Bumblebees’ behavior changes when under stressful situations. (ArtEvent ET/Shutterstock)

NEWCASTLE UPON TYNE, England — Bumblebees can get down in the dumps, too. Researchers from Newcastle University discovered that when physically stressed, bees begin to expect worse outcomes when faced with uncertainty, behaving in ways that mirror how humans respond when anxious or depressed.

This research, published in Proceedings of the Royal Society B, challenges our understanding of insect minds and raises fascinating questions about the evolution of emotions across different branches of the animal kingdom.

For years, scientists have debated whether non-human animals experience emotions. While most research has focused on mammals and other vertebrates, growing evidence indicates insects may have more complex inner lives than we previously thought.

How Do You Measure a Bee’s Mood?

The research team wanted to test this in bumblebees but how do you measure what a bee is “thinking”? They couldn’t simply ask the bees about their emotional state. Instead, they developed a clever experiment to observe how stress changed the bees’ behavior when faced with ambiguous situations.

This approach builds on the concept of “judgment bias,” the way emotions color our interpretation of neutral or ambiguous information. People experiencing anxiety or depression tend to expect the worst from uncertain situations.

Previous studies with insects used what scientists call a “go/no-go” test design. But this approach has problems. If a stressed bee doesn’t approach something, is it because of a negative outlook, or simply because stress reduced its motivation? To overcome this limitation, the researchers designed a test requiring bees to make an active choice between two alternatives, making their judgment clearer.

Training Bees to Make Choices

In the experiment, female worker bumblebees learned to associate different colors with different quality rewards. When shown one color (like blue), they found a high-concentration sugar solution (50% sucrose) in one location. When shown another color (like green), they found a lower-concentration reward (30% sucrose) in a different location.

After this training, some bees were subjected to simulated predator attacks. One group was shaken (placed in a container and vibrated on a laboratory shaker for a minute), another group was briefly trapped by a mechanical device (simulating being caught by a predator), while a control group remained unstressed.

Then came the real test: researchers showed the bees not only the original trained colors but also new, ambiguous colors falling between the two training colors on the color spectrum. Both possible reward locations now contained only water. Which would the bees choose?

Stressed Bees Expect the Worst

Stressed bees were much less likely to approach the location that had previously offered higher rewards when shown ambiguous colors. They consistently made more “pessimistic” choices, behaving as if they expected lower rewards even though they could still distinguish between the original training colors accurately.

When researchers separately tested feeding motivation by measuring how much sugar solution different groups of bees consumed, they found no differences between stressed and non-stressed bees. This confirmed the behavioral change wasn’t simply due to reduced hunger after stress.

To better understand the bees’ decision-making process, the researchers applied mathematical modeling. Their analysis determined that stressed bees weren’t confused about the colors. Rather, they had shifted their expectations about rewards.

Control bees remained optimistic, behaving as if they expected high rewards to be available about half the time. But stressed bees acted as if high rewards were much less likely. The models revealed they behaved as if high rewards might be available as little as 6% of the time.

“Our research suggests that, like other animals including humans, bees may experience emotion-like states when stressed, as demonstrated by a clear shift towards pessimism. When faced with ambiguity, stressed bees, much like someone seeing the glass as ‘half empty,’ are more likely to expect negative outcomes,” says study author Olga Procenko from Newcastle University, in a statement.

More Than Just Tiny Robots

From an evolutionary perspective, this pessimistic shift makes adaptive sense. After a dangerous encounter with a predator, playing it safe and expecting less from uncertain situations could help an animal avoid further danger. When the environment seems threatening, lowering expectations and taking fewer risks might increase survival chances.

Despite how small insect brains are, bees may experience internal states similar to emotions that influence their perception and decision-making in complex ways. Bees are critical pollinators facing numerous environmental stressors from pesticides to habitat loss and climate change. Scientists say the research is important as it means stress can impact how bees approach flowers and pollinate plants, as well as their ability to access high-quality rewards.

The humble bumblebee reminds us that the gap between insects and humans may be narrower than we once believed. Just like humans, they tend to be more pessimistic when under stress.