Landmark learning in bees

Summary

• 1. The experiments described here were undertaken to discover how bees use nearby landmarks to guide their way to a food source. Two major questions are raised. First, what do bees learn about the spatial layout of landmarks and food source? Secondly, how might this information help them reach their destination?

• 2. Single, marked bees were trained to collect sugar solution from a small and inconspicuous reservoir in a room in which extraneous visual cues had been reduced to a minimum. The position of the reservoir was defined by an array of one or more matt black landmarks. After bees had been trained, their flight path was recorded on videotape when the landmarks were present, but the food source absent. During such tests bees spent most of their time searching where the food source should have been.

• 3. Thus, if bees were trained to a reservoir whose position was specified by a single cylindrical landmark and tested with the same landmark, they searched at the expected site of the reservoir. However, when the size of the landmark was changed between training and testing, the area in which bees searched was displaced to one where the landmark appeared roughly the same size as the training landmark when viewed from the reservoir. These experiments suggest that bees learn no more than the apparent size and bearing of the landmark as seen from the food source, and that to return there they move to a position where their retinal image matches their remembered image of the landmark.

• 4. Experiments with more complex arrays of landmarks support the same hypothesis. A simple rule predicts a bee's search area when it is trained to a food source defined by the position of three landmarks and tested either with the same array, or with landmarks of different sizes, or with landmarks placed at different distances from the reservoir. The bee then always searches where the compass bearings of the landmarks on its retina were the same as they had been when it was stationed at the food source.

• 5. Tests with bees trained to either one or three landmarks suggest that the bearings of landmarks on the retina are learnt with respect to external compass bearings. Thus, a single, cylindrical landmark does not define direction. Nonetheless, bees searched in one location and not in a circle centred on the landmark. Bees trained to three landmarks only learnt the site of the reservoir if the array was kept in a constant orientation during training.

• 6. Computer models were devised to discover how bees might use a remembered image of the landmark array to direct their flight path to their destination. The models simulated a situation in which a bee takes a 2-dimensional snapshot of its surroundings from the position it wishes to retrieve and continuously compares this with its current retinal image. It then uses the difference between the two to guide its way. Different models of increasing complexity were explored until one was found which closely mimicked the bee's behaviour.