The Oxford English Dictionary gives three relevant definitions of the adjective “sentient”:

a.) That feels or is capable of feeling; having the power or function of sensation or of perception by the senses.

b.) Conscious or percipient of something.

c.) Physiology. Of organs or tissues: Responsive to sensory stimuli.

(“Sentience” itself is defined only as “The condition or quality of being sentient, consciousness, susceptibility to sensation.”)

The question that the Scientific American article below asks (and for once it’s written by a scientist in this field) is whether insects fit the definition of the first two definitions: do they have feelings and sensations experiencing qualia like pain, joy, pleasure, or the sensation of “redness”?  Or are insects merely chitinous robots that are programmed by evolution to act (to us) as if they have feelings—programmed reactions that we anthropormophize as similar to our own sensations? After all, you can be “responsive to sensory stimuli” (the third sense above) without actually feeling the sensory stimuli the way humans do.

Answering the question of whether a bee or a fly is sentient in the first two senses, or has consciousness (the ability to be sentient and perceive stimuli), is difficult. Some would say it’s impossible. After all, we all know that we ourselves have consciousness  and feel pain and joy, because we experience those things personally. But can I prove that, say, another person is conscious? Not directly, because we can’t get inside their brains. We infer that they’re conscious because they tell us they are; they are physically constructed with the same neurons that give us consciousness; and they act as if they experience qualia.  It’s inference, but of a Bayesian sort, and the question has high priors.

But can we extend this to other species?  Chittka uses the example of dogs:

Although there is still no universally accepted, single experimental proof for pain experiences in any animal, common sense dictates that as we accumulate ever more pieces of evidence that insects can feel, the probability that they are indeed sentient increases. For example, if a dog with an injured paw whimpers, licks the wound, limps, lowers pressure on the paw while walking, learns to avoid the place where the injury happened and seeks out analgesics when offered, we have reasonable grounds to assume that the dog is indeed experiencing something unpleasant.

This is a Bayesian approach to the question, and it’s really the only way to go. Yes, I think it’s highly probable that dogs, and most mammals, feel pain. But what about insects, reptiles and amphibians? They certainly avoid unpleasant stimuli and gravitate towards pleasant ones, which you could interpret as feeling joy, pleasure, or pain, but do they feel these sensations? If you say that the behavior denotes sentience, well remember that protozoans do these things, too (see below).

I’m fully aware that philosophers of mind have probably discussed this issue at length, and I haven’t followed that literature, so my musings here may seem childish to these philosophers.  But this Sci. Am. article (click below to read, or find it archived here) is not written for philosophers of mind but for people like me: folks interested in science and wanting to see what’s happening in other fields.  I found the article quite interesting, and for me it slightly raised the probability that insects can feel pain. But the answer remains far from settled—or even of having a high probability. And the author admits that. But he cites a number of cool studies.

Here are the lines of evidence that, to Chittka, raise the Bayesian probability that insects have sentience: experiencing pain, pleasure, and even joy.

a.) They learn and can do really smart things. (All quotes from Chittka are indented):

The conventional wisdom about insects has been that they are automatons—unthinking, unfeeling creatures whose behavior is entirely hardwired. But in the 1990s researchers began making startling discoveries about insect minds. It’s not just the bees. Some species of wasps recognize their nest mates’ faces and acquire impressive social skills. For example, they can infer the fighting strengths of other wasps relative to their own just by watching other wasps fight among themselves. Ants rescue nest mates buried under rubble, digging away only over trapped (and thus invisible) body parts, inferring the body dimension from those parts that are visible above the surface. Flies immersed in virtual reality display attention and awareness of the passing of time. Locusts can visually estimate rung distances when walking on a ladder and then plan their step width accordingly (even when the target is hidden from sight after the movement is initiated).

All of these responses, of course, could come from computers programmed to learn from experience, which is exactly what we and other animals are. Natural selection has endowed us with a neuronal network that will make us behave in ways to further our reproduction (or, sometimes, that of our group—like an ant colony). We can program computers to do this, too: robots that avoid aversive stimuli and gravitate towards good ones. And clearly we behave in such a way that furthers our reproduction, of which survival is one component. But do insects experience the world, with its pleasures and pains, by having qualia similar to ours?

