Who feels more pain, a person or a cat? A cat or a cockroach? It’s widely assumed animal intelligence and the capacity to feel pain are positively correlated, with brainier animals more likely to feel pain, and vice versa. But what if our intuition is wrong and the opposite is true? Perhaps animals that are less intelligent feel not only as much pain but even more.

Thinking about pain is psychologically challenging. It can be, well, a pain. “To have great pain is to have certainty; to hear about pain is to have doubt,” wrote Elaine Scarry in The Body in Pain. It’s all too easy to dismiss the pain of others while treating our own as unquestioned fact.

This disparity is even more true when it comes to perceiving the pain of animals, where Western society has placed Descartes before the truth. Animals, he famously claimed, are mere automata. They don’t feel pain like we do, and so, carrying the notion of human exceptionalism much too far, Descartes didn’t hesitate to cut animals open while they were alive, without concern for what they were clearly feeling. The same was true of other giants of early science, such as William Harvey, whose discovery of the heart’s role in circulating blood was based in large part on his own heartless vivisection of living dogs.

This argument begins with a simple question: What is pain for?

A correlate of this attitude, rarely challenged even today, is that the more similar animals are to us, the more likely they are to feel pain. And in proportion as they are “simple”—i.e., stupid—they can’t. I want to take issue with this and suggest a counterintuitive hypothesis: That animals with less cognitive capacity might feel at least as much and perhaps more pain than their smarter cousins. I vividly recall, as a child, watching with horror as my uncle threaded a worm on a hook. The victim wriggled with what in a human would unquestionably be agony, while my uncle reassured me, “It’s not feeling pain.” As an adult researcher (who should have known better), I’ve seen snakes, fish, and cockroaches spasm when subjected to electric shocks.

This argument, admittedly hypothetical, begins with a simple question: What is pain for? It leads to a similarly simple answer: It provides a valuable warning that something dangerous (literally, hurtful) is going on. You’ve stepped on a tack or touched a hot stove. Whether you’re an animal or a person, something has bitten or poisoned you, stepped on your toe or your tail. Pain can induce an individual to withdraw from a damaging situation, to protect a vulnerable and damaged body part, and to avoid further experiencing whatever produced the unpleasant sensation. It typically, but not always, resolves once the painful stimulus is removed, and it serves as a major indicator—often, the most demanding and important one—that removing the stimulus would be in the individual’s interest.

The absence of pain, accordingly, can itself be dangerous. This is why people suffering from Hansen’s Disease (“leprosy”) often lose their fingers, toes, or parts of their face, because one consequence of this illness is a loss of peripheral pain sensation. As unpleasant as it is and how terrible when severe and chronic, pain is important. It can be, paradoxically, our friend.

ROYAL PAINS: Human pain is multifaceted, but our big brains can outsmart it sometimes. Animals, argues David P. Barash, have no such luck. Chart by Usman Zafar Paracha / Shutterstock

Insofar as it is a crucial alarm signal, pain should be a cross-species universal, no less valuable for paramecia than for people. I agree with the argument made by Richard Dawkins in his book Science in the Soul, in a chapter titled “But Can They Suffer?,” that smaller-brained creatures just might have greater need for this signal. “Isn’t it plausible that an unintelligent species might need a massive wallop of pain, to drive home a lesson that we can learn with less powerful inducement?” Dawkins asked.

There are many benefits to having a big brain, something that evolutionary theory would predict given that complex neuronal networks are metabolically expensive to produce and maintain. The brains of Homo sapiens occupy roughly 2 percent of adult body weight, while accounting for 20 percent of our energy budget. Among those adaptive payoffs, the more functional the brain, the greater the learning capacity of those possessing it. Of course, this capacity is employed in all sorts of endeavors, including memory, among which one of the less often recognized benefits is the ability to recall physical circumstances that have been disadvantageous, even dangerous—that is, which are likely to have caused pain. Once they have experienced pain, those capable of remembering it have a distinct advantage. They can learn about the precipitating circumstances and avoid repeating them. Once burned, twice shy.

