Slime molds have memory – without a brain

University of Sydney researchers have shown that brainless slime molds can remember where they’ve been and navigate a maze using excreted chemicals as a memory system.

It’s strong support, they say, for the theory that the first step toward the evolution of memory was the use of feedback from chemicals.

“We have shown for the first time that a single-celled organism with no brain uses an external spatial memory to navigate through a complex environment,” says Christopher Reid from the University’s School of Biological Sciences.

“Our discovery is evidence of how the memory of multi-cellular organisms may have evolved – by using external chemical trails in the environment before the development of internal memory systems.”

It’s long been known that some insects, such as ants, navigate by leaving pheromone trails behind them, challenging the assumption that navigation requires learning or a sophisticated spatial awareness.

But this is the first time it’s been demonstrated that even an organism without a nervous system can navigate a complex environment with the help of externalised memory.

The process used by the slime molds, say the researchers, is similar to the way robots can navigate without a programmed map or the ability to build one. Instead, they respond only to feedback from their immediate environment to navigate obstacles and avoid becoming trapped.

The team based its test on one commonly used in robotics, requiring the slime mold to navigate its way out of a U-shaped barrier.

As the slime mould (Physarum polycephalum) moves, it leaves behind a thick mat of non-living, translucent, extracellular slime. When foraging, the slime mold avoids areas that it has already ‘slimed’ and explored.

“This shows it is using a form of external spatial memory to more efficiently explore its environment,” says Reid.

“We then upped the ante for the slime molds by challenging them with the U-shaped trap problem to test their navigational ability in a more complex situation than foraging. We found that, as we had predicted, its success was greatly dependent on being able to apply its external spatial memory to navigate its way out of the trap.”

It might be argued that slime molds have enough to deal with in life without being forced to navigate mazes. They form the exclusive diet of three types of beetle, for example: Agathidium bushi, A. cheneyi and A. rumsfeldi.