Brainless Slime Moulds Have Memory: Study

Researchers at the University of Sydney have shown how a slime mould can remember where it's been, and navigate through complex obstacles even without a brain. They do this using excreted chemical as a memory system.

This finding that was led by Christopher Reid from the University's School of Biological Sciences and a colleague from Toulouse University supports the theory that the first step towards 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," said Reid.

"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," said Reid.

"Results from insect studies, for example ants leaving pheromone trails, have already challenged the assumption that navigation requires learning or a sophisticated spatial awareness. We've now gone one better and shown that even an organism without a nervous system can navigate a complex environment, with the help of externalised memory."

The researchers drew inspiration from robots and based their design on that respond only to feedback from their immediate environment on order to navigate through obstacles. This "reactive navigation" method allows robots to navigate without a programmed map or the ability to build one and slime moulds (Physarum Polycephalum) use the same process.

In order for the slime mould to navigate its way out of a U shaped barrier, the researchers used a classic test of independent navigational ability that is commonly used in robotics. As the slime mould moves, it leaves behind a thick mat of non-living, translucent, extracellular slime. When it wanders the slime moulds avoids areas that it has already slimed. This action indicates that it can sense extracellular slime upon contact and will recognize and avoid areas it has already explored.

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

"We then upped the ante for the slime moulds 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."

The study is published online in the Journal Proceedings of the National Academy of Sciences.