When depressed or stressed we tend to have a pessimistic outlook on life. Everything seems dark and gloomy and it feels like life is conspiring against us. However, when things are going well, we feel optimistic and hopeful. Although very familiar to us, these emotional states are not unique to humans. Other vertebrates like dogs, sheep, rats and some birds, exhibit optimistic behaviours when housed in large enriched habitats and pessimistic behaviours when stressed or anxious. But when did these emotional states arise? Do invertebrate animals also become pessimistic when stressed? These questions prompted Melissa Bateson, Suzanne Desire, Sarah Gartside and Geraldine A. Wright, from Newcastle University, UK, to investigate whether stressed honeybees show evidence of pessimistic behaviour.
First, the team had to find a way of stressing the bees, so they decided to simulate a predatory attack by vigorously shaking the bees for 1 min. Then, they tested to see whether the attack had affected the insects' physiology. Knowing that the levels of key biogeneic amines, such as octopamine, dopamine and serotonin, change in stressed bees, the team tested their levels in the bees' haemolymph and found that they had decreased. The simulated attack had worked.
Next, the team tested how this stress affected honeybees' responses to odours that they associated with pleasant and unpleasant tastes. They wanted to find out whether a change in mood affected the bees' attitudes towards these odours. To do this, they collected bees from an outdoor colony at the university and trained each individual to associate an odour with a substance. One odour was always paired with a food reward (1 or 2 mol l–1 sucrose) while the other odour was paired with a punishment or a poor quality reward (0.01 mol l–1 quinine solution or 0.3 mol l–1 sucrose). Pretty soon, the bees learned to associate the odours with either the reward or the punishment, extending their mouthparts to the reward odour but withholding their mouthparts when presented with the punishment odour. Once the bees were trained, the scientists simulated a predatory attack on half of the bees by shaking them in the same way as before.
Five minutes after the simulated attack, the team tested the trained bees' reactions to five different substances: the reward odour, the punishment odour and three new scents, where the reward and punishment odours were blended together. Bateson and her team wanted to know whether shaken bees would be pessimistic about the odours and withhold their mouth parts more than their unstressed counterparts. Indeed, the bees that had gone through the simulated attack were more likely to behave pessimistically when they experienced the punishment odour and its most similar novel scent. These results show that when stressed, honeybees increase their expectations of a negative outcome.
Although invertebrates are not normally regarded as animals that can exhibit emotions, the combination of physiological changes and behaviour, in addition to an increased expectation of punishment, indicates a negative emotional state in agitated honeybees. This study by Bateson and her colleagues is the first to show that stressful conditions induce pessimistic behaviour in an invertebrate and the presence of ‘human-like’ emotions. Moreover, a ‘glass half empty’ attitude might be a good marker for negative emotions across species, from insects to mammals. Finally, the researchers suggest that a ‘glass half empty’ attitude in response to a stressful situation may have evolved as a way of protection from harmful conditions.