Cephalopods caught in the wild can learn to hunt crabs using only the reflection of their prey
Mirrors are delicate. Even humans are not born with an intuitive understanding of these; we need to learn how they work. Now scientists have discovered that the California two-spotted octopus (Bimaculoid octopus) can also learn to use mirrorsreport the researchers on June 3 in Current biology.
While brainstorming experiments on octopuses, Mary Kieseler, a neuroscientist at the University of Friborg in Switzerland, wondered whether these supposedly intelligent creatures could pass the mirror testwhich evaluates whether an animal can identify itself in a mirror. Because of the logistical complexity that testing mirrors for self-recognition underwater would require, Kieseler and his team decided to first study whether octopuses could use mirrors as a tool to do something they’re already good at. And octopuses are good at hunting prey.
The team began by habituating three wild octopuses to a mirror covering half of their tank. They let the octopuses hide from the mirror and even explore the other half of the tank behind it. Once the octopuses got used to seeing their reflection and eating in front of the mirror, the team gave them a task: find a hidden pot with a tasty crab inside, placed where the snack could be found using only its reflection in the mirror.
First, the octopuses approached the mirror, then turned around to find their prey. But after about 10 to 12 trials, each animal learned to crawl directly toward the crab without stopping the mirror.
When using real crabs, there was no way to know whether the octopuses were relying on smell or some other non-visual sense to hunt. So Kieseler and his team proposed one last test. Rather than using real crabs, the team used virtual ones.
They placed each octopus in a small three-sided chamber that blocked their view of anything other than a mirror at the front of the tank. A screen behind the room displayed videos of a crab appearing to move on either side of the back wall, creating a visible reflection for the octopus. To receive a real crab as a reward, the animals had to leave the room and move to the right side.
Motivating the octopuses to participate in the experiment was a challenge, especially if they weren’t hungry enough, Kieseler says. “They did many trials where they just fell asleep or sat in front of the mirror.” Each octopus would only make about one attempt per day.
Octopuses were able to choose the correct side in about 73% of trials with virtual crabs. In 59 percent of their correct trials, the octopuses even climbed over the side walls of the chamber to reach the crab stimulus rather than approaching the mirror.
This shows that octopuses can understand how a mirror represents the location of an object, “rather than impulsively moving toward the mirror’s reflection in hopes of getting a reward,” says Trevor Wardill, a neurobiologist at the University of Minnesota in Minneapolis, who was not involved in the research.
Octopuses adapt their navigation strategies in difficult environments with the tools at their disposal, the results suggest. And now that octopuses are known to learn to use the mirror, Kieseler hopes researchers will bring the mirror self-recognition test back into the tank.
