Octopuses are remarkably intelligent creatures, as was demonstrated by Inky the Octopus's famous escape from the National Aquarium of New Zealand through a drainpipe back to sea in 2016.
A new Dartmouth study shows octopuses can use mirrors to find food out of sight, demonstrating spatial cognitive abilities. The results are published in Current Biology .
"Our findings are the first to demonstrate that invertebrates can use mirrors to understand their environment to find prey," says lead author Mary Kieseler, Guarini '25, who conducted the research as a PhD student in the Department of Psychological and Brain Sciences at Dartmouth and is now a postdoc at Switzerland's University of Fribourg. "It's a skill that previously has only been documented in vertebrates, such as in some mammals and some birds."
The researchers trained three California two-spot octopuses (Octopus bimaculoides) in the Octopus Lab at Dartmouth to not attack a crab image that they see in a mirror but instead to infer and move to where the hidden stimulus was displayed behind them.
First, the octopuses were acclimated to the mirror in their habitat. Then, they were trained to understand how a mirror works using a live food reward—crab—which was placed in a glass jar that they could see in the mirror. To obtain the crab, the octopus had to make a 90-degree turn around a corner.
"We don't enter the world knowing how to use a mirror but learn how to use a mirror," says senior author and cognitive neuroscientist Peter Tse , a professor of psychological and brain sciences at Dartmouth. Just as new drivers learn to use a rearview mirror to track other vehicles, "Octopuses can also learn how to use a mirror to infer where things are in the world."
Octopuses have chemoreceptors that enable them to smell and taste by touch. So, for the experiment, the team used a virtual crab stimulus rather than a live crab.
The octopus was placed in a start box open to the top and front and shown the virtual crab image in a mirror directly in front of the animal. The virtual crab image was projected from behind the octopus on the left or right side. Instead of the octopus going to the mirror to try and obtain the virtual crab, it went to the projection site, requiring a 180-degree turn, where it then received a live crab reward. In some cases, the octopus would climb up and over the box to the side where the crab was projected rather than exiting the box and swimming around to the side.
The results show that octopuses travelled to the correct side approximately 73% of the time.
During the trials, the team manually tracked a spot between the eyes on the mantle, which is like the head of the octopus, from overhead. The researchers also calculated the length of the paths the octopuses used to seek the reward. While they did not always choose the shortest way of travel, they became faster at going to where the stimulus was based.
"Octopuses are among the most evolutionarily distant animals from humans, as our last common ancestor was a worm that lived 350 to 500 million years ago," says Kieseler. "Given that such a remote organism has independently evolved the means to use a mirror as a tool to process spatial cognition suggests that the underlying cognitive processes might be subject to convergent evolution, where different species evolve similar neural solutions to the same challenge."
The world in which octopuses live, mainly coral reefs and the ocean seafloor, are complex environments.
"Octopuses are like cats: they will sneak up on their prey and pounce, and they want to do so as fast as possible, so that they don't become preyed upon," Tse says.
"Hunters are very effective when they have a mental map of their territory, so that they know where they are in relation to their environments," says Tse. "Our work suggests that octopuses might also have internal maps, an internal representation of space."
However, according to the co-authors, additional research is needed to prove this.
