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Slime Moulds Get Smart

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Dr Tanya Latty has never lost her childhood fascination with “creepy crawlies”.

By Stephen Luntz

How can slime moulds make complex decisions when they don’t have a brain?

Dr Tanya Latty had more encounters with bears than she could count before coming to Australia to study slime moulds. She may now be safer, but her ego could have been dented with her discovery that these masses of brainless cells can be more effective than people at weighing up conflicting options before making optimal decisions.

“How individuals deal with multiple conflicting demands when they are choosing their food is an important aspect of foraging ecology in living things,” says Latty of the University of Sydney’s School of Biological Sciences. “Yet the body of knowledge we have on foraging behaviour has pretty much ignored neurologically simple organisms like the slime moulds.”

Latty has rectified this with the discovery, published in Ecology, that slime moulds can balance the quality of a food source against the level of light exposure. “Slime moulds don’t like too much light – they turn and grow away from areas that are well-lit,” Latty explains.

Slime moulds travel towards a food at 5 cm/hour. Latty offered slime moulds two patches of food with differing amounts of light and food quality (determined by the proportion of oatmeal in agar). The moulds consistently chose the better food in the same light, even if the oatmeal concentrations varied by only 2%. However, when light levels differed the moulds avoided the light unless they were tempted by a large difference in oatmeal concentration.

Latty admits that no one has any idea how such decisions get made without a brain, but is contemplating testing the extent of their capacity, perhaps by adding salt as a third factor.

Latty says she “always loved creepy crawlies”. As a child growing up in Canada, “I was always bringing home snakes and beetles to my poor mother”. She wanted to be a vet because it was the only career she knew “where you got to play with animals all day long. You don’t really hear about research work, don’t know that you can be running round the forests all day looking at beetles.”

Work experience at a local vet clinic during high school revealed that she was allergic to fur. She also learnt that veterinary work is repetitive, “so in addition to being sneezy I was really bored”. Having rejected veterinary science, Latty is now “really glad I made the choice I did. Vets don’t get to work with slime moulds.”

After a combined environmental science/biology degree at Trent University in Ontario, Latty started a Masters degree on pine beetles at the University of Calgary. The suppression of fires in Canadian forests has created what Latty calls “a beetle buffet” of overmature trees. Beetles spread from dying trees to attack younger pines, creating insect outbreaks of record size.

The pine trees produce a sticky resin to protect themselves from beetles, and can easily kill small numbers of beetles. It is only when beetles attack en masse that they can overwhelm a tree’s defence, killing it and leaving a feast for all.

Latty pondered the motivation for the first beetles mounting an attack. Unless they are joined by many others, these scouts are on a suicide mission.

“My first idea was that the first attackers got a better share of the resources. This turned out to be totally wrong,” Latty says. If they survive, the original beetles still get a worse cut of the wood than late arrivals.

Instead it seems that early adopters are driven by desperation. “If you’re a beetle flying through the forest, the best thing is to land on a tree that is dead or dying, but there are not a lot of those. So rather than just starving in mid-air, eventually the best approach is to attack a tree and hope that others come to support you.”

In the course of this research Latty had her bear encounters. “You just have to tell yourself you’re really calm and walk away,” she says.

Cougars don’t normally attack humans, but Latty says that her small size made her tempting. Fortunately when a big cat stalked her it waited until she was nearly at her car before making its charge. She was able to slam the door while the cougar circled in frustration. “To add insult to injury, it was wearing a radio tracking collar, so I almost ended up becoming some other scientist’s data point. I might have been the first scientist to be eaten by another’s research animal.”

While finishing her doctorate, Latty saw a Sydney University advertisement to work on “bees, ants and slime mould”. She didn’t expect to get the job since she lacked relevant experience, but was later told that she got the position mainly because of her enthusiasm,which is highly believable to anyone speaks to her.

Latty’s first role here was to study the trails laid down by Argentine ants, a species that has thousands of nests within one supercolony. She found that the trails between the nests are laid out with exceptional efficiency.

Another project required her to explain how honeybee swarms know where they are going. Her theory was that a small proportion of bees knew their direction and flew quickly through the swarm, with more confused colleagues falling in behind them. She tested this by sending distracter bees flying through the swarm, and found that this did indeed sow confusion among the swarm.

The work on slime moulds arose from studies of ant foraging behaviour. When ants find a high quality food source they will search intensively nearby. Lower quality food will prompt a less intense search, and if the food is really poor they will leave the area entirely. Latty likens this to panning for gold, and says that slime moulds behave in the same way. She’s interested in seeing whether fungi and plants do likewise.

Her mould work may have applications. Four years ago Dr Klaus-Peter Zauner of the University of Southampton demonstrated that it is possible for the slime mould’s photophobia to be used to make a robot avoid light. More recently, Dr Toshiyuki Nakagaki of Hokkaido University in Japan found that when oat flakes were placed on a map in the locations of cities in the Tokyo region, the slime moulds grew in a pattern replicating the subway system, suggesting that subway planners had got the system right.

“It’s amazing that a single-celled organism like the slime mould Physarum polycephalum is able to weigh up such options and decide on the best food option. It shows they are capable of making really complicated foraging decisions,” Latty says.

“Our work suggests that neurological complexity is not a prerequisite for complex foraging behaviour. So it turns out that choosing a top quality meal in a nice setting is a no-brainer – literally!”