Slime mould

TAKE a walk in the garden or woods in early summer or autumn and you may come across something on the ground that looks suspiciously like dog sick. It may be dog sick, but equally it could be a slime mould, a strange cross between an animal and a fungus that feeds on dead grass or leaves and thrives in the ground's moist warmth.

Why strange? Slime moulds are known as "social amoebas" but they don't behave like any other single-celled creature. The moulds are hard to classify because their life cycle is similar to a fungus's (they reproduce via spores), but they share more genes with animals than they do with, say, yeasts. We know about their animal genes because we now have the complete genetic blueprint for Dictyostelium discoideum, the most commonly studied slime mould (Nature, vol 435, p 43).

And then there's the outlandish way a slime mould hunts its prey. It moves about the damp soil as a single blob of protoplasm with many nuclei, gobbling up bacteria and particles of organic matter and growing through simple cell division. Then, as its growth outstrips its food supply, the creature sends out a chemical signal to other slime moulds, which gather together to form a multicellular super-organism. Covered in slime and often as big as a human hand, it crawls through the forest in search of food, reaching a top speed of about a centimetre an hour. Then, when resources run low, it finds a sunny spot to bask and transforms itself into a spore factory, dispersing its cells on the wind to better hunting grounds.

If you think that's clever, consider this: slime moulds appear to possess a basic intelligence. In 2000, researchers in Japan found that a slime mould called Physarum polycephalum could navigate a maze. They placed pieces of chopped-up slime mould in various corners of a plastic maze with two openings where they left oat flakes, a favourite food of slime moulds. The pieces of mould came together to form a single organism, but instead of filling the maze as the researchers had expected, the organism withdrew from dead ends and formed a single tube spanning the shortest distance between the two openings. "Clever and cunning" is how they described it.

How does an organism with no nervous system accomplish such a feat? It may be related to the mould's rhythmic contracting and relaxing when it forms into a tube, and the way these pulses vary when it comes into contact with food. Beyond that, where these enigmatic creatures get their computing power from is still pretty much a mystery.