If you’ve never watched bees carefully, you’re missing out. Looking up close as they gently curl and uncoil their tapered mouths toward food, you sense that they’re not just eating, but enjoying. Watch a bit more, and the hesitant flicks and sags of their antennae seem to convey some kind of emotion. Maybe annoyance? Or something like agitation?

Whether bees really experience any of these things is an open scientific question. It’s also an important one with implications for how we should treat not just bees, but the great majority of animals. Recently, studies by Geraldine Wright and her colleagues at Newcastle University in the UK have rekindled debate over these issues by showing that honeybees may experience something akin to moods.

Using simple behavioral tests, Wright’s research team showed that like other lab-tested brooders -- which so far include us, monkeys, dogs, and starlings -- stressed bees tend to see the glass as half empty. While this doesn’t (and can’t) prove that bees experience human-like emotions, it does give pause. We should take seriously the possibility that it feels like something to be an insect.

As invertebrates -- animals without backbones -- bees are representatives of a diverse group accounting for over 95 percent of animal species. But despite their prevalence, not to mention their varied and often nuanced behaviors, invertebrates are sometimes regarded as life’s second string, as a mindless and unfeeling band of alien critters. If that seems a bit melodramatic, just consider our willingness to boil some of them alive.

While there’s a good deal known about invertebrate neurobiology, these facts alone haven’t settled questions of their sentience. On the one hand, invertebrates lack a cortex, amygdala, as well as many of the other major brain structures routinely implicated in human emotion. And unsurprisingly, their nervous systems are quite minimalist compared to ours: we have roughly a hundred thousand bee brains worth of neurons in our heads. On the other hand, some invertebrates, including insects, do posses the rudiments of our stress response system. So the question is still on the table: do they experience emotion in a way that we would recognize, or just react to the world with a set of glorified reflexes?

To get a foothold on this fascinating question, Wright’s team, which was headed by Melissa Bateson, followed the lead of recent investigations on “pessimistic biases” in animals. In humans, this refers to the well known tendency to perceive threat or anticipate negative outcomes when anxious or depressed. For example, in tests where people are shown ambiguous sentences like “The doctor examined little Emily’s growth,” the anxious are less likely than others to conclude that Emily is fine, and that only her height was being checked.

Although it’s probably not surprising that bad moods and negative judgments go together, their correlation is useful. Casually, we rely on it to make informed guesses about how people are feeling from observing their actions and choices. Scientifically, we can use it to study the emotions of creatures who can’t directly tell us how they feel. The key here is to set up a controlled situation where animals encounter an ambiguous stimulus (a nonverbal version of the Emily sentence).

In the initial setup of Wright’s experiment, a group of bees was trained to associate two simple odor mixtures with two different foods. One mixture, which consisted of 1 part of hexanol to 9 parts of octanone, was repeatedly paired with sucrose, which bees find rewarding. The other odor mixture consisted of the same two chemicals in opposite proportions (9 parts hexanol to 1 part octanone), and was paired with quinine, a compound most of us find bitter, and that bees will actively avoid after tasting. After learning these odor-food associations, the bees responded as expected, uncoiling and extending their mouthparts in anticipation of food when the first odor mixture was presented, and retracting them at offers of the second mixture.

After this training, the scientists could test the bees’ mouthing responses to a series of ambiguous, intermediate odor mixtures to study perceptual judgments. But first, half of the bees got a trip to the vortexer.

It was probably as unpleasant for them as it sounds. In a procedure meant to simulate a badger attack on a hive, the bees were shaken for one minute in a benchtop machine used to vigorously mix chemicals. If anything would put bees in bad mood, this would be it.

Next, both shaken and unshaken bees were tested on five mixtures of hexanol and octanone at different concentrations. Unsurprisingly, both groups were more likely to advance their mouths to octanol heavy mixtures (which predicted sugar) than hexanol heavy mixtures (which predicted quinine). Interestingly though, the shaken bees were more reluctant to advance toward the mixtures than their unshaken counterparts. In an analogue of the classic half-empty vs. half-full scenario, in which an equal mixture of hexanol and octanone was presented, control bees gave the concoction the benefit of the doubt. They advanced their mouths in anticipation of food on more than half of trials. Shaken bees, on the other hand, were far more likely to recoil. The stress of shaking had turned them into pessimists who interpreted the ambiguous odor as half-threatening, rather than half-appetizing.

In addition to these behavioral measures, the scientists also tested for changes in bees’ systemic neurotransmitter levels after shaking. Transmitters with known roles in insect learning (octopamine), aversive conditioning (dopamine), and aggression (serotonin) were all reduced by the procedure.

Together, these behavioral and neurochemical tests reveal an unexpected dimension of bee cognition. Scientifically, we can say that bees have a persistent state of negative affect that is triggered by agitation, associated with system-wide changes in neurotransmitters and causes clear, measurable cognitive biases.

Can we draw a deeper conclusion than this? For now, no. Short of asking the bees how they’re feeling, or probing their minds with a yet un-built emotion-meter, we simply can’t know what being a bee feels like. However, Wright and her co-authors leave us with an intriguing plea for consistency, one that nudges us to think clearly on how we regard the minds and emotions of all creatures.

In other contexts, they imply, we’re instinctively willing to call a dog or a person anxious when we see behavioral evidence of pessimism. We see a “timid” but “personable” dog experience what is “quite likely” separation anxiety, test it for pessimistic biases, and upon finding these (as was done last year), conclude that dogs indeed feel anxious when left alone. “It is logically inconsistent,” Bateson and colleagues say, to conclude this “but to deny the same conclusion in the case of honeybees.”

Indeed it is. So you need to decide whether bees get a trial membership among the genuinely anxious, or if your dog’s pessimism implies nothing about its feelings. Probably not a tough choice for a dog owner.

Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.