Researchers trace a consistent sequence octopuses use to squeeze through holes

Findings could guide search-and-rescue robots through tight spaces

Researchers made octopuses crawl through holes of different sizes and analyzed video footage of what the animals did next. The animals ran a repeatable routine, the team reports in the Journal of Experimental Biology: one arm slips through first to explore the far side, three to five arms usually follow, and the octopus then pulls the rest of its body through by pressing back against the partition.

The point of the exercise is search and rescue. After a disaster like an earthquake or a train crash, robots can be sent into unpredictable environments to do lifesaving work — first finding out where survivors are, then reaching them and passing water or food, said Wenci Xin, a roboticist now at the Singapore-MIT Alliance for Research and Technology. But four-legged and wheeled robots have a hard time fitting into tight spaces, he said, and even undulating, snake-like robots fall short. “Even with this kind of long cylinder shape, a robot still has the limited functionalities to do the tasks,” Xin said. A snake-shaped robot is also too narrow to hold emergency supplies or larger mechanical components and sensors — which is why Xin and his colleagues, among them biologist Tamar Gutnick of the Vienna Zoo, turned to the octopus.

Except for a rigid capsule around the brain and a small hard beak used for eating, an octopus’s body is soft and pliable. “People are fascinated by the escape-artist aspect of octopuses,” Gutnick said. But how the animals actually manage their escapes had not been studied, she said. “We know that they can squeeze through all these spaces, but nobody’s actually looked at how they do this,” Gutnick said. “So what we did was take octopuses and systematically make them go through different-sized holes.”

In the experiment, conducted at a public aquarium in Vienna, the team enrolled a few common octopuses and several Mayan octopuses, each about the size of an apple. The researchers cut a hole in a clear acrylic partition, slid it into the tank, and recorded the animals’ attempts. “Once they find a hole, they often just naturally will try and go through it,” Gutnick said.

The video showed a consistent pattern. First, an octopus reached a single arm through the hole to explore the other side. Once the animal had determined the coast was clear, the arms followed. “They usually move three to five arms to the other side,” Xin said. If the hole was big enough, the octopus used those arms to pull the rest of its body through by pressing back against the partition. “This is a sequence of actions that is consistent,” Gutnick said.

The two species diverged when openings got small. If a hole was smaller than the distance between the animal’s eyes, Mayan octopuses would not even attempt the squeeze. The common octopus tried anyway: it stuffed all eight of its arms into the hole, then actively compressed its body cavity — internal organs included — to squeeze on through. “It’s very strange looking,” Gutnick said.

Gutnick said the octopuses appear to have a sense of their own shifting size, shape, and space as they work through an opening.

The next step is mechanical mimicry, which Xin said is no easy feat. “So we are thinking about whether we can bring this capability to the robots so that they can go through those kind of spaces that are really hard to reach,” he said. Applications might also include exploring sunken shipwrecks.

Mishu Duduta, an engineer at the University of Connecticut who builds robots inspired by living things and was not involved in the research, said the consistency of the behavior carries weight. “Consistently, all those octopi would reach and then pull themselves through, and that kind of has big implications for how we think of robots for search and rescue,” he said.

Robot designers have long debated whether a machine should push itself into a confined space or pull itself through, Duduta said, and the octopus results strengthen the case for pulling. Robots are usually sent to do “dangerous work, dull work or dirty work,” he said — and the octopuses, he said, point the way toward another kind of work: delicate work.