Distance estimation in the foraging honeybee Apis mellifera

During the past forty years the generally accepted mechanism of distance estimation by honeybees was the "energy hypothesis": honeybee foragers measure distances to food sources by gauging the amount of energy spent on the way to a feeder. This hypothesis was tested in several experiments. In experiments that increased energy expenditure to reach a feeding site, bees indicated in their dances a decreased distance. Bees were trained to forage from helium-filled balloons suspended 30, 60, and 90m above the ground at various distances from the hive. Energy needed to lift the body increased considerably but the distance indicated decreased by as much as 50%. From these results came the "optical flow hypothesis": honeybees use the retinal image flow of ground motion to estimate feeder distance. Outlines of objects move faster and farther across the retina when bees are closer to the ground. Bees do not have stereoscopic vision or other means for depth perception, therefore flight altitude is critical for distance perception. This optical flow hypothesis is supported by additional evidence. Foragers who flew from a hive on top of a high building (50m) to a feeder on the roof of a second high building (34m) indicated a much shorter distance than foragers that collected food at ground level. Bees who walk to feeding sites over patterns varying in the number of stripes danced differently, depending on the number of stripes encountered during the trip. Covering of the lower portions of the eyes with black paint led bees to fly at lower altitudes than control bees, and bees having the upper portions of the eyes covered. Bees keep optical flow constant by altering flight altitude. Most of the experiments used to support the energy hypothesis do not conflict with the optical flow hypothesis as long as one considers differences in flight behavior.