Which shark has the largest brain-to-body mass ratio?

The hammerhead shark has the largest brain-to-body mass ratio among the sharks commonly compared in this trivia question. Hammerheads, especially large species such as the great hammerhead, are regarded as highly encephalized sharks, meaning they have relatively large brains for their body size.

Their unusual cephalofoil—the broad, flattened head that gives them their name—does more than create a distinctive silhouette. It houses widely spaced sensory organs and eyes, improves visual coverage and depth perception, and provides room for an extensive network of electroreceptors used to detect prey.

Hammerheads also have well-developed regions involved in processing smell, movement and sensory information. Their brains are not simply large because their heads are wide: brain mass is measured separately from the surrounding cartilage and tissues. Researchers use brain-to-body ratios and related measures such as the encephalization quotient to make comparisons.

The answer should not be read as proof that hammerheads are automatically the most intelligent sharks. Brain-to-body ratios vary by species, age, body size and measuring method, and a ratio alone is an imperfect measure of intelligence. Still, hammerheads are the standard canonical answer for this comparison.