Octopuses can learn to use a mirror to find food hidden from direct view, a skill that until now had been documented only in vertebrates such as mammals and birds. In a study from Dartmouth College, researchers trained California two-spot octopuses to locate a food source they could see only in a mirror, and the animals chose the correct side about 73% of the time. The findings were published in the journal Current Biology.
Why this matters
Using a mirror to reason about the location of something out of sight is a demanding cognitive task. It requires understanding that a reflection corresponds to a real object elsewhere in the world, rather than reacting to the reflection itself. That ability had been seen in some mammals and some birds, but not in an invertebrate.
“Our findings are the first to demonstrate that invertebrates can use mirrors to understand their environment to find prey,” said lead author Mary Kieseler, who conducted the research as a PhD student in Dartmouth’s Department of Psychological and Brain Sciences and is now a postdoc at the University of Fribourg in Switzerland. The distinction she draws is important: the octopuses were not simply attacking a reflected image, but using the mirror as a tool to infer where food actually was.
The comparison the researchers reach for is everyday. “We don’t enter the world knowing how to use a mirror but learn how to use a mirror,” said senior author Peter Tse, a cognitive neuroscientist and professor of psychological and brain sciences at Dartmouth. Just as new drivers learn to track other cars with a rearview mirror, he said, octopuses can learn to use a mirror to work out where things are.
How the experiment worked
The team worked with three California two-spot octopuses housed in Dartmouth’s Octopus Lab. The animals were first given time to get used to a mirror placed in their habitat, then trained to grasp the relationship between a reflection and the real world. During training, a live crab was placed in a glass jar positioned so the octopus could see it only through the mirror; to reach it, the animal had to turn 90 degrees and move around a corner.
Testing required a careful design, because octopuses can smell and taste through touch using chemoreceptors, which could have tipped them off if real prey were used. To avoid that, the researchers relied on a virtual crab image instead. Each octopus was placed in a start box with a mirror directly in front of it, and the virtual crab appeared behind the animal, on its left or right, visible only in the mirror.
To earn a reward, the octopus had to recognize where the image really was and move toward that spot rather than approaching the mirror. The animals turned around and headed to the correct side, where they received a live crab, and some even climbed over the side of the box to reach the projected image. Tracking a point between the octopus’s eyes from overhead, the researchers found the animals chose the correct side about 73% of the time, and although they did not always take the shortest route, they grew faster at reaching the right location as trials progressed.
What it suggests about intelligence
The result carries implications for how intelligence evolves. Octopuses are among the animals most distantly related to humans; Kieseler notes that the last common ancestor was a worm that lived some 350 to 500 million years ago. That such a remote lineage independently developed the ability to use a mirror for spatial reasoning points to convergent evolution, in which different species arrive at similar neural solutions to the same problem.
The researchers connect the skill to how octopuses live. Their habitats, including coral reefs and the seafloor, are complex and full of obstacles, and Tse likens the animals to cats that sneak up and pounce on prey while avoiding becoming prey themselves. Effective hunters tend to carry a mental map of their territory, he said, and the work suggests octopuses may hold an internal representation of space as well.
Some questions are unresolved. The team emphasizes that more research is needed to determine whether octopuses truly maintain such mental maps, and the study rests on a small sample of three animals. Even so, the finding adds a notable ability to the growing list of octopus behaviors. For readers, it is a reminder that sophisticated cognition is not confined to animals built like us, and that the boundaries of what counts as a “mammal skill” keep shifting as researchers test creatures further from our own branch of the tree of life.
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*This article was researched with the help of AI, with human editors creating the final content.