But what if you’re an animal less endowed with intelligence, and with associated (and associative) memory? You might be doomed to repeat each painful experience, Groundhog Day style, because you are less able to connect the relevant dots leading to the distressing outcome. Not so for intelligent animals. Indeed, such animals should be able to learn what to avoid in proportion as they are neuronally endowed. The dummies, accordingly, would benefit more than the smarty-pants from an especially potent stimulus, a blast of something deeply unpleasant—call it “pain”—more likely to evoke whatever passes for memory and learning in their admittedly dim minds. If so, then they would benefit from a particularly loud alarm bell: More pain rather than less.

In recent years, research has demonstrated that animals can reason far more than previously thought. Consider the well-documented abilities of Alex, the African gray parrot studied by Irene Pepperberg. Alex, an acronym for Avian Learning Experiment, had a vocabulary of more than 100 words and could identify colors and shapes of objects. Or consider the cognitive capacities of border collie dogs such as Chaser, who mastered 1,022 nouns (one for each of her toys) and Rico, who could figure out logical puzzles with the acumen of a 3-year-old human. The ability to reason might well go along with the ability to suffer, but it is unlikely to be a prerequisite.

After all, we human beings feel immediate pain if we cut or burn ourselves, without reasoning about it. Our quick sensation of pain (labeled “nociception”) is typically associated with a reflex response, independent of our higher cognitive functions. Chickens subjected to internal bodily circumstances we associate with pain will preferentially eat food laced with analgesics. The sensory neurons of fish are physiologically indistinguishable from our own. Those neurons also respond to damaging stimuli, responses that are mitigated by administration of opioid drugs. Moreover, opioid receptors similar to our own have been identified in insects, crustaceans, mollusks, and even nematodes.

In a now-famous article titled “What Is It Like to Be a Bat?” philosopher Thomas Nagel concluded, essentially, we’ll never know. Nor will we ever know, for sure, what it’s like to be a fish, insect, or crustacean. But the evidence we do have suggests that as Shylock observes in The Merchant of Venice, if they are “pricked” they not only bleed, but feel pain.

Pain would have been among the most fundamental traits to have emerged.

How, then, do we feel pain? More accurately, what is the nature of the signals that we interpret as pain, and how do they function? So far as we know, pain—like all other mental experiences—is mediated by our neurons and “experienced” in our brains, even though it derives from events occurring elsewhere in our bodies. The human brain, surprisingly perhaps, is itself insensitive to pain, which is why pioneering neurosurgeon Wilder Penfield was able to invade the brains of conscious, unanesthetized patients and discover that stimulating different brain regions evoked distinct feelings and memories.

When most of us think of pain, it is what physicians and neurobiologists call “nociceptive pain,” typically the result of tissue injury when a peripheral body part has been burned, crushed, punctured, or sliced. Nociceptive pain can also derive from visceral structures such as the heart, liver, or—more commonly—the gastrointestinal tract, which is highly sensitive to inappropriate stretching. Whereas peripheral nociceptive pain is generally sharp, that deriving from our viscera tends to feel dully persistent and, unsurprisingly, deep. Not to be ignored when it comes to nociception is inflammatory pain, evoked by arthritis and other excessive immune responses.

Because of its universal and clinical significance (pain is responsible for nearly 50 percent of patient visits to physicians), we know a lot about this difficult subject, at least in our own species. We have two basic kinds of nerve fibers that carry pain signals up the spinal cord toward the thalamus via the spinothalamic tract. But before reaching your brain, they take different paths, with the quick, sharp pain fibers proceeding along a more lateral route, while the slower, duller signals go along what is designated the paleospinothalamic route, so named based on evidence that it’s evolutionarily more primitive.

