Spatial memory enhances an organism’s navigational ability. Memory
typically resides within the brain, but what if an organism has
no brain? We show that the brainless slime mold Physarum polycephalum
constructs a form of spatial memory by avoiding areas it
has previously explored. This mechanism allows the slime mold to
solve the U-shaped trap problem—a classic test of autonomous navigational
ability commonly used in robotics—requiring the slime
mold to reach a chemoattractive goal behind a U-shaped barrier.
Drawn into the trap, the organism must rely on other methods than
gradient-following to escape and reach the goal. Our data show
that spatial memory enhances the organism’s ability to navigate
in complex environments. We provide a unique demonstration of
a spatial memory system in a nonneuronal organism, supporting the
theory that an externalized spatial memory may be the functional
precursor to the internal memory of higher organisms.
extracellular slime | protist | reactive navigation | amoeboid organism