Bees can get emotional just like humans, a new study, which sheds light on the workings of the reward systems in our own brains, has shown.
The official definition of an emotion is a transient subjective state that is triggered by appraisal of the situation.
Humans usually express their feelings verbally, but other animals also show behavioural changes under different circumstances consistent with an emotional response. However, most of the species studied are mammals, and the work has focused exclusively on negative emotions and stress responses.
Now we can add insects, and specifically bumblebees, to the list of emotional beings. Writing in Science, QMUL researcher Clint Perry and his colleagues used a sucrose solution to train bees to prefer to fly towards a blue placard, where they were rewarded with a sweet drink, rather than a green one, where they got nothing.
Once the bees had learned the task, the researchers introduced an ambiguous stimulus comprising a placard that was both blue and green.
Before unleashing them on a foraging flight, the researchers rewarded half of the bees with a drop of sugar solution.
The hypothesis was that, if bees get emotional, then this reward should put them into a more positive frame of mind and make them judge the ambiguous placard as worthy of investigation so they should still make a "bee-line" for it.
More negatively-inclined bees, which hadn't been rewarded, they speculated, would take a more "glass half empty" attitude to the ambiguous stimulus and disregard it.
Just as predicted, this trick, which is based on similar psychology tests carried out on humans, resulted in the rewarded bees taking much less time to investigate the ambiguous placard compared with the unrewarded controls.
It wasn't that the sugar-dosed insects just had more energy, because tests showed that they weren't flying any faster or becoming otherwise more hyperactive compared with the unrewarded animals.
Next, the researchers probed how the sugar reward might be manipulating the bee mindset. In humans and other mammals the nerve transmitter dopamine is released in the brain in response to rewarding experiences, which helps to reinforce beneficial behaviours.
With this in mind the team dosed their bees with drugs to block a range of nerve transmitters. Bees treated with fluphenazine, which selectively interrupts dopamine signals in the brain, no longer responded to the sugar reward in their ambiguous stimulus task and behaved like the unrewarded control bees.
"This gives us new insights into how emotions evolved," says Perry. "It also shows that insects are not as behaviourally rigid as we used to believe, and makes us think about how emotion works, on a fundamental level, in general."