People are irrational. Example: You wouldn’t buy a new dress, or suit, that costs $100 (‘that’s far too much to spend!’) but you would buy one that was $300, but is now ‘reduced’ to $150 (‘but just look at how far down it’s come!’) Sound familiar? You’re not alone, it’s fairly well-known that humans are irrational (at least by those in advertising- some products in supermarkets are never meant to be bought- they’re just there to get you to buy other products more), but are other animals just as irrational?

Hummingbirds, starlings, and even bees have been shown to be irrational. But what about organisms even smaller than bees? A recent study1 found that even slime moulds (brain-less creatures, which live in damp places like mouldy leaves and logs) can be irrational.

Firstly, what do we actually mean by irrational? Well, in a rational world, you would go into tescos and all the products would have a value to you, that was independent of all the other products. So, choosing between a budget brand of pasta and tesco finest, perhaps the latter would look a bit too pricey, so you go for the budget. However, if another type of pasta, one that looks exceptionally posh, was now placed between the previous two, say, hand-crafted bowtie pasta freshly made this morning in Italy and carried over by carrier pigeon, now that tesco finest is looking fairly acceptable for dinner. However, this decision is irrational: the products themselves haven’t changed, but we have changed our view on their worth just through comparison with other products.

So, how might you test for this in slime-moulds (since, as far as I know, they don’t eat tortelloni)?

Physarum polycephalum, Thanks to Jerry Kirkhart for the photo

First, the slime mould was divided into two groups, one which was starved and one which was fed (apparently it likes oatmeal). Fragments could then be taken off each group- taking off a fragment makes a new individual, a bit like taking a ‘baby’ spider plant off the main plant and re-potting it. The individual was placed into an ‘arena’ (a rather dramatic way of saying a dish on the scientist’s desk) and given a choice between two food options, to see which one they would choose. There were three different food types (oatmeal in 3 different concentrations, low medium and high), placed in either light or dark conditions (slime moulds don’t like light), thus adding up to six different options that were given to the slime moulds in different combinations.

The slime moulds from the group which were fed chose the high concentration oatmeal mixture in dark conditions most frequently, and chose the low-concentration oatmeal mixture in the light conditions least often. The slime moulds which were starved had similar preferences, but were more willing to choose options in the disliked light conditions if they had a high concentration of oatmeal (slime mould hungry!).

The slime moulds were now put into another arena where they had the choice between three food options (like us adding in our third brand of pasta). The scientists knew which food options the slime moulds preferred when they had just two to choose between, and so now they wanted to see if adding in a third option of low value to the slime moulds would affect their preferences for the other two options.

It did. When the starved slime moulds were given an option between a food they liked a lot before, and one they did not like as much, when the third option was added of one they liked even less, they chose the first option much more than before. To anthropomorphise, this is because the first option is now ‘looking really good’ compared to the two lesser options. Thus their opinion of the first option changed based on a third option, even though they did not like the third option.

This shows that, like us, slime moulds make decisions through comparison to the other available options, rather than having an intrinsic value for things. Who knows what else we might have in common….



1 Latty, T., and Beekman, M. (2011). Irrational decision-making in an amoeboid organism: transitivity and context-dependent preferences. Proceedings Of the Royal Society B: Biological Sciences 278, 307.