Scientists from Australia and China have found that bumblebees exhibit a tongue extension response after consuming food if that food was perceived as pleasant by them. It was established that this reaction is not driven by dopamine, but its intensity increases when exposed to endocannabinoids.
Link to Hedonic Evaluation
A similar mechanism of reaction is observed in mammals, allowing researchers to link this phenomenon to the affective evaluation of food's hedonic value, meaning a signal indicating that the food brings pleasure.
Experiments on Bumblebees
The research team, led by Quinn Solvy from Southern Medical University in Guangzhou, conducted experiments on bumblebees (Bombus). Unlike the common understanding in literature regarding bees, bumblebees, and *Drosophila*, where reflexive proboscis extension is considered an indicator of motivation, scientists attempted for the first time to measure pleasure from eating.
Bumblebees were offered various aqueous solutions: pure, salty, bitter, sweet (20% sucrose), and very sweet (60% sucrose). The bees rejected the bitter, salty, and plain water. When consuming sweet solutions, they drank them with lapping movements of their tongues and then repeatedly extended their tongues while sipping drops. The number of such movements was higher in response to a higher sugar concentration. Salt or quinine solutions caused the insects to shake their heads and wipe their proboscis with their legs.
Separation of Motivation and Pleasure
To test whether the reaction was related to pleasure rather than just sweet taste, the insects were placed in a chamber at 40 degrees Celsius and induced dehydration. After this, all bumblebees drank plain and salty water, after which they extended their tongues, with a significant decrease in head shaking incidents. This showed that glossae extension did not depend solely on sweet taste but changed depending on the physiological state of the bumblebees.
Subsequently, researchers studied the possibility of separating tongue extension from motivation and food consumption. When bumblebees were offered a mixed stimulus (20% sugar and 5% salt), they drank but did not extend their tongues afterward, only slightly shaking their heads. Satiated bumblebees receiving a sugar solution (which equated to maintaining motivation) also hardly extended their glossa. Furthermore, the frequency of tongue extensions decreased if the bumblebees were first offered a 60% sugar solution and then a 20% one. The authors concluded that these two processes—consumption and tongue extension—are separated, suggesting that tongue extension reflects the affective evaluation of the stimulus, i.e., the absence of pleasure in the described cases.
Influence of Neurotransmitters
After this, scientists administered dopamine, octopamine (another neurotransmitter affecting motivation), or anandamide (an endocannabinoid) to the bumblebees and measured reactivity and tongue extension. All three neurotransmitters enhanced the insects' reaction to sugar: bumblebees began extending their proboscis even with 0.1–0.3% sugar solutions, whereas control groups reacted only to concentrations of 1–3% and above. However, tongue extension after eating was not altered by dopamine and octopamine, but increased by about double when exposed to anandamide.
Conclusion of the Study
The authors concluded that tongue extension after eating in bumblebees is a process unrelated to reflexive reactivity, which is traditionally considered a measure of motivation. In their opinion, bumblebees extend their tongues as a sign of pleasure, similar to mammals. Nevertheless, the mechanisms of such reactions in bumblebees have not yet been studied, unlike in mammals, where the enhancement of mimetic reactions occurs in certain brain areas upon administration of opioids or endocannabinoids.
Despite the authors' confidence in proving affective evaluation, they admitted that they did not discuss alternative interpretations. They note that glossae extension may be related not only to hedonic evaluation but also to the desire to repeat eating or searching for it. The text also mentions a contradiction between the behavior of satiated rats and bumblebees, which may indicate differences in the functioning of these processes in insects and mammals.
