Bumblebees balance pain perception with pursuit of food

The study was led by Professor Lars Chittka and colleagues, the findings were published in psychological and cognitive sciences.

Nociception and nocifensive behavior--the detection of and response to noxious stimuli--occur across many animal taxa, including insects. In mammals, neurons descending from the brain can modulate nociception and nocifensive behavior. The adaptive function of reducing nociception is to ensure that the subjective feeling of pain does not compromise the animal's performance in acquiring another motivational requirement. For example, if an animal is food-deprived and sustaining injuries from fighting with its prey, reducing nociception and pain could improve fighting performance, and thus the chance of alleviating starvation. In mammals, mental states can also drive the reduction of nocifensive behavior. For example, placebo effects, or imagining analgesia, reduces nociception and pain in humans. We tested whether an insect, the bumblebee (Bombus terrestris), is capable of context-dependent, centrally controlled reduction of nocifensive behavior.

Researchers used a motivational trade-off paradigm, where animals must flexibly trade-off two competing motivations. For example, hermit crabs require higher voltages of electric shock to evacuate their preferred shell species than for a less preferred one. Shock avoidance is traded off against shell preference. Bees also display motivational trade-offs in non-noxious contexts. However, whether bees trade-off noxious stimuli with other priorities is unknown.

They expanded on the motivational trade-off paradigm by ensuring that the trade-off relied on conditioned cues associated with the motivational stimuli, rather than the direct sensory experience of the stimuli themselves. Bees were given the choice between two high-quality feeders (containing 40 percent sucrose solution) and two alternative feeders. In the latter, different groups of bees were offered 10 percent, 20 percent, 30 percent, or 40 percent sucrose solution. Each group experienced the unheated temperature condition, with all feeders at room temperature (unheated), followed by the heated temperature condition, with the high-quality feeders at 55 celsius (heated) and the alternative feeders unheated. We predicted that, when the noxiously heated feeders dispensed higher sucrose concentrations than the unheated feeders, bees would have a lower tendency to avoid noxiously heated feeders.

Bumblebees avoided noxiously heated feeders less when these dispensed higher sucrose concentrations than unheated feeders. Unlike trade-offs described in other invertebrates, this trade-off relied on associative memories, rather than direct experience of the stimuli. Bees' ability to trade-off heat avoidance against sucrose preference indicates that conditioned motivational stimuli can influence nocifensive behavior, and the trade-off is mediated in the central nervous system. As in other animals, such an ability is viewed as consistent with the capacity to feel pain, although because of the subjective nature of pain experience, it is not a formal proof. Nonetheless, given the potential ethical implications of our research, the precautionary principle dictates that the possibility of insect pain and suffering should be taken seriously in insect research laboratories as well as insect conservation.

The heated and unheated feeders were coloured distinctly, either pink or yellow, and the heat and sucrose concentration could not be physically detected unless the bees had already chosen the feeder, suggesting that the bees' trade-off between a painfully hot stimulus and the pursuit of sucrose was likely driven by associative memory, rather than through direct sensation of the stimulus alone. According to the authors, the results suggest that bees are capable of context-dependent modulation of responses to noxious stimuli. (ANI)