Mammals are the champions1. The largest brain belongs to the sperm whale: 7 kg (17.5 pounds). The blue whale, the largest animal on the planet, being twice longer and thrice heavier, has a brain weighing 5 kg (12.5 pounds).
2. Human brain has an average weight of 2.7 pounds (1.2 kg), variations between 1.1 and 1.4 kg being considered normal. Our brain represents 2% of our weight, the largest brain in the animal world compared to the body size.
3. The brain of the men is, on average, 100 grams (0.3 pounds) heavier than that of the women.
4. The largest brain of a terrestrial animal is that of the elephant: 10.5 pounds (4.78 kg). Still, the brain of the elephant makes less than 0.1% of its body weight.
5. Amongst our relatives, the primates, apes have the largest brains. The mountain gorilla (Gorilla gorilla beringei) has a 0.95 pounds (430 g) brain. Still, that of the chimpanzee is larger compared to its body size: 0.77 pounds (350 g) (a chimpanzee male is 2.5-3 times lighter than a gorilla male).
The smallest primate brain is detained by the mouse lemur (Microcebus murinus) of Madagascar: 0.004 pounds (2 g) (this is also the smallest primate in the world).
6. The largest brain of a carnivore mammal is that of the walrus (Odobenus rosmarus): 2.4 pounds (1.1 kg).
7. Domesticated animals have smaller brains than those of their wild forebears. At a similar body weight, the brain of the domestic cat (with an average value of 0.06 pounds or 30 grams) is 23.9 % lighter than that of the wild cat, the brain of the dog is 29 % lighter than that of the wolf, and the brain of the pig is 34 % lighter than that of the wild boar.
8. The octopus is known to be the most intelligent invertebrate. Its brain is made by the largest neurons (brain cells) found in nature.
An octopus brain is formed by 500 million large neurons (while the human brain is made of roughly 100 billion smaller neurons), but the intelligence of this aquatic creature is comparable with that of the apes.
9. After humans, the nature's relatively largest brain is that of the dolphin: 2 kg. A dolphin weighing 120 kg (280 pounds) has a 1.7 kg (4 pounds) brain; compared to its size, this is almost thrice than a chimp.
10. Amongst vertebrates, the smallest brain in relation with the body size seems to have belonged to a dinosaur: Stegosaurus was 6 m (20 ft) long and weighed 2 tons. Its brain was just the size of a walnut! To compensate this, the spine presented a second "brain" in the pelvic area.