homo sapiens, which has evolved more than 20 times faster than all other mammal species.  

But humans are not the only species to buck this trend.  

All groups of mammals demonstrated rapid bursts of change, both towards smaller and larger brain sizes.  

For example, bats very rapidly reduced their brain size when they first evolved, but then showed very slow rates of change in relative brain size.  

This suggests that the demands of flight may have put an evolutionary constraint on the creatures.   

Further mystery  

There are three groups of animals that showed the most pronounced rapid change in brain size: primates, rodents, and carnivores.   

In these three groups, there is a tendency for relative brain size to increase in time.  

However, this is not a universal trend across all mammals, as previously believed.  

The study also showed there is something preventing brains from getting too big in larger animals.   

This could be due to brains beyond a certain size taking too much energy to maintain.   

However, because the researchers observed similar curvature in birds, the pattern seems to be a general phenomenon across species.  

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