A new study of 1,500 species shows that larger animals do not have proportionally larger brains, challenging old ideas and introducing a curve model for brain-body size relationships, with significant findings in primates, rodents and carnivores. Credit: SciTechDaily.com
Researchers have discovered that brain size in larger animals does not increase proportionally with body size, contradicting old beliefs.
The study, which included data from 1,500 kinddemonstrates a nonlinear relationship and highlights rapid evolutionary changes in brain size in primates, rodents, and carnivores.
Challenge to Traditional Beliefs About Brain Evolution
Researchers from the University of Reading and the University of Durham have compiled a huge dataset on the brain and body sizes of around 1,500 species, in an effort to clarify centuries of controversy surrounding the evolution of brain size.
Larger brains relative to body size have been linked to intelligence, sociality and behavioral complexity – with humans having evolved exceptionally large brains. The new research, published today (July 8) in Nature Ecology & Evolutionturns out that the largest animals do not have proportionately larger brains, challenging long-held ideas about brain evolution.
Professor Chris Venditti, lead author of the study from the University of Reading, said: “For more than a century, scientists have assumed this relationship was linear – meaning that brain size increases proportionally as an animal gets bigger. We now know that this is not true. The relationship between brain and body size is a curve, which essentially 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 confusing complexity in the relationship between brain and body mass. Our model has a simplicity that means previously elaborate explanations are no longer necessary – relative brain size can be studied using a single underlying model.”
Identifying evolutionary outliers
The study shows a simple relationship between brain and body size across all mammals, allowing researchers to identify the offenders – species that challenge the norm.
Our own species is also among these outliers, Homo sapienswhich has evolved more than 20 times faster than any other mammal, resulting in the enormous brains that define humanity today. But humans are not the only species to buck this trend.
All mammal groups showed rapid changes – both toward smaller and larger brain sizes. Bats, for example, reduced their brain size very rapidly when they first evolved, but then showed very slow changes in relative brain size, suggesting that there may be evolutionary constraints related to the demands of flight.
Exceptional growth in brain size in certain mammals
There are three groups of animals that show 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 for all mammals, as was previously thought.
Dr Joanna Baker, co-author of the study, also from the University of Reading, said: “Our results reveal a mystery. In the largest animals, there is something that prevents brains from growing too big. Whether this is because large brains are simply too expensive to maintain above a certain size remains to be seen. But since we also see similar curvatures in birds, the pattern seems to be a general phenomenon – whatever causes this ‘strange ceiling’ applies to animals with very different biology.”
Reference: “Co-evolutionary dynamics of mammalian brain and body size” by Chris Venditti, Joanna Baker and Robert A. Barton, July 8, 2024, Nature Ecology & Evolution.
DOI file: 10.1038/s41559-024-02451-3