What is the encephalization quotient?

• A. The ratio of the brain weight of an animal to the brain weight predicted for a comparable animal of the same body size.
• B. The percentage of metabolic energy used by the brain.
• C. The absolute brain weight divided by body weight.
• D. The ratio of heart weight to brain weight.

Answer: (A)

Encephalization quotient measures how large an animal’s brain is relative to what would be expected for its body size. It compares the observed brain weight to the brain weight predicted for a species of that body size, typically using a broad allometric model across many species. A higher quotient means the brain is larger than expected for that body size, which researchers use to infer potential cognitive differences beyond what body size alone would predict. This is why the correct description is the ratio of actual brain weight to the predicted brain weight for a comparable animal of the same body size. The other options describe brain-to-body weight ratios without adjusting for the expected brain size dictated by body size, or refer to brain energy use or a different organ ratio, which don’t capture the normalization that ECQ provides.