While scientists can't agree on a single definition of “intelligence,” they generally concur that humans are the most intelligent living creature on Earth. Because of this belief, some people assume that human beings also have the biggest brains – they assume that the bigger the brain, the smarter the animal. But that is not exactly true.

The average adult human brain weighs about 3 pounds – the same weight as the average brain of a dolphin (which is also a very intelligent animal). But there are animals with larger brains that are not considered to be as intelligent as a dolphin. For instance, a sperm whale has a brain that weighs about 17 pounds.

On the other hand, some relatively “smart” animals have small brains – for example, dogs and orangutans: A beagle's brain is about 2.5 ounces, and an orangutan's brain is about 13 ounces. In general, the larger the animal, the larger the skull, and therefore, the larger the brain.

So, back to the question: Does brain size affect an animal’s intelligence? The relationship between brain size and intelligence doesn’t have to do with actual brain weight. It has to do with the RATIO of brain weight to the entire body weight. For example, human beings have a ration of about 1-to-50. Most other mammals have a ratio of 1-to-180. For birds, the ratio is 1-to-220. So the brain takes up more weight in human beings than it does in other animals.

Intelligence also has to do with the different components of the brain. Humans have the largest cerebral cortex of all mammals, relative to the size of their brains. This area houses the cerebral hemispheres, which are responsible for higher functions like memory, communication and thinking. Birds, fish and reptiles, in comparison, have smaller cerebral cortexes relative to the size of their brains.

So a bigger brain doesn’t necessarily mean a smarter animal – it’s all in the ratio!