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?
A 2009 study in the journal Brain, Behavior and Evolution (opens in new tab) found that an especially tiny genus of ant has the largest brain for its body size. Brachymyrmex has an average body mass of up to 0.049 milligrams and an average brain mass of 0.006 milligrams. That means its brain is