Cetacean intelligence

Cetacean intelligence refers to the cognitive capabilities of the Cetacea order of mammals. This order includes whales, porpoises, and dolphins.

Brain size was previously considered a major 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 ⅔ or ¾ 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.

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, species all well known for their high intelligence, are the only others known to have spindle cells. 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.

Elephant brains also show a similar complexity to dolphin brains, and are also more convoluted than that of humans, and with a cortex "thicker than that of cetaceans". It is generally agreed that the growth of the neocortex, both absolutely and relative to the rest of the brain, during human evolution, has been responsible for the evolution of human intelligence, however defined. While a complex neocortex usually indicates high intelligence, there are exceptions to this. For example, the echidna has a highly developed brain, yet is not widely considered to be very intelligent.

...
Wikipedia