Cephalopods, Large Brains and the Cultural Brain Hypothesis: What a New Study Finds

Biologists have long suggested that a large brain in relation to body size could be linked to membership of a particularly social species.

Known as the social brain hypothesis, the idea holds up reasonably well for one branch of the animal family tree: our own.

We share that branch with numerous other social animals, including hoofed herd animals such as sheep and goats; pack carnivores such as wolves and lions; whales; dolphins; bats; and primates. Birds may fit the pattern too.

These animals generally display the relationship described by the social brain hypothesis: larger social circles are associated with larger brains, particularly the mammalian neocortex.

Why do cephalopods have large brains?

Yet another, very different group on a distant branch of the animal family tree is also renowned for large brains and complex behaviour: cephalopods. Squid, octopuses and cuttlefish belong to this group, although none is particularly well known for social skills.

Many cephalopods are actively hostile to others of their kind, and only a few, mostly squid, are known to assemble in larger groups. Even when they do, the gathering can sometimes turn into a bloodbath.

Cephalopods also die soon after laying eggs, so they do not even display the parenting behaviours that support the most basic social structures. Why, then, are their brains so big?

The authors of a new study published in iScience suggest that cephalopods could show that a different force is the dominant driver of brain size.

The cultural brain hypothesis

They point to the cultural brain hypothesis, first presented in a 2018 paper by economic psychologist Michael Muthukrishna and his colleagues. Muthukrishna, based at the London School of Economics and Political Science, is also a principal investigator on the new study.

The researchers note that the social brain hypothesis rests on correlation, which does not necessarily reveal the mechanisms involved.

"Correlations suggest possible factors in brain evolution, but by themselves they cannot tell us how or why brains evolved, nor disentangle cause from consequence among multiple confounding variables," first author and anthropologist Kiran Basava and the team write in the paper.

Put simply, the social brain hypothesis can only tell us so much.

"For decades the main story of why brains got big has been a social one where bigger brains evolve to manage bigger, more complex groups," Muthukrishna says.

"Cephalopods reveal that there's another path to bigger brains. They're often solitary, short-lived, sometimes even cannibalistic, and yet have large brains and intelligent behaviour."

The cultural brain hypothesis proposes instead that "brains have been selected for their ability to store and manage information, acquired through asocial or social learning".

Large social groups may therefore be one evolutionary pressure favouring bigger brains in animals, but they need not be the only one.

"Scientific dogma always needs to be questioned." – octopus psychologist Jennifer Mather

Habitat and brain size in cephalopods

In the new paper, the researchers found that habitat, rather than sociality, is more likely to be a key selection pressure behind the larger brains seen in cephalopods.

They brought together comparative brain-size data for 79 cephalopod species, alongside information about their ecology, behaviour and sociality. Species living on the sea floor and in shallower habitats tended to have larger brains.

The findings suggest ecological factors are a primary selection pressure for larger brains in environments where animals can access plenty of food and encounter comparatively complex landscapes.

Anyone who has watched a benthic, shallow-water octopus in action will recognise how well the theory describes these animals. Their soft molluscan bodies, free from the confines of an external shell, can adopt an endless range of shapes and degrees of leg co-ordination to make use of their surroundings.

They can hunt many kinds of prey, squeeze into many types of crevice and use a wide variety of tools. Most of their lives are spent alone, although they sometimes team up with other animals nearby. They also have very large brains for their body size.

By contrast, the cephalopods that do show social behaviour – squid, bobtail squid and cuttlefish – did not necessarily have larger brains when they were more social. That suggests the social brain hypothesis does not apply in this case.

"This should remind us that scientific dogma always needs to be questioned, and that once again it shows that cephalopods don't follow the predictable evolutionary paths," says octopus psychologist Jennifer Mather of the University of Lethbridge, who co-led the study.

The research still ultimately depends on correlation. Nevertheless, it suggests that brain size involves more than a species' degree of sociality.

"Our research started with a mathematical model we built years ago to explain human brain evolution that predicted a second path to big brains," Muthukrishna says.

"Solitary animals could evolve large brains if their environment was rich and complex enough to reward learning. Octopuses, squid, and cuttlefish let us test that prediction and the data fit. It turns out there's more than one path to evolving intelligence."

The study was published in iScience.

Carly Cassella fact-checked this article, and Peter Dockrill edited it. We take pride in our process, but we are only human. Please let us know if you notice an error.