Brain size was previously considered a major indicator of the intelligence of an animal.
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.
Recent discoveries concerning bird intelligence have called into question the usefulness of brain size as an indicator.
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 most promising suggests that cetacean brain size and complexity increased to support organisms in maintaining complex social relations with one another Dolphin group sizes vary quite dramatically.
River dolphins usually congregate in fairly small groups from 6 to 12 in number or, in some species, singly or in pairs.It is unknown whether every member of the group is acquainted with every other.However, large packs can act as a single cohesive unit – observations show that if an unexpected disturbance, such as a shark approach, occurs from the flank or from beneath the group, the group moves in near-unison to avoid the threat.For example, the echidna has a highly developed brain, yet is not widely considered to be very intelligent.Unlike terrestrial mammals, dolphin brains contain a paralimbic lobe, which may possibly be used for sensory processing.The basis of this social structure is the matriline, consisting of a mother and her offspring, who travel with her for life.