A related question is this: is consciousness like we have (feeling pain and joy) something that’s merely an epiphenomenon of having evolved a sufficiently complex nervous system, or is consciousness itself a product of natural selection to further our reproduction? We don’t know the answer, but it’s pretty clear that some of our conscious experiences, like pain, have evolved by selection. People who can’t feel pain as a result of neurological conditions or disease (like Hansen’s disease) quickly start getting infections, hurting their bodies without being aware, losing fingers, and the like. If you didn’t experience pain when putting your hand in boiling water, you’d damage your body. But if consciousness is just an epiphenomenon of a complex evolved nervous system, then we can’t automatically say that bees that act as if they’re conscious really are conscious.

I’m prepared to believe, based on what I said above, that mammals feel pain.  Maybe even reptiles or amphibians, though there are suggestions that fish don’t feel pain, at least in the way we do. All these creatures gravitate towards adaptive things and avoid nonadaptive ones, but again, they could be programmed to do so without the ancillary conscious experience that we have.

More evidence from Chittka:

b.) Insects act as if they can alter their consciousness:

Many plants contain bitter substances such as nicotine and caffeine to deter herbivores, but these substances are also found in low concentrations in some floral nectars. Researchers wondered whether pollinators might be deterred by such nectars, but they discovered the opposite. Bees actively seek out drugs such as nicotine and caffeine when given the choice and even self-medicate with nicotine when sick. Male fruit flies stressed by being deprived of mating opportunities prefer food containing alcohol (naturally present in fermenting fruit), and bees even show withdrawal symptoms when weaned off an alcohol-rich diet.

Again, seeking out things that are good for you, like curing you of illness or infection, could be programmed, either directly or as part of programs involved in “learning what gets rid of harmful conditions”. Now if bees are partial to coffee and cigarettes because it gets them high, then yes, it seems to show that they want to alter their consciousness, which implies that they have consciousness. But these facts aren’t that convincing to me, because nicotine and caffeine may have other beneficial physiological effects.

c.) Bees appear to be “optimistic”. Here’s the experiment Chittka adduces to support  that:

We trained one group of bees to associate the color blue with a sugary reward and green with no reward, and another group of bees to make the opposite association. We then presented the bees with a turquoise color, a shade intermediate between blue and green. A lucky subset of bees received a surprise sugar treat right before seeing the turquoise color; the other bees did not. The bees’ response to the ambiguous stimulus depended on whether they received a treat before the test: those that got the pretest sugar approached the intermediate color faster than those that didn’t.

The results indicate that when the bees were surprised with a reward, they experienced an optimistic state of mind. This state, which was found to be related to the neurotransmitter dopamine, made the bees more upbeat, if you will, about ambiguous stimuli—they approached it as they would the blue or green colors they were trained to associate with a reward.

This is not a meaningless experiment, but to me shows only that bees conditioned to approach a color after a sugar reward are more likely to approach something like that color than those who weren’t conditioned.  To call this “optimism” seems to me hyperbolically anthropomorphic.

d). Bees appear to experience “joy”.  This experiment is more suggestive to me:

Other work suggests that bees can experience not only optimism but also joy. Some years ago we trained bumblebees to roll tiny balls to a goal area to obtain a nectar reward—a form of object manipulation equivalent to human usage of a coin in a vending machine. In the course of these experiments, we noticed that some bees rolled the balls around even when no sugar reward was being offered. We suspected that this might be a form of play behavior.

Recently we confirmed this hunch experimentally. We connected a bumblebee colony to an arena equipped with mobile balls on one side, immobile balls on the other, and an unobstructed path through the middle that led to a feeding station containing freely available sugar solution and pollen. Bees went out of their way to return again and again to a “play area” where they rolled the mobile balls in all directions and often for extended periods without a sugar reward, even though plenty of food was provided nearby. There seemed to be something inherently enjoyable in the activity itself. In line with what other researchers have observed in vertebrate creatures at play, young bees engaged more often with the balls than older ones. And males played more than females (male bumblebees don’t work for the colony and therefore have a lot more time on their hands). These experiments are not merely cute—they provide further evidence of positive emotionlike states in bees.

It’s hard to understand these results without thinking that bees, like panda cubs, are playful, messing around with balls that give them pleasure. And since bees don’t experience balls in their natural state, they could be enjoying the novelty. On the other hand, they could simply be encountering something they haven’t experienced, and are following neuronal instructions to manipulate it to see how it operates, which could be useful knowledge in the future. This second interpretation means that no “pleasure” need be involved. Remember, play behavior in animals is often there to prepare them for what happens when they become adults, and isn’t just there for pleasure.

