A new study on cephalopods suggests that large brains may evolve due to environmental factors rather than social interactions. Researchers analyzed brain sizes across 79 species and found links to habitat complexity, not sociality. This prompts a rethink of why animals like octopuses develop complex nervous systems.
The social brain hypothesis, proposed by Robin Dunbar at the University of Oxford about 30 years ago, posits that larger brains in mammals such as primates, dolphins, and camels arise from the demands of managing social connections. However, cephalopods—including octopuses, squid, cuttlefish, and nautiluses—present a puzzle. These creatures exhibit intelligent behaviors and relatively large brains despite leading solitary lives with minimal parental care or group dynamics.
To explore this, Michael Muthukrishna at the London School of Economics and colleagues compiled data on 79 cephalopod species, measuring brain size as the total volume of the central nervous system. Octopuses, for instance, possess nine brains: one central in the head and semi-independent ones in each of their eight arms. "What could be more different from humans than this kind of alien species on our planet, with its wacky multi-appendage brain with arms?" Muthukrishna remarked.
The analysis, detailed in a May 2024 bioRxiv preprint (DOI: 10.1101/2024.05.01.592020v5), revealed no correlation between brain size and sociality. Instead, species in shallower waters and seafloor environments—richer in calories, manipulable objects, and interactions—tended to have larger brains. Deep-sea drifters, by contrast, showed smaller ones. "That relationship is quite robust," Muthukrishna said, though he cautioned that data covers only about 10 percent of the roughly 800 extant cephalopod species.
Dunbar noted that octopuses lack coherent social groups, so their brains avoid the extra workload. Paul Katz at the University of Massachusetts Amherst suggested a possible "deep-sea phenomenon" akin to island dwarfism, but emphasized it might be mere correlation. He also pointed to historical competition with fish as a driver of cephalopod brain growth over 500 million years since diverging from vertebrates.
This aligns with Muthukrishna's cultural brain hypothesis, which incorporates ecological pressures and information processing alongside social factors, as seen in prior studies on whales and dolphins. "Big brains aren’t just down to sociality," he concluded. Dunbar added that abundant calories enable brain expansion, freeing cognitive resources for diverse uses, much like in humans for reading and mathematics.
