Larger animals do not have proportionally larger brains, and humans buck this trend, according to a new study published in Nature Ecology and Evolution Has revealed.
Researchers from the University of Reading and Durham University have compiled a huge dataset on brain and body size from around 1,500 species to clear up centuries of controversy surrounding the evolution of brain size.
A larger brain relative to body size is linked to intelligence, sociability and behavioural complexity, and humans have evolved exceptionally large brains. New research, published today (Monday 8 July), reveals that larger animals do not have proportionally larger brains, challenging long-held beliefs about brain evolution.
Professor Chris Venditti, lead author of the study from the University of Reading, said: “For over a century, scientists have assumed that this relationship was linear – that is, that brain size increases proportionally as the animal gets bigger. We now know that this isn’t true. The relationship between brain size and body size is a curve, which basically means that very large animals have smaller brains than expected.”
Professor Rob Barton, co-author of the study from Durham University, said: “Our results help to resolve the puzzling complexity of the relationship between brain and body mass. Our model has a simplicity that makes previously elaborate explanations unnecessary: relative brain size can be studied using a single underlying model.”
Beyond the Ordinary
The research reveals a simple association between brain and body size across all mammals, allowing researchers to identify offenders – species that defy the norm.
These outliers include our own species, Homo sapiens, which has evolved more than 20 times faster than all other mammal species, giving rise to the enormous brains that characterize humanity today. But humans are not the only species that bucks this trend.
All mammal groups showed rapid changes, both toward smaller brains and toward larger brains. For example, bats very rapidly reduced their brain size when they emerged, but then showed very slow rates of change in relative brain size, suggesting that there may be evolutionary constraints related to the demands of flight.
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 over time (the “Marsh-Lartet rule”). This is not a universal trend across all mammals, as previously believed.
Co-author Dr Joanna Baker, also from the University of Reading, said: “Our results reveal a mystery. In larger animals, there’s something stopping brains from getting too big. Whether this is because large brains beyond a certain size are simply too expensive to maintain remains to be seen. But as we also observed a similar curvature in birds, the pattern seems to be a general phenomenon – whatever causes this ‘curious ceiling’ applies to animals with very different biology.”