Octopuses prompt rethink of why animals evolve big brains

Octopuses may have large brains due to environmental, not social, factors.

Large mammalian brains are generally thought to be linked to social behavior, an idea known as the social brain hypothesis. The more social connections members of a species have, the larger the brain must be to manage those connections, a model that applies to groups such as primates, dolphins, and members of the camel family.

But there are animals like cephalopods – octopuses, squid, cuttlefish and nautiluses – that have relatively large brains, show signs of intelligent behavior and yet live largely solitary lives, with little parental care, complex group dynamics or social learning.

To study what might be behind these larger brains, Michael Muthukrishna of the London School of Economics and his colleagues gathered data on 79 cephalopod species for which brain information is available. They took brain size to be the total volume of the animals’ central nervous system. This was necessary because an octopus, for example, has nine brains, not just one: a central brain in the head that controls the nervous system, and a smaller, semi-independent brain in each of its eight limbs.

“What’s more different from humans than this kind of alien species on our planet, with its wacky brain with multiple appendages and arms?” said Muthukrishna.

The data collected by the team showed no link between brain size and sociality, but revealed that cephalopods generally have larger brains when they live in shallower, seafloor habitats, which have a greater range of things to interact with, or objects to manipulate or even use as tools, and are higher in calories. Species that float in deep, featureless water tend to have smaller brains.

“This relationship is quite strong,” says Muthukrishna. “But this is a cautious conclusion,” because brain data is only available for about 10 percent of the 800 cephalopod species living today.

“It’s interesting that there is no social effect on the brain in octopuses, but that’s not surprising,” says Robin Dunbar of the University of Oxford, who proposed the social brain hypothesis around 30 years ago. Because octopuses don’t live in cohesive social groups, their brains don’t need to do the extra work that entails, he says.

Paul Katz, of the University of Massachusetts at Amherst, says it’s certainly possible that whenever cephalopods evolve to live in deeper waters, their brain size decreases. “It’s like every time you see animal species stuck on islands, they get smaller. There’s an island phenomenon, so there might be a deep-sea phenomenon as well,” he says, but adds that it might just be a correlation.

Muthukrishna previously published a study on whales and dolphins, suggesting that brain size predicts the extent of their social and cultural behaviors, as well as ecological factors such as prey diversity. The fact that cephalopods, which diverged from vertebrates more than 500 million years ago, exhibit a similar pattern — which he also modeled in humans — is evidence for an idea called the cultural brain hypothesis, developed by Muthukrishna and his colleagues, which describes how ecological informational and selection pressures can also produce large and complex brains, he says.

“Big brains aren’t just about sociality,” says Muthukrishna.

“I completely agree that explanations for why humans have big brains are based on what we know about humans, so if you really want to understand the evolution of big brains, you should study distant species,” says Katz. But it’s difficult to look at the behavior of current species and make a statement about what was happening 500 million years ago, when cephalopod brains would have evolved in a very different predator-prey environment before fish became widespread, he says.

Additionally, other evidence indicated that cephalopod brains grew larger due to competition with fish, Katz says.

Dunbar says octopuses can typically need a lot of brain power because they have eight arms that they can use independently. “We don’t really know what an octopus brain is, because it kind of has a little brain in each arm, but a lot of what the brain does is manage the body and make it do what it needs to stay alive,” he says.

Bigger brains evolving where more calories can be found makes sense, however, Dunbar says. “You can’t increase the size of your brain unless you solve the energy problem. Once you have the big brain in place, you can use it for lots of different things. This is precisely why humans can read, write, and do intelligent calculations when that wasn’t in our evolutionary environment of selection.”