The latest study published in Nature Ecology and Evolution has debunked the notion that the largest animals always have larger brains, with humans being the exception to this rule.
A team of researchers from the University of Reading and Durham University compiled a vast amount of data on brain and body sizes from approximately 1,500 species to shed light on the age-old debate surrounding brain size evolution.
The significance of having larger brains relative to body size has been linked to intelligence, social behavior, and complexity in behavior, with humans having notably evolved with disproportionately large brains. The new findings, released today (Monday, 8 July), challenge the conventional wisdom by revealing that the biggest animals do not necessarily have larger brains in proportion to their size.
Professor Chris Venditti, the lead author of the study from the University of Reading, explained that for over a century, scientists had assumed a linear relationship between brain and body size, assuming that as an animal grows larger, its brain also grows proportionally. However, the study now indicates that this is not the case, revealing a curvilinear relationship where very large animals have relatively smaller brains than expected.
Professor Rob Barton, a co-author of the research from Durham University, added: “Our findings provide a clearer understanding of the intricate relationship between brain and body masses. Our model simplifies previous complex explanations, indicating that relative brain size can be studied using a single fundamental model.”
Breaking the Mold
The study unveils a straightforward correlation between brain and body sizes in all mammals, enabling the researchers to identify species that defy the established patterns.
One such outlier is our own species, Homo sapiens, which has evolved more rapidly than all other mammal species, leading to the remarkable brain size that distinguishes humanity today. However, humans are not the only creatures deviating from the norm.
All mammalian groups exhibited sudden bursts of change in brain size, both shrinking and expanding. For instance, bats swiftly decreased their brain size upon emergence but later displayed sluggish changes in relative brain size, hinting at possible evolutionary restrictions linked to flight demands.
Three groups of animals showed the most significant rapid changes in brain size: primates, rodents, and carnivores. In these groups, there is a tendency for relative brain size to increase over time (known as the “Marsh-Lartet rule”), contrary to the previously widely accepted belief.
Dr. Joanna Baker, another co-author from the University of Reading, remarked, “Our findings uncover a perplexing phenomenon. In the largest animals, something hinders brain expansion beyond a certain point. The possibility that maintaining excessively large brains may be too resource-intensive remains to be explored. Furthermore, as a similar curvilinear trend is observed in birds, it appears to be a widespread occurrence, with diverse animals exhibiting this ‘curious ceiling’ driven by different biological factors.”
