Why are human brains the biggest?
Category: Biology Published: October 24, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
The brains of humans are not the biggest compared to all other animals. The average human brain has a mass of about 1 kg. In contrast, the brain of a sperm whale has a mass of 8 kg and that of an elephant has a mass of 5 kg. You may be tempted to think that bigger brains means smarter and that because humans are the smartest animals, we must have the biggest brains. But biology does not work that way. When an animal has a larger overall size, all of its organs must be generally bigger just to keep it alive. A sperm whale weighs 35 to 45 metric tons and stretches about 15 meters long. With so much biological tissue to take care of, the sperm whale needs a giant heart to keep blood pumping to all this tissue, giant lungs to provide oxygen to all this tissue, and a giant brain to coordinate it all. The sperm whale needs a giant brain not because it is intelligent, but because it has so much low-level functions to carry out with such a huge body.
Intelligence is instead more strongly linked to the brain size relative to the body size. If a large brain is housed in a small body, there is less low-level signal processing and control that the brain has to carry out. As a result, the brain can be freed up for more higher-order thinking. But even this picture is over-simplified. Intelligence seems to depend on many things: absolute brain size, relative brain size, connectivity, energy expenditure, etc. But as a rough model, relative brain size is indeed correlated to intelligence within main groups of organisms. While humans do not have the biggest brains overall, we do have the biggest brains relative to our body size among the mammals.