Scientists have traditionally held the belief that larger animals tends to have a larger brains. However, recent research challenges the simplicity of this assumption, offering fresh perspectives on the evolution of brain and body size across species.
Array of animal brains and sizes in animal kingdom
The array of brain shapes, sizes, and structures across the animal kingdom is remarkable. Generally, a larger brain is associated with higher cognitive capabilities, yet this size largely corresponds with the overall size of the animal.
When animals increase in size, so does their brain, but the relationship isn’t perfectly proportional – a concept referred to as allometric scaling.
Mammalian size varies widely, from the minuscule bumblebee bat to the colossal blue whale. To make any meaningful statements about the evolution of brain size, one must consider how it has evolved in tandem with body size.
Research on mammalian species
Our latest research aimed to unpack this relationship by employing advanced computational methods to analyze data from over 1,500 mammalian species. We explored the evolutionary trajectory of brain sizes in correlation to their respective body sizes.
Surprisingly, our findings suggest a non-linear, curved relationship between brain and body size, plateauing beyond a certain body size threshold.
This contradicts previous assumptions of a more straightforward relationship and sheds light on why some large animals have brains that are small in proportion to their body size.
Investigation on some large animals having relatively small brains
Our investigation into why some large animals have relatively small brains revealed that this phenomenon could not solely be attributed to the energetic costs of sustaining a large brain or to neuron density, which might afford greater processing capabilities without an increase in brain size. The precise reasons behind this finding remain elusive.
Additionally, our study identified mammalian species that deviate significantly from the established brain-body size relationship, highlighting lineages with rapid brain size evolution.
Human brain vs animal brain
Notably, Homo sapiens stand out for having brains that evolved over 20 times faster than the average mammalian species, contributing to our species’ exceptional cognitive abilities.
Apart from humans, other mammals like rats, bears, dolphins, and elephants, known for their intelligence, along with species like wallabies, marmots, and quolls, have also experienced rapid increases in brain size.
In contrast, the evolution of bats shows a significant initial decrease in brain size, with minimal changes thereafter, suggesting potential evolutionary constraints possibly linked to flight.
Marsh-Lartet rule
Our findings also reveal that carnivores, rodents, and primates are the most frequent subjects of rapid brain size evolution, typically resulting in larger brains proportionate to their body size.
This supports the Marsh-Lartet rule, suggesting an evolutionary trend towards larger relative brain sizes, although not universally observed across all mammalian species.
Evolution of Brains
Significantly, primates, including humans, represent a lineage where brain size has consistently increased, from the earliest primates whose brains composed merely 0.5% of their body size to our significantly larger brains today.
This research underscores the notion that simple explanations often underlie nature’s complexities. Nonetheless, the dynamic of natural selection introduces complexity, demonstrating that the evolution of brain and body size can break from established patterns, leading some species to evolve beyond the expected curves.