Three California two-spot octopuses in a Dartmouth College lab learned to turn away from a mirror, not toward it, in order to reach food they could see only as a reflection. Trained over repeated trials, the animals picked the correct side of their tank roughly 73% of the time, a result researchers say is the first documented case of an invertebrate using a mirror as a tool to find something outside its direct line of sight.
The study, led by then-PhD student Mary Kieseler and senior author Peter Tse of Dartmouth’s Department of Psychological and Brain Sciences, worked with three octopuses of the species Octopus bimaculoides housed in the university’s Octopus Lab. It was published in the journal Current Biology in June 2026.
A crab hidden everywhere except in the mirror
The researchers built a setup that forced each octopus to solve a specific spatial puzzle rather than simply react to a reflection. Each animal started inside an open-topped box with a mirror positioned directly in front of it. A projected image of a crab, standing in for a live one, appeared behind the octopus on either its left or right side, visible only through the mirror’s reflection. To reach the reward, the octopus had to work out where the crab actually was, turn around, and move toward the correct side, sometimes climbing over the edge of the box rather than going around it. Instead of lunging at the glass or the mirror itself, as an animal reacting only to the image in front of it might do, the trained octopuses moved toward the real, physical location the reflection was pointing to, according to the ScienceDaily summary of the Dartmouth research.
A live crab was used only during an earlier training phase, in which the animals learned the basic relationship between the mirror and the space around it by watching a real crab in a jar reachable by turning 90 degrees around a corner. Researchers switched to the projected image for the actual test trials because octopuses can smell and taste through touch, meaning a real crab’s scent could have let the animals cheat by using chemical cues instead of the mirror. Across trials, the octopuses became faster at finding the correct side as the experiment progressed, even though they did not always take the shortest physical path to it, a detail reported in the Current Biology paper describing the experiment.
This is not the classic mirror self-recognition test
The famous mirror test, in which an animal marked with a spot investigates its own body after noticing the mark in a reflection, measures something different: whether an animal recognizes that the image in the mirror is itself. What the Dartmouth octopuses demonstrated instead is called mediated perception, the ability to link a visible reflection to a real, occluded location in the environment. Researchers describe mediated perception as a skill some scientists regard as a precursor to self-recognition rather than self-recognition itself, a distinction that keeps the octopus result well short of claiming the animals recognized themselves in the glass.
“Our findings are the first to demonstrate that invertebrates can use mirrors to understand their environment to find prey,” Kieseler said. “It’s a skill that previously has only been documented in vertebrates, such as in some mammals and some birds.” Tse, describing how the skill develops rather than existing from birth, compared it to a familiar human experience: “We don’t enter the world knowing how to use a mirror but learn how to use a mirror,” he said, noting that octopuses, like new drivers learning to read a rearview mirror, “can also learn how to use a mirror to infer where things are in the world.”
An animal 350 million years removed from humans solved the same problem
What makes the finding notable to the researchers is less the mirror use itself and more which animal accomplished it. Octopuses and humans last shared a common ancestor, described by the researchers as a worm-like organism, somewhere between 350 million and 500 million years ago, making cephalopods among the most evolutionarily distant animals from humans capable of complex behavior. “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,” Kieseler said.
Tse connected the skill to how octopuses hunt in the cluttered, obstacle-filled environments of reefs and the seafloor. “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,” he said. “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. Our work suggests that octopuses might also have internal maps, an internal representation of space.”
The mental-map question remains open
Neither Kieseler nor Tse claims the study proves octopuses carry a full cognitive map of their surroundings the way some birds and mammals appear to. The researchers tracked each octopus by following a fixed point on its mantle from overhead cameras and measured the paths the animals took to the reward, finding that speed improved with practice even without consistently efficient routes, a pattern that leaves open exactly how much spatial reasoning underlies the behavior. The team, whose work was supported by Dartmouth’s own account of the study, has said additional research is needed to determine whether the mirror-guided navigation reflects a genuine internal map of the tank or a narrower, task-specific rule the octopuses picked up through repetition, a question that will likely shape how the finding is interpreted as more cephalopod cognition studies follow it.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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