Some authors, including prominently Michael Tye, have argued that cartilaginous fish (sharks and rays) may not feel pain because we have not found nociceptors in them and because of anecdotes where sharks appear to disregard injuries. I’ve heard a few EA’s repeating this claim and his arguments and actually think the counterarguments to his position are fairly decisive. Cartilaginous fish are not the most heavily fished kind of fish, so this post is not optimal from that perspective, but some of the arguments I gave here are also reasons for thinking various other animals may not feel pain, so I think this post was worth writing.
These authors claim not only that sharks do not experience conscious pain in a relevantly similar way to humans, but that they do not even show a sort of ‘proto pain.’[i] By which I mean that they claim that they do not even avoid potential bodily damaging stimuli in the standard way. I think that this former claim is a fair bit more plausible than the latter claim.
What does it mean for an animal not to feel pain or proto-pain? I think the human case is instructive. We know that humans who have congenital insensitivity to pain tend to have tragically short life expectancies and frequently have their bodies mutilated because their inability to recognize body damage as it’s occurring and be appropriately motivated to prevent it. These people have the advantage over other animals of being able to cognitively recognize that body damage is something they want to avoid independently of the associated pain and use their intelligence to make that happen. They also have the advantage the modern medical system as well as friends and family who can help them. Other animals that did not experience pain would probably fare far worse.
Especially since cartilaginous fish are long-lived (some may live even longer than humans), we would expect them to benefit. It’s difficult to see how they wouldn’t quickly kill themselves without a motivation to back down in fights, avoid obstacles, or avoid eating poisonous animals. If cartilaginous fish truly don’t have anything that even functions like nociceptors that they can use for detecting damaging stimuli then they could not even respond with appropriate reflexes in response to damaging stimuli.
Being in an aquatic environment might be helpful because an animals buoyancy in water reduces damage associated with falls or collisions and large bodies of water may reduce extremes of temperature and noxious chemicals will us compared to a terrestrial lifestyle. It’s also true that humans who are insensitive to pain are at risk of mutilating their own body with their hands, whereas I’m not sure if sharks are flexible enough to be able to bite themselves.
Merely disregarding injuries is probably not sufficient reason to believe that a certain kind of animal doesn’t feel pain. Many mammals from the weasel family, such as honey badgers and wolverines, are very tough and often disregard injuries, but I suspect they feel pain and it would be a major point of evolutionary discontinuity if they didn’t at least feel proto-pain.[ii] Maybe they feel less pain in certain circumstances or their motivational system is different in some other way to account for this toughness.
It’s true that it would sometimes be beneficial for animals not to feel pain or proto-pain, and animals that benefit from ‘tough’ lifestyles. Pain can distract animals sometimes cause animals to be too cautious. However, because of the large benefits from pain, I expect that evolution would handle these situations by causing the animals to feel less pain in certain circumstances, rather than not feeling it at all.
It probably makes sense for various kinds of animals to not feel pain (or not find that pain experience aversive) in certain circumstances. Humans are sometimes motivated to exercise to exhaustion or get into fights. This may be handled through us feeling reduced pain from endorphins or from us feeling a lot of counterbalancing excitement. Some animals have an even greater need for this. For example, male praying mantises are often eaten by the female praying mantises after mating, and this problem may be solved in a similar way.[iii]
If we imagine only adult individuals of a large species of shark, it is more easy to imagine them getting by without the experience of pain or proto-pain. However, many species of cartilaginous fish are small and some are filter feeders, but these fish it seems like there would be little active benefit not feeling pain and plenty of downside.
We know that many invertebrate species are capable of reinforcement learning on the basis of noxious stimuli, will flee from noxious stimuli, and show various other features associated with the experience of pain. So we know that whatever is required to show at least this form of proto-pain that many invertebrate show it is not that complex.
Cartilaginous fish tend to have much larger brains than other fish (of similar size to birds and marsupial mammals). Manta rays are a kind of cartilaginous fish that have huge brains (in both absolute and relative terms) and may have passed the mirror test. For this reason it would be strange if cartilaginous fish did not show at least that much evidence of proto-pain.
One possible position someone could take is that cartilaginous fish that are smaller or don’t live such rough lifestyles do feel pain or proto-pain, but larger sharks don’t. However, we have not found nociceptors in those animals either, so you would have to commit to the claim that absence of nociceptors is not a sufficient reason for believing that animals don’t feel pain and believe that all of the kinds of cartilaginous fish that are small or don’t leave rough lifestyles independently evolved pain or proto-pain.
Another possible position is that cartilaginous fish do not feel pain or proto-pain, but that they do go through fear (or proto-fear). I think this is a more plausible claim than the claim that they merely don’t feel pain or proto-pain because it gives them some mechanism for avoiding damaging stimuli. However, I still think this claim is implausible.
It seems like the usual way we come to fear certain stimuli is because they become associated with pain. If sharks didn’t feel pain or proto-pain it’s not clear how they would come to reliably fear stimuli that evolution would want them to avoid. It also just doesn’t seem like fear or proto-fear by itself would do enough work and helping at the animals avoid damaging stimuli. It seems like it might only help them avoid predators and not really any other damaging stimuli. If cartilaginous fish do experience conscious fear, that would make them make them moral patients even if they did not feel physical pain.
If an animal has too much exposure to damaging stimuli it will die or be crippled. Animals that can’t detect damaging stimuli and respond accordingly are not paying attention to what is evolutionarily most important. When you consider that cartilaginous fish are complex enough that you would expect that they could easily accomplish this, it is implausible that evolution would not give them this ability.
I suspect nociceptors may be present in these animals but we just haven’t found them yet or that they may simply use a different mechanism for detecting bodily damage and feeling pain.[iv] It is in fact not yet certain that cartilaginous fish do not in fact have nociceptors. More study is needed.
[i] There are other questions about how plausible it is that these animals experience proto-pain but not conscious pain, but those are the questions I'm concerned with in this post. For the record I think that as an animals response to noxious stimuli animal looks increasingly similar to the response of a human paren or other animal we believed to be conscious), the likelihood that the animal is missing whatever feature that would makes it proto-pain in their case but conscious pain in our case, gets increasingly small.
[ii] "Likewise, for the fabled African ratel, or honey badger. Ratels, relatives of wolverines, are medium-sized black-and-white, tough-skinned intrepid animals that routinely feed on all sorts of prey, including poisonous snakes (reputed to be unable to bite through the tough skin), chase lions and other carnivores from their prey to claim the kill, and are best known for their love of honey and bee brood. Ratels, like bears, learned that a certain number of stings cause no meaningful damage, and thereby they have learned to overcome the pain. This is a tricky game for ratels. Bee stings are truthful as well as painful. Enough stings, about 4 for a mouse, or an estimated 140 stings for a ratel, can kill. Until around a hundred stings, the ratel is safe. No one knows how well ratels can count in the ratel–bee brinkmanship game, but they likely can sense when a dangerous level of envenomation is near. The game can be tricky, however, for some ratels have misjudged and paid the ultimate price of being stung to death." The Sting of the Wild
[iv] Consider this passage from Eisemann et al. 1984: "The apparent absence from insects of any known or likely candidate nociceptors is of doubtful significance. Whilst it could be argued that the evolution of specific nociceptors accompanies a developing capacity to experience pain states, alternative mechanisms could possibly subserve this function. For example, nociceptive information could be decoded from abnormally high-frequency discharges from non-nociceptive mechano-, chemo-, and thermo-receptors, such as has been proposed for nociception in some mammalian viscera[...]. "