Clint Perry can tell if a bee is in a good mood.

The little bugs will fly faster toward uncertainty — as though predisposed to believe that unfamiliar things will turn out fine. They recover faster after being attacked. Their dopamine levels are elevated.

Bumblebees may be mere invertebrates, with no cortex and only a tiny fraction of the neurons we humans have in our heads. But they seem capable of surprisingly complex emotions, Perry and his colleagues write in the journal Science. In their paper published Thursday, the researchers describe how they were able to induce something very much like optimism in a tiny, seemingly unsophisticated bee.

To start their experiment, Perry's team trained their bees to associate rewards with certain stimuli. A green square on one side of their habitat marked a cup full of sucrose solution — a sweet treat to a bumblebee. A blue square on the other side of the habitat marked a cup full of mere water. Quickly, the insects learned to make a literal beeline toward the green square and avoid the blue one.

Next, the researchers gave half of their bees an extra jolt of the sucrose solution. This was to put the insects in a good mood; research on humans and other animals has found that eating something sweet makes creatures feel better — one study of newborns found that they cried less and were less sensitive to pain when fed sugar water.

Other research has suggested that when humans are in a positive emotional state, they're more likely to expect good things. This makes sense — if you're already happy about that delicious pastry you just ate, or your favorite sports team making the playoffs, you'll probably be inclined to see the glass as being half full. In scientific terms, that's known as cognitive bias: Having experienced one good thing, we automatically expect more.

But would bees be the same? To test this, Perry and his colleagues created an "ambiguous" signal — they paired the cup with a turquoise square and placed it in an unfamiliar part of the habitat. Then they watched to see what happen.

Those who had been primed with sugar zipped right through, while their less lucky comrades meandered. And when they were "attacked" by a simulated predator (two sponges that momentarily clamped down and trapped them), the rewarded bees were quicker to go back about their business. This is what an optimistic bee looks like: Having gotten one good thing, it is more confident, less fearful and eager for more.

To figure out the biological basis of this optimism, the scientists treated the bees with several chemicals that block various neurotransmitters associated with motivation, pleasure and rewards. One of them, a dopamine antagonist called fluphenazine, completely canceled out the positive effect of the sugar treat. That suggests that higher dopamine levels from the sugary jolt is what made the bees so inclined to expect the best.

In an analysis for Science, animal cognition researchers Michael Mendl and Elizabeth Paul said it's possible that the so-called "optimism" may be just a sugar high. How can we know that the bees weren't flying faster because they were energetic after being fed?

But Perry and his colleagues think this is unlikely. The bees' behavior didn't speed up overall in response to the sugar solution; they were as slow as ever getting to the blue card that signaled unappealing water.

The fact that optimism — or something like it — exists in an invertebrate suggests that the feeling may have an evolutionary root, Mendl and Paul write. One function of "affective states," they say, could be to inform animals' decision-making strategies when balancing the need for food with the fear of predators.

The finding also opens up a host of other questions: How exactly does dopamine induce the optimistic state? To what extent can this state be compared to optimism in humans? And most of all, if bees can act optimistically, can they be said to feel optimistic?

"Whether 'emotion-like' states in insects are accompanied by emotional feelings remains unanswered," Mendl and Paul write, "but the possibility of insect consciousness is now the topic of exciting new theories and vigorous debate."

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