Octopuses that live in shallow water – like the common octopus – usually have large brains
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Octopuses may have large brains due to environmental—not social—factors.
Large brains in mammals are generally thought to be associated with 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 handle those connections, a pattern that holds for groups including primates, dolphins and members of the camel family.
However, there are animals such as cephalopods—squids, octopuses, cuttlefish, and nautiluses—that have relatively large brains, show signs of intelligent behavior, and yet live largely solitary lives with little to do with parental care, complex group dynamics, or social learning.
To find out what might be behind these bigger brains, Michael Muthukrishna at the London School of Economics and his colleagues collected data on 79 species of cephalopods for which brain information is available. They took brain size as the total volume of the animals’ central nervous system. This was necessary because, for example, the octopus has nine brains, not just 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 crazy brain with many appendages and arms?” says Muthukrishna.
The data the team collected showed no link between brain size and sociability, but did reveal that cephalopods generally have larger brains when they live in shallower and seafloor habitats that have a greater variety of things to interact with or objects to manipulate or even use as tools, and are richer in calories. Species that swim in the shapeless deep sea tend to have smaller brains.
“The relationship is quite strong,” says Muthukrishna. “But that’s a cautious finding,” because brain data is available for only about 10 percent of the 800 species of cephalopods alive today.
“It’s interesting that there’s no social brain effect in octopuses, but it’s not surprising,” he says Robin Dunbar at the University of Oxford, which he designed the social brain hypothesis sometime 30 years ago. Because octopuses don’t live in cohesive social groups, their brains don’t have to do the extra work involved, he says.
Paul Katz at the University of Massachusetts Amherst says it’s certainly possible that each time cephalopods evolved to live in deeper water, their brain size shrank. “It’s like every time you see animal species that get stranded on islands, they’re shrinking. There’s an island phenomenon, so there might be a deep-sea phenomenon,” he says, but adds that it could just be a correlation.
Muthukrishna previously published a studies whales and dolphins, suggesting that brain size predicts the breadth 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, show a similar pattern—which has also been modeled in humans—is evidence for an idea called the cultural brain hypothesis. developed by Muthukrishna and his colleaguesthat describes how informational and ecological selection pressures can also produce large, complex brains, he says.
“Great minds are not just about sociability,” says Muthukrishna.
“I absolutely agree that explanations for why humans have large 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’s difficult to look at the behavior of contemporary species and make a statement about what was going on 500 million years ago, when the cephalopod brain was evolving in a very different predator-prey environment before fish spread, he says.
Moreover, other evidence pointed to it the brains of cephalopods became larger as a result of competition with fishsays Katz.
Dunbar says that octopuses in general may need a lot of brainpower because they have eight arms that they can use independently. “It’s kind of unclear what the brain of an octopus is because it has a bit of brain in each arm, but a lot of what the brain does is run the body and keep it in what it needs to stay alive,” he says.
But bigger brains evolving where more calories can be found makes sense, Dunbar says. “You can’t increase the size of your brain unless you solve the energy problem. Once you have a big brain in place, you can use it for many different things. Which is exactly why humans can read and write and do smart math when it wasn’t in our evolutionary environment of choice.”
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