Watching a column of ants marching towards food is an impressive and somewhat mesmerising sight. Back and forth they go, almost as if they have a GPS system to guide them. Researchers have long known that ants use landmarks for navigation, despite having fairly poor sight. But what do the ants do if the landmarks look similar? A group from the University of Sussex have been tackling this question.
So how do ants find their way back to a food site or their nest? They simply store a view of the surrounding landmarks, like taking a snapshot. When they return they recall this snapshot and match it with the image on their retina of the landscape. `It's a simple and economic way of using visual memories to navigate by,' explains Tom Collett.
Previous work had suggested that ants store a wide-angle snapshot while looking directly at a landmark. So if ants were given two landmarks on either side of a feeding site that cannot be seen simultaneously, they might store two snapshots, one per landmark, to help them return to the site. Paul Graham,Virginie Durier and Collett wondered how ants might recall the correct snapshot when looking at a particular landmark(p. 393).
The team initially trained ants to search for food midway between two differently sized black cylinders against a white background. Multiple visits meant that the ants had the opportunity to form clear snapshots of the landmarks: one snapshot would be of the small cylinder, while the other would be of the large. They had previously found that ants move towards a landmark that appears smaller than expected until it matches the memorised snapshot. Indeed, when the team changed the two cylinders for a pair of equal size and tested the ants, the ants still searched close to the centre as though they did not know which cylinder was which. As Collett emphasises, `There is a premium on retrieving the right snapshot.' So how do the ants avoid such confusion?
To see if they use extra visual information to help distinguish similar landmarks, the training was repeated but with a patterned background. A patterned curtain was placed so it was to the left of the cylinder in one snapshot while in the other it was to the right. This time when the cylinders were changed for two of equal size in the test, the ants searched closer to the cylinder that corresponded to the large training cylinder. Collett and his colleagues suspect that the curtain primes the recognition of the correct landmark, so that the ants can identify which of the two identically sized cylinders corresponds to the small cylinder and which to the large. But how isn't clear and will be the focus for future work by Collett and his team.
So next time you see ants marching purposely across your kitchen, don't reach for the ant powder, mess with their landmarks and confuse them instead!
