Bumblebees may have an inner life, according to a new study that captured their facial expressions after tasting different solutions.
Researchers used slow-motion video to observe the insects' reactions.
They found that bees stick out their tongues after tasting something pleasant, but shake their heads and wipe their mouths when encountering something distasteful.
The study, published in the journal Proceedings of the National Academy of Sciences, was a collaboration between Macquarie University and Southern Medical University in China.
Scientists presented bumblebees with droplets containing various concentrations of sugar, plain water, salt, or quinine, and recorded their physical responses.
Professor Andrew Barron from Macquarie University, a co-author of the paper, noted that after tasting sweet solutions, the insects displayed a behavior termed "post-consumption glossa."
"Facial expressions are an important window into the internal states of animals," Barron said.
"What we found is that bees show responses with their mouthparts to solutions that indicate their subjective like or dislike of those solutions.
It tells us there is an inner life to the insect."
Establishing evidence for pleasure or pain in animals without language is challenging, so scientists rely on behavioral markers like facial expressions to evaluate positive and negative experiences.
While similar indicators of liking or disliking are widely accepted in mammals like rats and primates, the concept remains controversial when applied to insects.
"If a rat gets a salty taste it doesn't like, it wipes its mouth parts, wipes its whiskers, wipes its tongue," Barron said.
"And we see something similar in a bee."
To confirm these responses were not merely chemical reflexes, the researchers tested 18 different bee colonies under varying circumstances, including full states, heat stress, and under the influence of various drugs.
The reactions altered based on context.
Heat exposure shifted their responses to water or salty solutions from neutral or averse reactions to positive ones.
Barron compared this to a human being offered an electrolyte drink, noting it might taste unpleasant normally unless a person had just completed a long run on a hot day.
Associate Professor Thomas White, an entomologist from the University of Sydney not involved in the study, described research into insect sentience as a fast-moving field.
White emphasized that this study is fascinating because it investigates the positive side of life, whereas most historical research has focused on negative experiences like pain or fear.
"The picture is increasingly pushing towards a view that insects, or many insects, have some simple capacity to feel the world, not just to assess it and detect it and process information, but to actually have a point of view," White said.
According to White, this finding challenges the intuition of many people regarding where the line is drawn in the animal kingdom concerning moral and ethical responsibilities.
