This is pretty amazing. Single-celled slime molds have been shown to learn to anticipate events after repeated stimuli. In this case:
As the cells crawled across an agar plate, the researchers subjected them to cold, dry conditions for the first 10 minutes of every hour. During these cool spells, the cells slowed down their motion. After three cold snaps the scientists stopped changing the temperature and humidity and watched to see whether the amoebas had learned the pattern. Sure enough, many of the cells throttled back right on the hour in anticipation of another bout of cold weather.
This sort of learning has been demonstrated before, in organisms as simple as earthworms and planarians. But even then, these animals have at least a rudimentary “brain” consisting of specialized neurons. Slime molds, on the other hand, are independent single-celled organisms. They do, however, have the ability to aggregate to form complex systems, migrating as a single unit and forming a stalk to sporulate. This new finding suggests that these emergent properties can also produce intelligent behavior, without the need for a nervous system.
For a bonus: detailed and colorful pictures of slime molds, with a focus on their fruiting bodies.