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In this issue |
On the Edge (p. 559)
kathryn{at}biologists.com
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Lehrer had always been impressed by the honey bee’s ability to learn. She knew that no one had systematically analysed how bees recognise simple shapes. So, armed with this knowledge and both species, she and Campan set out to see if they could train honeybees to do what had been thought impossible 70 years before, and find out what the bees were looking for.
They trained the bees to recognise five different shapes: a disk, a square, a square that had been rotated to make a diamond, and two triangles (one pointing up and the other down). Bees were trained to recognise each of the shapes in turn, but the two species were trained in different ways that depended on their life styles.
Lehrer trained the foraging honeybees to recognise one of the five shapes by rewarding them with sugar. Once they were trained, Lehrer gave them a choice of two shapes, but only one led to the reward. Lehrer gave bees that successfully recognised a shape a positive score, and marked them down when they picked the wrong one.
Campan carried out a parallel experiment in France with the leafcutter bees. This time their reward was getting home. They used a shape as a gateway, passing though a hole in its centre, to find their nest. Once the bees had learned which shape led them home, Campan replaced the single entry with a choice of two, to see if they could recognise their ‘home’ shape. Again, the bees were scored, gaining a positive mark if they chose the shape they were trained to recognise, or a negative mark if they went to the alternative entrance.
The idea was to determine whether the bees discriminate by comparing the areas of the shapes, or by extracting information about some aspect of the shape’s perimeter to distinguish between them. The cunning twist was that Campan and Lehrer tried to confuse the bees. They tested them with shapes that had the same surface areas, and pairs of identical shapes that had different orientations. This cut down the number of parameters that the bees could use to recognise a shape that led to a reward.
Watching which shapes the bees chose, Lehrer and Campan realised that neither species learned the area of the shapes. Some other feature was setting the shapes apart. But which?
The diamond and the square had the same number of angles and straight edges, so bees should confuse the shapes if these parameters were important. But both species showed no sign of confusion. Campan and Lehrer realised that it was the positions and orientations of individual edges that gave the game away.
The final proof that the bees were looking at the edges came when Campan and Lehrer repeated the experiments using patterned shapes set in front of a patterned background. In this situation, the shape appears to move as the insect approaches, making the edges stand out. Even then both species could tell a triangle from a square.
Whether the bees were tested with solid shapes, or the shape as moving edges, honeybees always scored higher than the less sophisticated leafcutters. And Lehrer isn’t surprised that they can do the impossible, again; after all, bees are simply super-learners!
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