Brain size puzzle solved as humans buck evolutionary trend

The biggest animals don’t have proportionally bigger brains — while humans buck the trend — a study published in Ecology and evolution of nature revealed

Researchers from the University of Reading and Durham University have compiled a huge dataset of brain and body size from around 1,500 species to shed light on centuries of controversy over the evolution of brain size.

Larger brains relative to body size are associated with intelligence, sociability, and behavioral complexity—humans have evolved exceptionally large brains. New research reveals that the largest 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: “Scientists assumed for more than a century that this relationship was linear – meaning that brain size increased proportionally the larger the animal. Now we know this is not true, the relationship there is a curve between brain and body size, 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 unravel the puzzling complexity in the brain-body mass relationship. Our model is simple, which means that previously elaborate explanations are no longer necessary – the relative size of the brain can be studied using a single basic model.”

Beyond the ordinary

Research reveals a simple link between brain and body size in all mammals, which has allowed researchers to identify rule-breakers – species that challenge the norm.

Among these outliers is our own species, Homo sapiens, which evolved more than 20 times faster than all other mammalian species, resulting in the massive brains that characterize humanity today. But humans aren’t the only species bucking this trend.

All groups of mammals showed rapid bursts of change—both toward smaller and larger brains. For example, bats reduced their brain size very quickly when they first appeared, but then showed very slow rates of change in relative brain size, suggesting that there may be evolutionary constraints related to the requirements of flight.

There are three groups of animals that have shown the most 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 (“Marsh-Lartet rule”). This is not a universal trend across all mammals as previously believed.

Dr. Study co-author Joanna Baker, also from the University of Reading, said: “Our results reveal a mystery. In the largest animals, there is something that prevents brains from getting too big. Whether it’s because big brains beyond a certain size are simply too expensive for maintenance remains to be seen, but since we see similar curvature in birds, this pattern seems to be a general phenomenon – what causes this ‘strange ceiling’ applies to animals with very different biology.”