Do insects feel pain? Many of us probably ask ourselves this question. We swat mosquitoes, step on ants, and spray poison on cockroaches, assuming, or perhaps hoping, that they can’t – but can they? As someone who studies the physiology behind insect behaviour, I’ve wondered about it myself. Those thoughts motivated me to examine the question from the perspective of evolution, neurobiology and robotics.
To find out whether insects feel pain, we first need to agree on what pain is. Pain is a personal subjective experience that includes negative emotions. Pain is different from nociception, which is the ability to respond to damaging stimuli. All organisms have nociception. Even bacteria can move away from harmful environments such as high pH. But not all animals feel pain. The question, then, is do insects have subjective experiences such as emotions and the ability to feel pain?
We’ve probably all observed insects struggling in a spider’s web or writhing after being sprayed with insecticide; they look like they might be in pain. Insects can also learn to avoid electric shocks, suggesting that they don’t like being shocked. However, just as I was appreciating how much some insect behaviour looked like our pain behaviour, I realized that Artificial Intelligence (e.g. robots and virtual characters) can also display similar behaviours (e.g. see (https://www.youtube.com/watch?v=YxyGwH7Ku5Y). Think about how virtual characters can realistically express pain in video games such as “The Last of Us” (e.g. https://www.youtube.com/watch?v=OQWD5W3fpPM). Researchers have developed circuits allowing robots and other AI to simulate emotional states (e.g. ‘joy’, ‘anger’, ‘fear’). These circuits alter how the robot/virtual character responds to its environment (i.e. the same stimulus produces a different response depending on the AI’s ‘emotion’). However, this does not mean that robots or virtual characters are ‘feeling’ these emotions. AI shows us that behaviour may not be the best guide to an insect’s internal experience.
Given that behaviour seemed an unreliable guide, I then looked for neurobiological evidence that insects feel pain. Unfortunately, the insect brain is very different from the human brain. However, once we understand how our brains perceive pain, we may be able to search for circuits that are functionally similar in insects. Research in humans suggests that pain perception is created by complex neural networks that link up the necessary brain areas. These types of networks require massive bidirectional connections across multiple brain regions. Insect brains also have interconnections across different brain areas. However, these interconnections are often quite modest. For example, the mushroom bodies in the insect brain are critical for learning and memory. Although the mushroom bodies contain thousands of neurons, in fruit flies, for example, they have only 21 output neurons. In humans, our memory area, the hippocampus, has hundreds of thousands of output neurons. The lack of output neurons in insects limits the ability of the insect brain to sew together the traits that create pain in us (e.g. sensory information, memory, and emotion).
Finally, I considered the question from an evolutionary perspective. How likely it is that evolution would select for insects to feel pain? In evolution, traits evolve if the benefits of a trait outweigh its costs. Unfortunately, nervous systems are expensive for animals. Insects have a small, economical, nervous system. Additional neurons dedicated to an ‘emotional’ neural circuit would be relatively expensive in terms of energetics and resources. If it is possible to produce the same behaviour without the cost, then evolution will select for the cheaper option. Robots show that there could be cheaper ways.
The subjective experience of pain is unlikely to be an all-or-none phenomenon. Asking whether insects feel pain forces us to consider what we would accept as a subjective experience of pain. What if it was devoid of emotional content? What if cognition is not involved? If insects have any type of subjective experience of pain, it is likely to be something that will be very different from our pain experience. It is likely to lack key features such as ‘distress’, ‘sadness’, and other states that require the synthesis of emotion, memory and cognition. In other words, insects are unlikely to feel pain as we understand it. So – should we still swat mosquitoes? Probably, but a case can be made that all animals deserve our respect, regardless of their ability to feel pain.
Adamo, S. (2019). Is it pain if it does not hurt? On the unlikelihood of insect pain. The Canadian Entomologist, 1-11. doi:10.4039/tce.2019.49 (Paper made available to read for FREE until Sept. 16, 2019 in cooperation with Cambridge University Press)