Researchers Find Evidence of Emotion-like States in Bees

Bumblebees can experience positive emotion-like states, say the authors of a study recently published in Science. Behaviors compatible with emotion-like states appeared to be influenced by dopamine, a neurochemical involved with emotions in mammals.

Researchers from Queen Mary University of London, England, conducted four experiments to investigate bumblebees’ responses to an unexpected reward, a droplet of sucrose solution. Sugar consumption induces positive emotions in humans, say the authors, so they explored whether sugar can produce a similar state in bees. Because bees cannot express emotion, the researchers tested quantifiable behaviors that would fit the criteria for emotion-like states.

The first experiment tested whether bees exhibit judgment bias. In a judgment bias model, subjects learn that one cue leads to a positive outcome and another cue leads to a negative outcome. Subjects in a positive emotional state tend to treat ambiguous cues optimistically, as though they will also lead to a positive outcome. The researchers trained bumblebees to enter a cylinder marked with a blue placard by placing a positive reward, sucrose solution, within that cylinder. A cylinder in a different location marked with a green placard contained no reward, only water. Once the bees were trained to aim for the blue placard, the researchers presented them with an ambiguous cue: a blue-green placard in an intermediate location. Before showing the bees the blue-green placard, they gave half of the bees a droplet of sucrose solution. The bees that consumed sugar entered the cylinder marked with the blue-green placard more quickly than those that did not.

To see if the sugar-consuming bees were simply seeking an additional reward rather than exhibiting something like a positive emotion, the researchers tested them with placards of a completely different color and location. In this case, sugar consumption had no effect on the speed with which the bees entered the cylinder. The authors concluded that the bees’ response to the blue-green placard did not reflect a general expectation of reward.

The aim of the second test was to find out if sucrose consumption increased bees’ activity level, which might affect their decision-making speed. The investigators found that thorax temperature (an indication of metabolic rate) was higher in bees receiving sucrose, but flight time and speed were not significantly different between bees that did and did not consume sugar. Therefore, they write, “unexpected rewards did not affect bees’ overall activity level.”

The third experiment explored whether the effect of sucrose would hold up in different contexts—in this case, during a simulated predator attack. The researchers trained bees to forage for sugar at a feeder. Then they gave the bees either a droplet of sucrose or nothing (control) before the bees entered a tunnel leading to the feeder. The investigators mimicked an attack by a crab spider by restraining each bee for a few seconds in the tunnel with a padded trapping device and then releasing the bee. Bees that received sucrose resumed foraging more quickly than the control bees. The authors concluded that sucrose attenuated the bees’ aversive response to a “predator” attack, analogous to the improvement in negative mood that people can experience after eating sweets.

In the fourth experiment, the researchers treated bees with a topical dopamine antagonist (fluphenazine) and repeated the first and third experiments. The dopamine antagonist reversed the effects of pretest sucrose consumption. Dopamine is a neurotransmitter involved in reward processing in mammals, and the authors concluded that dopamine may serve a similar function in bees.

The experiments illustrate the adaptive function of emotional states to influence behavior, say the authors, but they do not indicate that bees have feelings. “We didn't show that they feel happy,” said Dr Clint J. Perry, the lead author, in an interview.

The results “support the hypothesis that the fundamental elements of emotion exist in many species,” write the authors. “Understanding and investigating the basic features of emotion states will bring us a step closer to determining the brain mechanisms underlying emotion across taxa.”

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC.