Bees are remarkable navigators. Flitting around the environment, they match snapshot memories of landmarks surrounding luscious flowers with their current view to successfully direct themselves back to blooms: and they do all this with a brain the size of a microdot.
Laura Dittmar and her colleagues from Bielefeld University, Germany, explain that this strategy works fine when landmarks stand out well from the background, but what happens when the landmarks blend in with the surroundings? Dittmar, Martin Egelhaaf and Norbert Boeddeker decided to find out how bees locate a transparent feeder guided only by landmarks that blend in (). But first the team decided to check that their bees used snapshot memories of conspicuous landmarks to locate goals.
Building a 1.95 m diameter flight arena covered with blotchy red paper, Dittmar and Emily Baird trained bees to fly into the arena and locate an almost invisible acrylic glass feeder surrounded by three conspicuous red pillars (landmarks). Then the duo began filming the insects with high-speed video as they altered the landscape. They removed one of the red pillars and filmed the bee as it tried to locate the Perspex feeder with only two pillars for guidance. Then they replaced the cylinder and removed another to see how the bees coped using a different pair of landmarks.
Dittmar saw that the bees initially memorised the location of the entrance to the arena before exploring the pillar landmarks by flying in straight lines, making sharp turns and flying sideways. Dittmar admits that many of these manoeuvres were so fast that they were only visible in the high-speed movies. However, when Dittmar and Baird removed a pillar, the bees' search strategy changed completely. This time the bees focused their search on the remaining pillars and spent longer searching.
After analysing the bee's flight paths, Wolfgang Stürzl calculated stationary bee's eye views of the arena to find out how the insects navigated and saw that the bees try to match their current view of two pillars with the snapshot memory of the three pillars. The team realised that the bees were able to match up their view of the two remaining pillars with their snapshot memory at several locations in the arena, so the insects spent extra time searching these locations until they correctly matched the two remaining pillars with their memory and located the feeder.
Having confirmed that the bees used snapshot memories when navigating with conspicuous landmarks, the duo covered all of the cylinders in the blotchy red paper that covered the flight arena and filmed the bees to see how they fared when their landmarks blended in with the background. Amazingly, they had no problem. ‘I was really surprised to see the bees finding the nearly invisible food source when the landmarks were hardly detectable,’ says Dittmar.
So how did the bees do it? Calculating the stationary bee's view of the blotchy pillars, Stürzl could see that the pillars were almost invisible as they blended in perfectly with the background. However, when he calculated the way that the pillars moved across the insect's eye at different locations in the arena, and compared it with the way that the image of the pillars moves over the eye when the insect is in its final approach to the feeder, Stürzl found a good match. Not only do the bees match static snapshot memories to locate conspicuous landmarks but also they memorise motion snapshots, that include information about the relative movement of objects against the background, to pick out navigation landmarks that would otherwise blend in.
