Research suggests that the large brains of octopuses are influenced more by environmental conditions than by social interactions.

It is widely accepted that larger mammalian brains correlate with social behavior, a theory known as the social brain hypothesis. The premise is that the more social connections a species has, the larger their brains must be to handle those interactions. This trend is evident among primates, dolphins, and camelids.

In contrast, cephalopods—like octopuses, cuttlefish, and nautiluses—exhibit significant intelligence despite mostly living solitary lives, with limited parental care and minimal social learning.

To delve deeper into the reasons behind the substantial brain size of these creatures, Michael Muthukrishna and researchers from the London School of Economics analyzed data from 79 cephalopod species with available brain information. They quantified brain size based on the total volume of an animal’s central nervous system, considering that octopuses actually possess nine brains: one central brain and semi-independent brains in each of their eight arms.

“This species is a stark contrast to humans, showcasing unique appendages and behaviors,” Muthukrishna notes.

The findings revealed no direct correlation between brain size and sociability. However, they did uncover that cephalopods generally have larger brains when inhabiting shallow waters, where they encounter a wide array of objects to manipulate and use as tools, along with rich calorie availability. Conversely, species dwelling in featureless deep-sea environments tend to have smaller brains.

“The correlation is quite strong,” Muthukrishna states, “but it’s imperative to approach these findings cautiously,” as only about 10 percent of the existing 800 cephalopod species have brain data accessible.

“The absence of a social brain effect in octopuses is intriguing yet expected,” explains Robin Dunbar from Oxford University, who proposed the social brain hypothesis around three decades ago. He argues that because octopuses do not inhabit cohesive social groups, their brains lack the necessity to manage complex social dynamics.

Professor Paul Katz from the University of Massachusetts articulates the possibility that evolution may have led to smaller brain sizes each time cephalopods adapted to deep-sea environments. “It’s reminiscent of species dimensions reducing on isolated islands; the same could apply to species in the deep ocean,” he mentions.

Muthukrishna’s previous research proposed that brain size not only predicts the extent of social and cultural behaviors but also reflects ecological factors such as prey diversity. Thus, the parallel patterns between cephalopods, having diverged from vertebrates over 500 million years ago, and humans bolster the cultural brain hypothesis. According to Muthukrishna and colleagues, this hypothesis illustrates how ecological pressures and information acquisition lead to the development of larger, more complex brains.

“It’s not solely about social instincts when it comes to large brains,” Muthukrishna asserts.

“I wholeheartedly agree that exploring why humans possess large brains must be informed by our understanding of current species. However, unraveling the evolutionary history of large brains, particularly with cephalopods, is challenging, especially given the radically different predator-prey dynamics when their brains began evolving,” Katz explains.

Additionally, various studies indicate that competitiveness with fish may have spurred cephalopod brain growth, Katz asserts.

Dunbar emphasizes that octopuses may require substantial brainpower for their independent-use of eight arms. “Understanding an octopus’s brain is complex due to its unique structure, but a significant part of its brain’s function is to manage its intricate body mechanics necessary for survival,” he states.

Furthermore, Dunbar notes that it is logical for larger brains to evolve in environments abundant in calories. “You can’t increase brain size without addressing energy consumption. Once you have a more substantial brain, its applications become vast, which is why humans can engage in writing, reading, and complex mathematics—skills not inherently present within our evolutionary contexts.”