Octopuses Used Mirror Reflections To Find Hidden Food, A First For Invertebrates

For the first time, an animal without a backbone has used a mirror to find something it couldn’t directly see. The animal is an octopus, and what happened in a lab tank at Dartmouth College is making researchers rethink what kinds of brains can solve a tricky spatial problem.

Scientists trained three California two-spot octopuses to locate a hidden food reward using only a mirror’s reflection as a guide. Each octopus learned to read the reflection, figure out where the real target was located behind it and out of view, and navigate there deliberately. Using a mirror to locate hidden objects had previously been documented in vertebrates, including mammals and birds, but not in invertebrates. Octopuses don’t share that vertebrate blueprint. Their evolutionary lineage split from ours more than 520 million years ago.

Chimpanzees, elephants, pigs, dogs, and some birds have been tested on whether they can use mirrors to solve problems or locate hidden food. Because this ability had been shown mainly in vertebrates, researchers didn’t know whether an invertebrate brain could pull off the same kind of mirror-guided navigation. These three octopuses answered that question.

Octopuses Had to Ignore What They Could See to Find What They Couldn’t

Published in Current Biology, the experiment was designed to confirm the octopuses were using the mirror and not picking up on other cues. Each octopus was placed inside a small open-topped start chamber along the back wall of a large tank. A mirror spanning the tank’s width sat at the midline, facing the back wall, and the chamber walls blocked any direct view of what lay behind the octopus.

A projected image of a crab, one that octopuses respond to with real predatory behavior, was displayed on either the left or right side of the back wall. From inside the chamber, the only way to see it was to look in the mirror. To earn a live crab as a reward, the octopus had to exit the chamber and move to the correct side, matching where the virtual crab appeared in the reflection. No humans were present during trials; experimenters watched remotely through cameras in an adjacent room.

Octopuses Learned to Override Their Own Hunting Instinct

When an octopus spots something that looks like prey, its instinct is to go straight for it. During the first mirror-use learning trial, when a live crab in a jar was visible only in the mirror, all three animals moved toward the reflection as though the crab were actually there. But over roughly 10 to 12 learning trials per animal, each learned to override that impulse and move toward the actual location of the reward on the opposite wall.

Across all trials, the octopuses chose the correct side about 73% of the time, well above what random chance would produce. Multiple statistical tests confirmed the result was not a fluke. One octopus showed a slight preference for moving right regardless of where the crab appeared, but even accounting for that, all three animals performed better than chance. As trials progressed, they also got faster, a sign they were becoming more confident and efficient.

Climbing Over Walls to Reach Food They Couldn’t Even See Yet

Most telling was what the octopuses did with their bodies. In 59% of correct trials, instead of walking around inside the start chamber toward the back wall, they climbed up and over the side wall to reach the target.

That path took them away from the mirror reflection before they had any direct view of where the crab actually was. If the octopuses were simply reacting to what they saw, it makes no sense. But if they were using some kind of internal sense of the tank’s layout, as the authors suggest, climbing over the wall becomes a reasonable shortcut. The study cannot prove that explanation on its own, but the behavior points in that direction. Researchers describe this as evidence of “spatial mapping.” “These findings extend mirror-use capabilities to invertebrates,” the authors write, “demonstrating that cephalopods can employ mirror reflections for spatial navigation.”

What Octopus Intelligence Tells Us About How Smarts Evolve

Octopuses developed sophisticated brains and behaviors entirely independently from the line that produced mammals, birds, and humans. Yet in a tank in New Hampshire, three of them demonstrated a cognitive ability that looks remarkably similar to what researchers have documented in those other animals.

Scientists call this convergent evolution, when two unrelated groups independently arrive at the same solution to a shared problem. Here, that problem is navigating a world where not everything is visible at once. The ability to use mirrors for navigation, appearing in both octopuses and vertebrates despite separate evolutionary histories, points to something worth considering: this kind of spatial navigation may be a basic requirement for any animal that needs to hunt, hide, and survive in a world full of obstacles.

None of this means octopuses recognize themselves in mirrors. But the study raises the question of whether they may have other mirror-related abilities worth testing.

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