Desert memories and route guidance - for ants
The famed ability of ants to traverse hot desert sand from a location 100m from the nest is further tested in a new scientific paper. Travelling from your home to your feeding ground can sometimes be a tortuous route. Desert ants such as Cataglyphis fortis can only use stereotyped visual routes but an individual can become "disengaged" from her cues, even when all visual signals are present.
Training of the insects in this paper's experiments involved using a single isolated landmark on a simple route, but repeating their route often confused them. The clues as to how the memory acts in these circumstances are now more obvious. The memory gained from one trip provides guidance for future trips, but how does the ant decide which memory is correct for each route? Bound for her nest, a food carrier will only use memories of homeward journeys. She will use the alignment of images gained on previous trips to travel automatically in the usual direction. This particular species also use guidance from the directions and cumulative distances travelled. This clever path integration (PI) system means that distance travelled can be subtracted from total distance to give the animal an estimate of direction and distance remaining.
Using the PI and its visual memory, the ant can assume she is in the nest when the PI tells her this. So the experimenter places the individual back at the food site, leaving the ant with a PI "zero vector." This interesting state still allows the ant to use visual memory to negotiate around the single landmark. It's essential to avoid anthropomorphic thinking so an "ant-centric" explanation is offered by the author, Matthew Collett of Exeter University. He has published his absorbing paper in the Proceedings of the Royal Society B, as - "A Desert Ant's memory of Recent Visual Experience and the Control of Route Guidance".
Using route memories, individuals taken after their homeward journey began returning to the nest again. Matthew suggests that this return movement by the ants is always interrupted. They stop after 3-5 metres and assess or search their immediate vicinity. He calls the next stage a second switch, in which they resume the normal route. Ant-centric thinking would imply a changed navigational state in the ant. The ant's normal homeward trek is never interrupted in this way.
To prove that ant-centric thinking was valid in this case, the home run was extended. From 12 or 18m behind the feeder, a PI state of zero would again be achieved at distance of 14m. Confusion only happened when the ant was at the further 18m distance. So the PI state couldn't then explain why the ant was searching or assessing the situation. Instead, there was some connection with the repetition of the approach to that single landmark. Their memory told them they had already travelled towards it. Some residual memory of the travel prevented the PI from fully operating.
The conclusions to be drawn are that several classes of memory process are involved. Long term memory retains visual routes for months if necessary; there could also be very short term memories of 5-10 second duration. They put the situation into a context or part of a sequence. Memory of repetition is a longer term memory of current visual data, resembling the "counting" seen in honey bees. The mammalian style episodic memory in our brains is absent. These tiny organisms can modify behaviour differently but very successfully with just a little confusion. There is a lot to be learnt, as usual, from the observation of the ant.