The brain-to-body mass ratio is a metric used by scientists to compare brain size across different species. This calculation divides an animal’s brain mass by its total body mass, yielding a proportion that can be standardized for comparison. While a large animal will naturally possess a large brain simply to manage its vast physical structure, the relative size of the brain compared to the animal’s overall mass hints at the proportion of neural tissue available for more complex processing. Analyzing this ratio helps researchers understand the biological factors that correlate with intelligence.
Measuring Relative Brain Size
Comparing the raw mass of a brain to the mass of the body provides a preliminary metric of relative brain size. However, this simple ratio has a significant limitation because brain size does not increase linearly with body size across different species. This phenomenon is due to allometric effects, where the body’s proportions change as size increases, meaning larger animals tend to have a smaller ratio. As a result, simply dividing brain mass by body mass can inaccurately group species with vastly different cognitive capabilities.
To correct for the misleading effects of body size, scientists developed a more sophisticated measure known as the Encephalization Quotient (EQ). The EQ compares an animal’s actual brain mass to the brain mass that is statistically expected for an animal of its body size. The expected brain mass is determined by establishing an average line of best fit based on the brain and body masses of numerous reference species.
An animal with an EQ of 1 has the brain size expected for its body mass, while an EQ greater than 1 suggests a greater potential for advanced cognitive function. Humans, for instance, have one of the highest EQs, typically around 6 or 7. This means the human brain is six to seven times larger than the brain size expected for an average mammal of the same body weight. The EQ serves as a refined tool that accounts for the evolutionary changes in brain development that have occurred across diverse animal taxa.
The Animal with the Highest Ratio
When considering the raw brain-to-body mass ratio across the entire animal kingdom, the record holder is a tiny ant from the genus Brachymyrmex. The brain of this ant makes up an astonishing amount of its total body mass, giving it a raw ratio of approximately 1:8. This means the ant’s brain constitutes roughly 12% of its entire physical weight, a proportion necessary for its complex social behaviors and navigation.
Among mammals, the Treeshrew holds the record for the highest raw brain-to-body mass ratio. The Treeshrew has a brain that makes up about 10% of its total body weight, giving it a ratio of approximately 1:10. This disproportionately large brain size illustrates the general principle that smaller animals often have a larger relative brain size. Their small body size means the neural tissue required for basic survival functions takes up a much larger percentage of their total mass.
Absolute Size Versus Relative Size
The animal with the largest absolute brain mass is the Sperm Whale, whose brain can weigh up to 9 kilograms, dwarfing the average human brain mass of about 1.4 kilograms. However, the whale’s immense body mass of up to 40 metric tons results in a very low brain-to-body mass ratio, sometimes as small as 1:5,100. Similarly, the African Elephant has the largest brain of any land animal, but its overall body size still results in a relatively small ratio of about 1:560. These large animals require massive brains primarily to coordinate the vast number of muscle fibers and sensory inputs associated with their huge bodies.
The contrast between the absolute size winner (Sperm Whale) and the relative size winner (Brachymyrmex ant) demonstrates why the simple raw ratio is an imperfect measure of cognitive complexity. The ant has a relative brain size 637 times larger than the whale, yet the whale exhibits far more complex social intelligence. The Encephalization Quotient was developed to bridge this gap, recognizing that the sheer number of neurons available for abstract thought is what matters.
Beyond Volume What Really Determines Intelligence
The size of the brain, whether absolute or relative, only tells part of the story when evaluating an animal’s cognitive abilities. Scientists have shifted focus to the internal architecture of the brain, as features within the neural tissue correlate more closely with intelligence.
Neural Density and Folding
One of the most important factors is the total number of neurons present, particularly in the cerebral cortex, which is responsible for higher-order functions. The human brain contains a massive concentration of neurons in its cortex compared to other mammals, which is a stronger predictor of cognitive performance than total volume alone. Another key feature is cortical folding, or gyrification, which increases the surface area of the cerebral cortex without requiring a larger skull size. This folding allows for a far greater number of connections and a higher density of gray matter.
Network Efficiency
The efficiency of the neural network also plays a role, determined by the ratio of gray matter to white matter. Gray matter handles information processing, while white matter consists of the myelinated axons that transmit signals between brain regions. The organization and complexity of the dendritic trees also influence the speed and efficiency of information transfer. Research has shown that in humans, a thicker cerebral cortex in certain frontal and temporal regions correlates with higher intelligence scores.