If you've ever watched a bee and wondered if she's happy, you're not alone. Similar questions have fascinated biologists for generations, including Charles Darwin, who argued in 1872 that "even insects express anger, terror, jealousy and love."

Now, almost 150 years later, scientists have found signs of optimism — and possibly happiness — in bumblebees. It's still unclear what this feels like for bees, or how it compares with complex human emotions. But for such tiny brains to experience even a "positive emotion-like state," as the researchers describe it, is a big deal.

Aside from what it reveals about insects, they explain in the journal Science, this discovery could shed new light on the nature of emotion itself.

"Investigating and understanding the basic features of emotion states will help us determine the brain mechanisms underlying emotion across all animals," says lead author Clint Perry, a biologist at Queen Mary University of London, in a statement.

Sweet emotion

So what puts bees in a good mood? Good food, namely sugar. Similar to how people often feel happier after eating dessert (to a point, anyway), bees get a seemingly emotional boost from sweets, Perry and his colleagues report.

To demonstrate that, they first built a chamber containing artificial flowers — actually just small tubes colored blue or green. They trained 24 bumblebees to enter this chamber through a tunnel, at which point the bees had to decide which "flower" to investigate first. The researchers hid a 30 percent sugar solution in the blue tubes, while green tubes held plain water rather than a reward. Bees are astute foragers, and they soon learned to favor blue tubes over green.

And then came a curveball: The researchers sent the bees into the chamber again, only now the tube was an ambiguous color, like blue-green. As the bees passed through the entry tunnel, half were given a droplet of 60 percent sugar solution, while the other half got nothing, just like the previous test. Bees that received this pre-experiment pick-me-up behaved differently in the chamber, flying to the unfamiliar flower up to four times faster than bees whose entry involved no sugar.

A bumblebee drinks a droplet of sugar water during one of the study's experiments. (Photo: Clint J. Perry/QMUL)

That suggests the snack improved bees' moods, making them more hopeful about a confusing situation. Follow-up experiments support that interpretation, the researchers say, indicating pre-fed bees weren't just more energized or readier to forage, but felt an insect version of optimism. Both groups were equally swift when they knew a tube contained food, for example, and equally sluggish when they knew it didn't. Their suspected moods only became evident amid uncertainty.

In one of the other experiments, Perry and his colleagues simulated a spider attack — a common threat for bumblebees in the wild — with a mechanism that grabbed the bees and temporarily held them. When it finally let go, bees that had been primed with sugar water took less time to recover and start foraging again.

The researchers even found they could end bees' good moods by giving them a drug called fluphenazine, which blocks the effects of dopamine in the brain. Dopamine is a neurotransmitter that plays a key role in the brain's reward system, and it's known to improve mood in humans. Since an anti-dopamine drug seemed to kill the bees' buzz, this backs up the idea that sugar had made them "happy" in the first place.

"Sweet food can improve negative moods in human adults and reduce crying of newborns in response to negative events," says study co-author Luigi Baciadonna, a Ph.D. candidate at Queen Mary University of London. "Our results suggest that similar cognitive responses are occurring in bees."

Let it bee

Like most insects, bees are far more sophisticated than they seem, from colony-building socialites to solitary volcano dwellers. And aside from what it might help scientists learn about emotion in general, this study casts insects in a more relatable light — and that could compel people everywhere to be nicer to bees.

A wide range of bee species around the world are now in decline, including many bumblebees, due to a mix of threats such as insecticides, invasive parasites and disease. We already know that's bad for us, since bees are vital pollinators of native plants and food crops, but the prospect of emotion adds another twist, says study co-author Lars Chittka. We should also consider the suffering of individual bees, whether we're simulating spider attacks in a lab or spraying insecticides in our yards.

"The finding that bees exhibit not just surprising levels of intelligence, but also emotion-like states," Chittka says, "indicates that we should respect their needs when testing them in experiments, and do more for their conservation."