Our story doesn’t end in the thalamus. From here, impulses spread to at least two regions of the cerebral cortex, the insula and the anterior cingulate. Current thinking among neurologists and neurobiologists is that within the insula, we somehow distinguish straightforward pain from so-called homeostatic sensations such as nausea and itching, whereas the anterior cingulate cortex mediates the emotional sense of pain’s unpleasantness. (Personal note: Pain sensations are also perceived in the secondary somatosensory cortex, mapped by neuroscientist Clinton Woolsey, with whom I studied raccoon brains at the University of Wisconsin, Madison.)

In addition to nociceptive pain there also lurks neuropathic pain, caused by damage or irritation to nerve fibers themselves, as when we bump our “funny bone,” actually the ulnar nerve. Finally, there is so-called “nociplastic” pain, aka central sensitization, in which receptors in the brain become hypersensitized to chronic pain signals as a result of which the experience is often multifocal and difficult to isolate at a specific body region, and is typically more intense than can be attributed to an obvious physical cause. It is increasingly recognized, nonetheless, as “real,” manifested in some forms of chronic back and neck pain as well as fibromyalgia. More recently, it has been implicated in long Covid.

Pain, unlike love, may not be many-splendored, but it is multifaceted. In addition to tracing its complex pathways, scientists have identified a mind-numbing array of pain-mediating neurotransmitters in other animals, as in humans, although less is known about how pain works in “lower” species, where they seem likely to experience something similar, and perhaps even more intense than we do.

“Why should the law refuse its protection to any sensitive being?”

The complex brains of vertebrates, however, have much in common with the neural networks of invertebrates. For example, endorphins (the pain-sensing neuromodulators affected by opiates) are found not only in vertebrates but also in mollusks, crustaceans, insects, and even flatworms, which lack brains altogether. Moreover, just as opiates are known to reduce nociception in many animal species, opiate antagonists such as naloxone have been found to reverse this effect even among invertebrates, as it does in human beings. Roundworms avoid extremes of heat, just like mammals, and even single-celled organisms retreat from certain chemicals, depending on the acidity or alkaline content.

Cross-species continuity is the key take-home message from evolutionary biology. When it comes to the most basic and adaptive traits, we are all cut from the same underlying cloth. The mechanisms of pain were doubtless elaborated and diversified as organisms with more complex central nervous systems evolved. Given the adaptive value of pain, that sensation would not only be conserved over evolutionary time, but ancestral, among the earliest and most fundamental traits to have emerged. This makes it, well, pretty much insufferable to deny other animals the experience of pain that we know all too well.

The 19th-century philosopher Jeremy Bentham was the earliest and most influential Western thinker to propose a non-theological argument for animal rights. In his Introduction to the Principles of Morals and Legislation, Bentham wrote that animals “on account of their interests having been neglected by the insensibility of the ancient jurists, stand degraded into the class of things.” He went on to ask, “Why should the law refuse its protection to any sensitive being?” and concluded that “The time will come when humanity will extend its mantle over everything which breathes.”

That time has come, even if not extended widely enough to our animal cousins. From 1993 to 2012, the common octopus was protected in the United Kingdom under the “Animals (Scientific Procedures) Act,” legislation that was extended in 2012 to include all cephalopods, consistent with an EU directive that states “there is scientific evidence of their [cephalopods] ability to experience pain, suffering, and distress.” Vivisection, once standard procedure, is now unacceptable torture. Industrial-scale animal rearing is increasingly scrutinized and may someday be delegitimized. The late philosopher Bernard Rollin of Colorado State University was a leader in getting laws on the United States books that required vets and livestock workers to pay attention to pain among animals under their charge.

Some people extend concern about non-human pain to plants, which probably goes too far. But if my argument, limited for now to our animal relatives—all of them—causes you pain, just think how that wriggling worm probably feels.

David P. Barash is professor of psychology emeritus at the University of Washington. His most recent book is Threats: Intimidation and its Discontents.

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