This comparison seems more favorable if one excludes the large amount of insulating blubber (15-20% of mass). In some dolphins, it is less than half that of humans: 0.9% versus 2.1%.
The brain to body mass ratio (not the encephalization quotient) in some members of the odontocete superfamily Delphinoidea (dolphins, porpoises, belugas, and narwhals) is greater than modern humans, and greater than all other mammals (there is debate whether that of the treeshrew might be second in place of humans).This is slightly greater than that of humans (1,300–1,400 grams) and about four times that of chimpanzees (400 grams). Bottlenose dolphins ( Tursiops truncatus) have an absolute brain mass of 1,500–1,700 grams.Orcas ( Orcinus orca) have the second largest known brain mass of any extant animal.Sperm whales ( Physeter macrocephalus) have the largest known brain mass of any extant animal, averaging 7.8 kg in mature males.Comparison of actual brain size with the size expected from allometry provides an encephalization quotient (EQ) that can be used as a more accurate indicator of an animal's intelligence. Allometric analysis indicates that in general, mammalian brain size scales at approximately the 2⁄ 3 or 3⁄ 4 exponent of body mass.
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. However, many other factors also affect intelligence, and recent discoveries concerning bird intelligence have called into question the influence of brain size. Brain size was previously considered a major indicator of the intelligence of an animal.
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