Research has shown that the large brain size of octopuses may be due to environmental influences, and not necessarily to the nature of their social behavior.
The "social brain" hypothesis posits that brain size increases with the number of social interactions , a theory supported by monkeys and dolphins. However, cephalopods such as octopuses and squid also possess large brains despite their solitary lifestyles and lack of complex social interactions.
Michael Mutukrishna of the London School of Economics and his colleagues studied 79 species of cephalopods, measuring the total size of their central nervous systems. They found that the brains were larger in species living in shallow waters or near the bottom, where there are more things to interact with and manipulate. In deep-sea environments, the brains are typically smaller.
Motokrishna said that this association is fairly strong, but brain data are only available for 10% of the 800 cephalopod species.
Professor Robin Dunbar believes the lack of a social brain in octopuses is due to their solitary lifestyle. Paul Katz of the University of Massachusetts hypothesizes that brain size may decrease when they colonize deep waters, as happens to animals on islands.
Motokrishna has previously shown that the brain size of whales and dolphins predicts their social behavior and environmental factors. The similarity with cephalopods supports the "cultural brain" hypothesis, in which informational and environmental factors also contribute to the evolution of large brains. However, it is difficult to draw conclusions about the behavior of modern species within the context of cephalopod evolution over 500 million years ago.
Dunbar adds that octopuses require a lot of brain resources because of their eight independently controlled limbs.
