The human brain can bring rubber to life with one simple trick: Cover a person’s hand and place a rubber hand next to it, then stroke their hand and its facsimile, and suddenly the person will begin to “feel” the touch in the fake appendage. The rubber hand illusion helps demonstrate how our senses work together to create the feeling of ownership over our bodies, a fundamental part of self-awareness. And now, a new study in Current Biology demonstrates that octopuses also fall for the rubber arm trick—the first documentation of the phenomenon outside of mammals.
Researchers at the University of the Ryukyus placed a plain-body octopus (Callistoctopus aspilosomatis) into a tank filled with polymer beads. Whenever the cephalopod sat on the beads, the team covered one of the limbs with a fake arm affixed to an opaque sheet and then gently stroked both arms at the same time. When the researchers pinched the fake arm with tweezers, the octopus reacted defensively by changing its body color, retracting its arm, or escaping. But in trials where only the fake arm was stroked, both arms were stroked out of sync, or the real arm was in a different position than the fake one, the mollusk showed little or no response.
The findings suggest octopuses perceive a similar level of body ownership as humans and other mammals, highlighting the complexity of the squishy eight-armed creatures’ cognition. The spineless animals have a complex nervous system that evolved independently from vertebrates—and also seem to have multisensory representations of their own boneless bodies.