http://www.highnorth.no/Library/Myths/br-si-bo.htm

Brain size, body weight and intelligence

The brain of the sperm whale weighs 7,800g, the elephant's weighs 7,500g, man's weighs 1,500g, the dolphin's 840g, and the brain of a mouse weighs 0,4g. If these figures are used to determine intelligence, then the sperm whale and the elephant are five times as intelligent as man, who in turn is twice as intelligent as the dolphin, which in turn is 2,000 times as intelligent as a mouse. Should we rank animals in order of how large their brains are in relation to their body weight, then the mouse would come out on top with its brain comprising 3.2%, the dolphin's 0.9% and the sperm whale's 0,021%. Neither absolute brain weight nor the relationship between brain weight and body size provide us with sensible criteria for comparing the intelligence of different species.