Binyamin Hochner

No brain strain.
Octopuses move their arms like humans do to avoid taxing their nervous systems.

Fetch Like an Eight-Armed Man

With no rigid parts, an octopus can bend its arms into positions that would snap the bones of even the most flexible human contortionist. Usually, though, the creatures opt to keep things simple, moving their arms much like humans do. Scientists suspect this is to avoid overwhelming their nervous systems, which would have trouble coordinating a large number of bends and rotations. Now a team of researchers has figured out how octopuses go easy on their brains.

When an octopus moves an object from one place to another--say, a piece of food from the ocean floor to its mouth--it's known as a "point-to-point" movement. To study how the creatures perform such movements without stressing their nervous systems, a team led by neurobiologist Binyamin Hochner of the Hebrew University in Jerusalem, Israel, filmed them scooping up morsels floating in their tanks. As had been observed before, the octopuses created three bends in their arms, which acted as joints.

The team placed electrodes at different locations along the octopuses' arms to measure neural activity. This revealed that, when the octopus reaches out, two waves of muscle contraction travel down the arm in opposite directions. One begins at the arm's tip and another at the body; where the waves meet determines the position of the middle joint. The "wrist" forms behind where the suckers grasp the food, and the "shoulder" forms at the arm's base. An octopus's suckers allow it to hold an object anywhere along its arm, so the nervous system ensures that the elbow forms midway between the position of the food and the mouth. The "elbow" accounted for most of the arm movement, but the shoulder also contributed to moving the wrist close to the base of the arm; a final rotation around this joint placed the food in the octopus's mouth.

Because the octopus nervous system only has to compute the movements of three joints, the problem is much simpler than it would be if the creature actually harnessed its unlimited flexibility, says Hochner, whose team reports its results today in Current Biology.

The study is "exciting" because it shows a novel mechanism which reduces the number of variables that an octopus has to control to move its arms, says muscle expert Bill Kier of the University of North Carolina, Chapel Hill. Such strategies could be used by engineers to design flexible robotic arms, he says.

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