Octopuses prompt rethink of why animals evolve big brains

Octopuses may have big brains because of environmental – not social – factors.

Large brains in mammals are generally thought to be linked to social behaviour, an idea known as the social brain hypothesis. The more social connections that members of a species have, the bigger the brain needs to be to manage those connections, a pattern that holds for groups including 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 behaviour and yet live largely solitary lives, with little in the way of parental care, complex group dynamics or social learning.

To investigate what might be behind these larger brains, Michael Muthukrishna at the London School of Economics and his colleagues have brought together data on 79 species of cephalopods 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 merely one: a central one in the head that controls the nervous system, and a smaller, semi-independent brain in each of its eight limbs.

“What could be more different from humans than this kind of alien species on our planet, with its wacky multi-appendage brain with arms?” says Muthukrishna.

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

“That relationship is quite robust,” says Muthukrishna. “But this is a cautious finding,” because there is brain data available for only about 10 per cent of the 800 species of cephalopod that live today.

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

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

Muthukrishna previously published a study of whales and dolphins, suggesting that brain size predicts the breadth of their social and cultural behaviours, as well as ecological factors like the diversity of prey. The fact that cephalopods, which diverged from vertebrates over 500 million years ago, display a similar pattern – which he has also modelled in humans – is evidence for an idea called the cultural brain hypothesis, developed by Muthukrishna and his colleagues, which describes how information and ecological selection pressures can also produce large, complex brains, he says.

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

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

Moreover, other evidence has indicated that cephalopod brains grew larger as a result of competition with fish, says Katz.

Dunbar says octopuses may generally need a lot of brain power because they have eight arms that they can use independently. “It’s a bit unclear what an octopus brain is, because it kind of has a bit of a brain in each arm, but a lot of what the brain does is manage the body and keep it doing what it needs to do to stay alive,” he says.

Bigger brains evolving where more calories can be found does make sense, though, says Dunbar. “You can’t increase the size of your brain unless you solve the energy problem. Once you’ve got the big brain in place, you can use it for many different things. Which is precisely why humans can read and write and do clever maths when that wasn’t in our evolutionary environment of selection.”