Cetacean Intelligence - Brain Size

Brain Size

Brain size was previously considered an indicator of the intelligence of an animal. However, many other factors also affect intelligence. Recent discoveries concerning bird intelligence have called into question the usefulness of brain size as an indicator. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the 2⁄₃ or 3⁄₄ exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalization quotient (EQ) that can be used as another indication of the animal's intelligence.

• Sperm whales (Physeter macrocephalus) have the largest brain mass of any extant animal, averaging 7.8 kg in mature males.
• Bottlenose dolphins (Tursiops truncatus) have an absolute brain mass of 1500-1700 grams. This is slightly greater than that of humans (1300-1400 grams) and about four times that of chimpanzees (400 grams).
• The brain to body mass ratio (as distinct from encephalization quotient) in some members of the odontocete superfamily Delphinoidea (dolphins, porpoises, belugas, and narwhals) is second only to modern humans, and greater than all other mammals (there is debate whether that of the treeshrew might be second) In some dolphins, it is less than half that of humans: 0.9% versus 2.1%. This comparison seems more favorable if the large amount of blubber (15-20% of mass) that dolphins require for insulation is omitted.
• The encephalization quotient varies widely between species. The Orca has an EQ of 2.57, the franciscana dolphin of 1.67, the Ganges River dolphin of 1.55, the bottlenose dolphin of 4.14, and the tucuxi dolphin of 4.56. These are less than the human EQ of 7.44, but some are greater than that of chimpanzees at 2.49, dogs at 1.17, cats at 1.00, and mice at 0.50.

• The majority of mammals are born with a brain close to 90% of the adult weight. Humans are born with 28% of the adult weight, chimpanzees with 54%, bottlenose dolphins with 42.5%, and elephants with 35%.

The discovery of spindle cells (neurons without extensive branching, known also as "von Economo neurons", or VENs) in the brains of the humpback whale, fin whale, sperm whale, killer whale, bottlenose dolphins, Risso's dolphins, and beluga whales is another unique discovery. Humans, the great apes, and elephants are the only other species known to have spindle cells, species all well known for their high intelligence. Spindle neurons appear to play a central role in the development of intelligent behavior. Such a discovery may suggest a convergent evolution of these species.