For an animal with a brain the size of a microdot, bees are capable of some remarkably sophisticated feats, such as learning the locations, colours and scents of luscious flowers. But it is their brain size that makes them so attractive to scientists. Martin Giurfa from the University of Toulouse, France, explains that scientists investigate the insect's tiny brain to understand more about the neural basis of learning and memory. According to Giurfa, scientists train bees to recognise odours in the lab by teaching them to associate particular scents with a tasty sugar-water reward so that they extend their tongue (proboscis) whenever they catch a whiff of the same scent. However, this appetitive approach didn't work properly for the bee's visual sense. Giurfa and Jean-Christophe Sandoz decided to try a different tactic. Could bees be trained to memorise colours by associating them with a minor shock – a small electric shock to be precise (p. 3577)? Giurfa explains that years before, his PhD supervisor, Josué Núnez, had discovered that mildly shocked bees extend their stings, so initially he decided to see whether bees could be trained to extend their stings when they recognised an odour if the scent had been delivered with a weak electric shock.
‘Some people said that would never work because when a bee stings a person the bee dies,’ recalls Giurfa. However, despite the doubts, Giurfa and Sandoz proved that bees could learn that an odour was followed by a mild electric shock and extend their sting. The team also knew that the shock had to be as gentle as possible. ‘We wanted the bee to respond reliably and not to be damaged by the stimulation,’ he explains, so they settled on a mild 2 s long 7.5 V shock to protect the bees from injury.
Next, Giurfa and Sandoz teamed up with graduate students Theo Mota and Edith Roussel to test whether bees could be trained to extend their stings when they recognised a colour.
Gently strapping a bee to a copper stage, the team showed it a green screen and then applied the gentle shock 3 s later: the bee's sting popped out. The team also showed the bee a blue screen, without applying the mild shock. After repeatedly showing the two screens to the bee in random order (green accompanied by the shock and blue without) they showed the bee the green screen, this time without the gentle shock, to see if she would recognise it and extend her sting. She did. And she never extended the sting when she saw the blue screen. She could distinguish between the two colours.
Then the team tested to see if the bees could be trained to distinguish between two blue screens (439 nm and 440 nm) that were indistinguishable to human eyes. This time, one of the blues was associated with the shock and the other was not. Again, the bees only extended the stings when they saw the blue that had been associated with the mild shock during training. Finally, the team tested whether the bees could be trained to distinguish between colours with different intensities, and again the insects passed with flying colours.
So, restrained bees in the lab can be trained to distinguish and memorise colours if they are coupled with a gentle shock and this new training regime offers scientists the opportunity to discover how bees use their memories to avoid enemies and other risks in the environment. ‘Now we have a protocol in which you can do both [visual and odour cues] so that we can study multimodal learning in the brain,’ explains Giurfa.