Again, it’s hard to judge from such studies whether bees are feeling pleasure in the way we do. But to me this makes it marginally more likely.

Finally,

e). Bees appear to weigh pain against pleasure, and change their behaviors when the balance is altered.  Here’s another experiment:

We decided to do an experiment with only moderately unpleasant stimuli, not injurious ones—and one in which bees could freely choose whether to experience these stimuli.

We gave bees a choice between two types of artificial flowers. Some were heated to 55 degrees Celsius (lower than your cup of coffee but still hot), and others were not. We varied the rewards given for visiting the flowers. Bees clearly avoided the heat when rewards for both flower types were equal. On its own, such a reaction could be interpreted as resulting from a simple reflex, without an “ouch-like” experience. But a hallmark of pain in humans is that it is not just an automatic, reflexlike response. Instead one may opt to grit one’s teeth and bear the discomfort—for example, if a reward is at stake. It turns out that bees have just this kind of flexibility. When the rewards at the heated flowers were high, the bees chose to land on them. Apparently it was worth their while to endure the discomfort. They did not have to rely on concurrent stimuli to make this trade-off. Even when heat and reward were removed from the flowers, bees judged the advantages and disadvantages of each flower type from memory and were thus able to make comparisons of the options in their minds.

To me, this really shows nothing more than that animals are attracted to adaptive stimuli and repelled by harmful ones, with the addition of being able to balance harms versus advantages. (This is like the “flight distance” of animals, with some individuals able to give more weight to attractive stimuli. That’s probably how cats got domesticated!) But it doesn’t tell us whether animals are feeling the pain or attraction the way we do.

And we should remember that even protozoans show avoidance of some external stimuli and can be induced by electrical shocks to avoid light. So these animals can be trained. Do they feel pain or pleasure? I doubt it—not protozoa!  They may not show “play” behavior, but perhaps they can be trained to weigh aversive versus adaptive stimuli, as in section “d” above.  I doubt anybody would conclude with confidence that protozoa feel pain the way we do (they don’t have a nervous system) or are even conscious.

Against the doubts that I’ve raised, Chittka offers a counterargument:

Critics could argue that each of the behaviors described earlier could also be programmed into a nonconscious robot. But nature cannot afford to generate beings that just pretend to be sentient. Although there is still no universally accepted, single experimental proof for pain experiences in any animal, common sense dictates that as we accumulate ever more pieces of evidence that insects can feel, the probability that they are indeed sentient increases.

The first sentence is what I have said already. And I’m willing to go along with the third sentence, too: as we learn more, the Bayesian probability that other species experience pain or pleasure can increase or decrease.

But I’m not willing to go along with the idea that “nature cannot afford to generate beings that just pretend to be sentient.”  What does he mean by “afford”? My interpretation is this: he’s saying that natural selection cannot produce organisms that act as if they’re sentient unless they really are sentient. And I cannot see any support for that, for we already know that protozoans act as if they experience qualia, but almost certainly don’t. And saying “pretend to be sentient” is pretty anthropormorphic! It implies, for example, that programmed robots that do what bees do are “pretending to be sentient” when in fact we know they are NOT sentient.

Finally, that leads to the Big AI Question: if we generate robots sufficiently complex that they respond exactly as humans do in complex situations requiring consciousness, does that mean that they have become conscious?  I say “no”, but others disagree.  After all, there are those panpsychists who say that even electrons and rocks have a rudimentary form of consciousness.

I’m writing this on the fly, so forgive me if my thoughts are half-baked.  I do think that Chtittka’s experiments are clever, and, over time, may give us a sense of sentience in other species. But I’m not yet ready to throw in with him on the claim that insects are conscious.  It’s enough for me now to realize that they do experience some aspects of the environment as things to be avoided. And that is why I have always anesthetized my fruit flies before killing them. (When I was an undergrad I used to take them to the biology department roof and let them go, but my advisor Bruce Grant nixed that on the grounds that I was polluting the natural gene pool of Drosophila.)

The last bit of Chittka’s paper is a thoughtful analysis of how these kinds of studies should condition our behavior towards insects. But even if they don’t feel pain, aversion or attraction itself should help us confect a philosophy of “insect ethics.”

h/t: Howard, who brought this paper to my attention and wanted my take on it. I’m sending him this link as my take.