Animals acquire an understanding of the world through experience and instruction. If we touch a hot stove, we burn ourselves and learn to keep our fingers away from glowing red coils. Alternatively, we can avoid the drama of direct experience by heeding the warnings of our parents. Both options are effective and help to guide our future behaviors. And until now, both were believed to require a complex and coordinated nervous system. However, in a recent issue of Proceedings of the Royal Society, Series B, David Vogel and Audrey Dussutour, from Toulouse University in France, show that even brainless slime molds can learn and teach.
Few people would look towards slime molds as the poster-child of intelligence. Yet, over several decades of work, these multi-nucleate bags of goo have been found to be capable of surprising feats of cleverness. They can navigate complex mazes, optimize nutritional challenges and even find their way through a miniature version of the Tokyo subway system; they show swarm, or collective, intelligence. But can they learn?
To test this, Vogel and Dussutour tempted slime molds with a tasty treat that could only be reached by traversing a bridge containing high levels of salt. At first, slime molds hesitated before crossing the salt bridge; they moved across it slowly and haltingly, like dipping a single toe into a frigid sea. However, after repeated exposure, they learned to ignore the noxious, although harmless, stimulus. In other words, they had become habituated to the salt and learned to ignore it so that they could more rapidly reach the food. Further, after the salt was removed, the slime molds forgot what they had learned and reverted to salt aversion; they had recovered their justified caution.
Habituation and recovery may not seem especially advanced. However, they require a form of basic learning coupled to a type of memory. Given this, the team reasoned that this physical memory could potentially be transmissible. When slime molds meet each other they can undergo cellular fusion, whereby the two formerly independent individuals join forces to form a larger single entity. If fused cells can share nuclei and cytoplasm, why not their memories too?
When the team fused a habituated slime mold with an unhabituated one, they found that the unhabituated individual gained the salt habituation of the other. Something, as yet unidentified, thus carried durable memories from one individual to another. But the transfer took some time. If the pair was separated after only an hour of fusion, no transfer occurred, while by 3 h, transfer was complete. Furthermore, habituation persisted in the formerly naive slime mold even after it was separated from its habituated partner, confirming that transferred memories are at least as durable in the recipient slime mold as they are in the donor.
Our world is filled with sensory inputs that we experience and share. But imagine yourself stripped of sight, sound, taste or smell. How would you learn from others? How would you teach what you know? You couldn't simply push your brain, with all its knowledge, into another's head. Yet, this is essentially what slime molds do. By fusing together, they share information. And, as the authors have shown, as more individuals fuse, the faster they learn. But this is also an area filled with questions. Among many others, how are memories formed and carried? Do some individuals cheat by acquiring information while withholding their own hard-won knowledge? Are all memories transmissible? And finally, does any of this matter in nature? I hope the authors are as eager to learn the answers to these questions as I am.