Auto-Repelling Octopus Arms Could Inspire Better Bots

With 400,000 neurons governing its movement, a single octopus arm has a “mind of its own.”

That’s because those neurons don’t take orders from the brain—they exist independently of the octopus’s cognitive command center. An octopus arm, then, can be amputated and still function normally for about an hour. It isn’t just some fancy trick—octopuses delegate control to their tentacles in order to avoid information overload, since having eight arms would otherwise mean that 3.2 million neurons flood into the brain at once. That’s way too much for an organism to handle.

For scientists, one question has persisted: if octopuses’ arms have so much freedom and autonomy, why aren’t they constantly getting tangled? Each arm has 40 million tactile and chemical receptors—most of them lined along the rims of each “sucker,” which adheres to pretty much any surface it comes in contact with—so it’s remarkable that the arms avoid getting ensnared in snarls or tied up in knots. But now, scientist might have an answer as to how it works.

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An octopus's arms repel each other in order to avoid getting tangled.

Here’s Jason Goldman, writing for io9:

The researchers, led by Hebrew University neurobiologist Nir Nesher, got their first clue when they noticed that the suckers on amputated octopus arms never attached onto the arm itself or onto another amputated octopus arm. It was as if the arms—which, devoid of the rest of the octopus’s body, are limited to simple reflexes—were actually avoiding making contact with other octopus arms. A more carefully controlled experiment revealed that twenty-one amputated arms never grasped other amputated arms, but they had no problem grasping amputated arms with the skin removed.

Nesher and his colleagues concluded that it must be something about the octopus’s skin that’s repelling octopus suckers. They first tested amputated arms on Petri dishes doused in either octopus skin extract or fish skin extract. The amputated arms clung to the fish extract more firmly, indicating that suckers can detect a chemical signal that makes it harder for them to grab on to their own skin.

Then the researchers presented the octopuses with their own amputated arms and with amputated arms from other octopuses. Here’s Ed Yong, writing for Phenomena:

They can even tell if an amputated arm belonged to them or to another octopus. If they sensed another individual’s severed arm, they would often explore it, grab it, and hold it in their beaks in an unusual posture that the team called “spaghetti holding”. (Common octopuses will cannibalise their own kind, so a floating arm is fair game.) But when they sense their own severed limbs, they typically avoided it, and only rarely treated it like food.

Here, an octopus holds an amputated arm from a different octopus as though it were food.

Contrast that video with this one, in which an octopus is given its own amputated arm:

An octopus grasps the arm only at the amputation site, where flesh is exposed, and then brings the arm to its beak (not the flesh part of its mouth like it would do with food.) More often, though, the octopus avoided touching its own amputated arm altogether.

Left to their own devices, octopus arms connected to the same body will repel each other. But these examples show that sometimes (in the case of amputation, for instance), the octopus brain itself can overrule the arm’s independent “decisions.”

Goldman comes to an interesting conclusion regarding these tendencies. He writes that they illustrate how “complex behaviors can emerge from a set of simpler reflexes.” In other words, it’s not that octopuses are self-aware—their instincts just generate behaviors that look like they’re complicated cognitive processes. Thus, cognition isn’t necessary to explain all forms of self-recognition.

Here’s Yong again:

The octopus… well… embodies this idea. Its brain governs many of its decisions and exerts control upon its arms, but the arms can do their own thing, including getting out of each others’ way. The animal doesn’t need to know the location of each of its arms to avoid embarrassing entanglements. It can let its arms do the work of evading each other.

But of course, when push comes to shove, the octopus’s central brain has the final say. Understanding this type of body “awareness” in the natural world could have implications for soft robotics:

This concept might be useful for designing robots. A typical robot, like Honda’s ASIMO, relies on top-down programs that control his every action. He can pull off pre-programmed feats like dancing or running, but he trips over even minor obstacles. He’s inflexible and inefficient. By contrast, Boston Dynamics’ Big Dog relies on embodied cognition. His springy legs are designed to react to rough terrain without needing new instructions from his central processor.

A robot with body part that can interfere with each other, though, could be more efficient than any of these options, and could set the stage for a new generation of bio-inspired devices.

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