Octopus Intelligence: How a Soft-Bodied Mind Works
Octopuses have about 500 million neurons, most of them in their arms, and solve puzzles, use tools, and escape tanks. Explore how their alien intelligence works.
Octopuses are among the most intelligent invertebrates. They have roughly 500 million neurons, about as many as a dog, and unlike in vertebrates, around two-thirds of those neurons sit in the eight arms rather than in the central brain. They solve puzzles, unscrew jars, carry coconut shells as portable shelter, and can tell individual people apart. Because the octopus lineage split from ours more than half a billion years ago, they show that complex minds have evolved more than once.
A very different nervous system
An octopus has a central brain wrapped around its oesophagus, with large optic lobes behind the eyes, but much of its nervous system is spread through the arms. Each arm holds a chain of ganglia that can run reflexes and movement patterns on its own, and the suckers carry receptors for both touch and taste. A severed arm can keep responding to stimulation for some time. The likely arrangement is that the brain issues general goals, such as reach that crab, while the arms work out the details. That distributed control suits a body with no skeleton and almost unlimited ways to move.
Cephalopod nerves also include giant axons for fast escape jets. The giant axon of the squid, a close relative, was the preparation in which Alan Hodgkin and Andrew Huxley worked out the action potential in the 1940s and 1950s. An octopus also has three hearts, two pumping blood through the gills and one around the body, and its blue blood carries oxygen on copper-based hemocyanin rather than iron-based hemoglobin.
Problem solving and tool use
Octopuses have been seen opening screw-top jars from the inside, learning by watching other octopuses in laboratory experiments, and navigating mazes. The veined octopus was filmed in 2009 collecting discarded coconut shell halves, carrying them across the seafloor, and assembling them into a shelter, which was reported in the journal Current Biology as a case of tool use in an invertebrate. They are also famous escape artists. In 2016 an octopus nicknamed Inky slipped out of its tank at the National Aquarium of New Zealand and made its way down a drainpipe to the sea. In laboratory tests, giant Pacific octopuses have reacted differently to individual keepers, which suggests that they can recognise people.
Camouflage as a form of control
The skin of an octopus contains thousands of chromatophores, pigment sacs stretched open by tiny muscles under direct control of the nervous system, along with reflective cells that add shimmer. The animal can change colour, pattern, and even skin texture in a fraction of a second, matching its background. Strangely, octopuses seem to have only one type of light receptor in the eye, so they should be colour-blind by the usual definition, and how they match colours so well is still debated. Some of their relatives go further and make their own light, as in bioluminescence.
Genes and RNA editing
The genome of the California two-spot octopus was sequenced in 2015. It has about 2.7 billion bases and roughly 33,000 protein-coding genes, more than humans have, with large expansions in gene families linked to the nervous system. Cephalopods also edit the RNA copies of thousands of genes in their nerve cells, rewriting the instructions after they are copied from DNA. This may be a way of fine-tuning proteins quickly without changing the genes, though the trade-off appears to be slower genome evolution.
Short lives, big questions
Most octopus species live only one to five years, and many females die soon after their eggs hatch. They do not raise their young, so they cannot pass on what they have learned, which makes their intelligence a puzzle: it has to be built again in every generation. The best-supported explanation is natural selection in a sea full of predators and prey: a soft body with no armour makes cleverness more valuable. The most recent common ancestor of octopuses and humans was probably a simple worm-like animal living more than 550 million years ago, so octopus intelligence evolved independently. Their neurons work on the same basic principles as ours, including electrical signals and chemical messengers (synaptic transmission), yet the result is organised very differently.
The question of whether octopuses can suffer has had real policy impact. In 2022 the United Kingdom's Animal Welfare (Sentience) Act recognised cephalopods as sentient beings, following a review commissioned from the London School of Economics.
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animal intelligence cephalopods cognition neurons octopus