Jules Smith-Ferguson is exploring how even a unicellular organism can form memories and make complex decisions.
Ethograms is a monthly column published in collaboration with the Australasian Society for the Study of Animal Behaviour (ASSAB), showcasing the work of early-career researchers. Johanne Martens is a PhD candidate at Deakin University, and an outreach officer for ASSAB.
A yellow, slimy alien creeps over the damp forest floor. It doesn’t have eyes, or even a head. It may seem motionless to your untrained, impatient eye. But with the aid of time-lapse filming, you can see a shape shifter, tentacles crawling over logs, testing, searching, hunting food. Bacteria, fungi — nothing is safe from the hungry, many-headed slime. This alien has no brain, no nerves, but it can make decisions, and even has a memory.
This slimy creature is a protist, named Physarum polycephalum, which translates into “many-headed slime”. During certain life stages, this amoeboid organism can extend a network of interconnected veins, and wander off on the hunt for food, even though it only consists of one single, but multinucleate, cell. Once it has eaten enough, it will enter its reproductive stage and form spores. If conditions get too dry, it can produce a protective armour-like crust and survive until conditions improve. When it is nice and damp again, the alien walks on.
Jules Smith-Ferguson, from the University of Sydney, is investigating this fascinating creature for his PhD, to find out more about how this unicellular organism ticks. He says that he is interested in “how complex the behaviour of creatures without a brain can get”, and whether we are wrong to assume that some complex behaviours require a brain.
Smith-Ferguson completed his honours in philosophy of biology; his research focussed on minimal cognition in plants, and on similarities in the behaviour of these non-neuronal organisms and organisms with a brain. His fascination with slime moulds started when he attended a talk about slime mould research by his now-supervisor, Madeleine Beekman. Slime moulds, he explains, show some remarkable behaviours “for something which is essentially a big bag of slime, with no nervous system and the ability to be chopped into little pieces and come back together as one individual”.
For a long time, behavioural and cognition research has focussed on organisms that are fairly similar to us, meaning that they have a brain and a nervous system: animals, mainly vertebrates. But we know much less about what makes creatures tick that are very, very different from us.
The many-headed slime, for example, has both an internal and external memory system: Once they have explored an area, they will leave behind the outer shell of their tube system — their tentacles, or “extracellular slime”, as Smith-Ferguson calls it — and bring along only their living protoplasm. This way, they can ‘map’ areas that they have already explored, similar to a pheromone trail that ants leave behind. This is a form of external memory. The slime mould is “putting information into the environment so that it doesn't need to store it internally”, says Smith-Ferguson. “[It’s] like writing a shopping list instead of trying to remember every individual item you need to buy.”
But it gets even more fascinating: “Something particularly interesting is that, at least in Physarum polycephalum, they seem to use this external memory system in a negative sense — that is, they avoid areas covered in extracellular slime, as it suggests the area has already been explored and exploited,” explains Smith-Ferguson. So this is a tactic which is opposite to leaving a pheromone trail to a food source; it’s a bit like writing a shopping list for the things you want to avoid in the shops.
Slime moulds have been widely studied in recent years, with a focus on their movement, behaviour and decision-making. They can, for example, anticipate unfavourable conditions before they occur, which is pretty impressive for a brainless organism that only consists of one cell. They can find the shortest path through a maze. For one famous study, published in the journal Science, researchers laid out food in a pattern that resembled a map of cities in the Tokyo area. The efficient network Physarum established between these food sources was surprisingly similar to the local rail network. According to the researchers, slime mould can, therefore, form “networks with comparable efficiency, fault tolerance, and cost to those of real-world infrastructure networks”, which can help with future network development.
Smith-Ferguson hopes that his project will give insights into mechanisms of information attainment, storage and retrieval, similar to the network studies by his colleagues, which target applications for artificial intelligence and information networks.
He is currently testing whether he can increase the chance of a slime mould making a specific decision. This could be choosing a high-quality, but risky, food source over a low-quality but safe one, or vice versa. For a slime mould, a desirable but risky food could be some oat flakes presented in a bright spot (slime moulds don’t like light, as it puts them in danger of drying out). Smith-Ferguson applies a selective pressure on the slime moulds, by only selecting individuals that make a specific choice — for example, choosing a high-quality, but risky, food. “The idea is that hopefully, over time, by continually selecting slime which make the decision you are applying pressure on, we can change the ratio of individuals making this decision,” he says.
Smith-Ferguson also wants to find out whether slime moulds are capable of associative learning — that is, if they can associate other cues, like light, location or odour, with a food reward. “If we can show associative learning in the slime mould, it would fundamentally change the way we view learning and memory, especially given that associative learning has now been demonstrated in plants and artificial networks/machines,” says Smith-Ferguson. This study is still in the pilot phase, as one major step is to find a stimulus that is relevant for the slime mould.
This research on the “bag of slime” could have a big impact on how we see the creatures that inhabit the planet with us. “At the core of my research is a motivation to colour in the spectrum of intelligent behaviours that range across the domains of life,” says Smith-Ferguson. “There has been a distinct shift in the last few decades towards appreciating the complex abilities of a vast array of organisms, including bacteria, plants and protists, and if I can contribute to this cause then I'll view my research as successful."
Edited by Andrew Katsis