Which animal has the largest brain relative to its body size?

If terms like “bird brain” and “harebrained” are any indication, animal brains present fascinating and diverse examples of one of the most complex organs known to science. Animal brains differ not only in overall size but also in their size relative to the animal’s body mass.

At 18 pounds (8 kilograms), on average, the sperm whale (Physeter macrocephalus) has the largest brain, but has an overall body mass of 45 tons (40 metric tons), giving it a brain-to-body-mass ratio of 1:5,100. But which animal has the biggest brain relative to its body size?