Biologists have long believed that having a large brain, relative to your body, might go hand-in-hand with being part of a particularly social species.
This is called the social brain hypothesis, and it holds up pretty well for a particular branch of the animal family tree: ours.
We share this branch with a number of other social animals: hoofed herd animals like sheep and goats; pack carnivores, like wolves and lions; whales; dolphins; bats; primates; and possibly birds too.
All of these animals tend to follow the relationship described in the social brain hypothesis: Bigger social circles correlate with bigger brains, specifically with the mammalian neocortex.
But a few other, very different kinds of animals, on a very different branch of the tree, are also known for the size of their brains, and the complexity of their behaviors: cephalopods.
These include squids, octopuses, and cuttlefish, none of which are particularly known for their social skills.
In fact, many cephalopods are actively hostile towards other members of their group, and only a few (mainly squid) are known to gather in larger groups at all. Even then, it's sometimes a bit of a bloodbath.
Cephalopods also die shortly after laying eggs, which means they don't even have the kinds of parenting behaviors that underlie the most basic of social structures.
In which case, why are their brains so darn big?
The authors of a new study published in iScience propose that cephalopods may be evidence that something else is the dominant driver of brain size.
They refer to the cultural brain hypothesis, first introduced in a 2018 paper by economic psychologist Michael Muthukrishna and colleagues.
Muthukrishna, who is based at the London School of Economics and Political Science, is also a principal investigator on this new research.
In the new study, the researchers point out that the social brain hypothesis is held up only by correlation, which doesn't necessarily explain the mechanisms behind it.
"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 team write in the new paper.
In other words, there are limits to what the social brain hypothesis can tell us.
"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 behavior."
The cultural brain hypothesis proposes that "brains have been selected for their ability to store and manage information, acquired through asocial or social learning".
In other words, large social groups may indeed be one of the pressures that favor a larger brain in animal evolution.
But it's not necessarily the only one.
"Scientific dogma always needs to be questioned." – octopus psychologist Jennifer Mather
In the new paper, researchers identified that habitat, not sociality, is more likely to be a key selection pressure for the larger brains we see in cephalopods.
They compiled comparative data about the brain size of 79 cephalopod species, along with details about their ecology, behaviors, and sociality.
Those that live on the sea floor, and in shallower habitats, tended to have larger brains.
The results suggest that ecological factors are a primary selection pressure for larger brains, in environments where animals can access a lot of food, and encounter comparatively more complex landscapes.
Anyone who has seen a benthic, shallow-water octopus in action will know this theory describes them well.
Their soft molluscan bodies, freed from the structure of an exterior shell, can take on an endless variety of shapes (and degrees of leg co-ordination) to make use of what's around them.
They can hunt many different kinds of prey, fit in many different kinds of crevasses, use many different kinds of tools. They spend most of their lives flying solo – though sometimes they will team up with other animals in their vicinity.
And, they have really big brains relative to their body size.
Meanwhile, cephalopods that do display social behaviors – squid, bobtail squid, and cuttlefish – did not necessarily have larger brains the more social they were. This suggests the social brain hypothesis does not apply here.
"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," octopus psychologist Jennifer Mather from the University of Lethbridge, who co-led the study, says.
Of course, this research still ultimately relies on correlation. But it does suggest there's more to brain size than a species' level of sociality.
Related: Squid Brains Are Nearly as Complex as Dog Brains, Researchers Claim
"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 research was published in iScience.
This article was fact-checked by Carly Cassella and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.