octopus female arm stretch

Common octopus beginning the arm-stretch experiment; image courtesy of Laura Margheri et al.

Almost as fast as you can say "go-go-gadget arm," an octopus can stretch its arm more than twice its normal length—without the help of any cyborg attachments. What's more, according to new research, female common octopuses (Octopus vulgaris) are able to stretch their arms even more than the males—on average, three times resting length. This striking sex difference was a surprise to the team of scientists studying these animals as inspiration for a new breed of robots.

Octopus arms are made from a structure like that of the human tongue. Known as a muscular hydrostat, this mechanism allows octopuses to bend and stretch their arms in crazy ways, all the while keeping the total volume the same. In the wild, this ability helps the animal to reach into small crevasses to forage for and capture food—and occasionally to rope in a mate. And for the robotics field, mimicking this talent could usher in a vast new range of robot capabilities.

Cecilia Laschi, a roboticist at The BioRobotics Institute at the Scuola Superiore Stan'Anna in Livorno, Italy, and collaborator on the new research, was kind enough to show me around her labs in the summer of 2011. She is one of the leads on the OCTOPUS Integrating Project, a consortium of scientists working to build an entirely soft-bodied robot octopus. One of the researchers in her lab, Laura Margheri, explained to me that in order to replicate stretchiness in a robotic arm, they first needed to understand how the animals themselves used their appendages.

Measuring any specifics on a live octopus can get tricky. Biomechanics are often tracked by placing dots or other markers on an animal's body and then using high-speed cameras to capture motion. But octopuses are not the most obliging research subjects. As Margheri and her co-authors noted in the new research paper, "It is impossible to apply artificial markers because the animal tends to remove objects from its body." Additionally, the octopus's quick, color-changing ability can confound video analysis software.

So, Margheri showed me a solution she had helped devise: a simple clear Plexiglas tube that could be inserted into a tank—with the octopus on one end and a tasty morsel of food on the other. The length of an arm stretch could then easily be measured along the tube, which had two video cameras trained on it.

But what they hadn't yet discovered when I visited was that not all of their octopus subjects could extend their arms a proportional distance. The new findings, which will appear later this year in Journal of Experimental Marine Biology and Ecology, show that females are capable of longer arm extension than males. Margheri and her colleagues are not sure what might account for this sex difference. Smaller octopuses also could stretch their arms proportionally longer than the larger octopus specimens, a trend that could be due to "a greater need of nutrients, higher agility and energetic metabolism," the researchers speculated.

The study tested seven females and 12 males all collected from the Bay of Naples, near the facility where the research was conducted. Resting arm length was measured when the animal was jetting casually across a tank. The octopuses learned quickly how to reach one arm up the tube to snatch a piece of crab, anchovy or sardine that was waiting on a movable target. After just a few days of training, each octopus underwent 30 to 60 trials over the course of two to three days to see how far it could stretch its arm for food.

And not all arms are created equal in the common octopus. The third left arm is the longest, and it was employed more often than other arms for the reaching experiments. For males, the third right arm is the hectocotylus, which functions as its sperm-delivery extremity, and turns out not to be terribly stretchy. But the males might reserve this crucial appendage for mating and not risk it for mundane tasks like feeding—or laboratory experiments.


Illustration courtesy of Ivan Phillipsen