Sharks probably do feel pain: a reply to Michael Tye and others

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[...]. ”