Brains Beyond Body Size
A sperm whale's brain weighs 7.8 kg; a human's weighs 1.35 kg. Yet no neuroscientist argues that sperm whales are cognitively superior. The key insight is that larger bodies demand larger brains simply to manage more muscles, sensory neurons, and homeostatic circuits. The encephalization quotient (EQ) corrects for this by comparing actual brain mass to the mass predicted by allometric scaling — the 'extra' brain tissue above the body-size baseline correlates with behavioral complexity.
The Allometric Baseline
Harry Jerison established in 1973 that mammalian brain mass scales as Br = 0.12 × M^0.76. This power law defines the expected brain mass for a given body size. Species above the line (positive residuals) are more encephalized than average; species below are less. The simulator plots this regression line and lets you position any species to see where it falls relative to the mammalian trend.
The Human Outlier
Humans are the most extreme outlier in the mammalian brain-body dataset, with an EQ of 7.4 — our brains are over seven times larger than predicted. This excess comes at enormous metabolic cost: 20% of resting energy consumption for 2% of body mass. The evolutionary pressure driving this expensive tissue expansion — social competition, ecological intelligence, cultural transmission, or some combination — remains one of the most debated questions in paleoanthropology.
Beyond EQ: Neuron Counts
Recent work by Suzana Herculano-Houzel has shown that neuron count may matter more than brain mass. The human cerebral cortex contains 16 billion neurons — more than any other species, including elephants (5.6 billion cortical neurons despite 3× larger brains). Primates pack neurons more densely than other mammals due to a different neuronal scaling rule, giving humans a cognitive edge that EQ alone does not fully capture.