Features Relevant to Invertebrate Sentience, Part 1

Ex­ec­u­tive Summary

In this, the first of three posts on fea­tures po­ten­tially rele­vant to in­ver­te­brate sen­tience, we as­sess 10 anatom­i­cal and evolu­tion­ary fea­tures and 5 types of nox­ious stim­uli re­ac­tions. Here are some high-level take­aways:

  1. Neu­ron count and brain size are of­ten over-em­pha­sized in su­perfi­cial dis­cus­sions of sen­tience.

  2. No­ci­cep­tors (spe­cial­ized periph­eral sen­sory cells used by the body to de­tect po­ten­tially harm­ful stim­uli) are found in a di­verse range of an­i­mals in­clud­ing fruit flies, sea hares, and ne­ma­todes. The pos­ses­sion of no­ci­cep­tors may be a nec­es­sary con­di­tion for painful ex­pe­rience, but it is not a suffi­cient con­di­tion.

  3. Cen­tral­ized in­for­ma­tion pro­cess­ing of some kind is prob­a­bly a nec­es­sary con­di­tion for con­scious­ness.

  4. Phys­iolog­i­cal re­sponses to nox­ious events don’t tell us much about valenced ex­pe­rience.

  5. Sim­ple re­ac­tions to nox­ious events, such as im­me­di­ate with­drawal, also don’t tell us much about valenced ex­pe­rience.

  6. More com­plex re­ac­tions to nox­ious events, such as long-term pro­tec­tive be­hav­ior, might tell us some­thing about valenced ex­pe­rience.

In­tro­duc­tion and Pro­ject Overview

This post is the third in Re­think Pri­ori­ties’ se­ries on in­ver­te­brate[1] welfare. In the first post we ex­am­ine some philo­soph­i­cal difficul­ties in­her­ent in the de­tec­tion of morally sig­nifi­cant pain and plea­sure in non­hu­mans. In the sec­ond post we dis­cuss our sur­vey and com­pila­tion of the ex­tant sci­en­tific liter­a­ture rele­vant to in­ver­te­brate sen­tience,[2] as well as the strengths and weak­nesses of our ap­proach to the sub­ject. In this post we ex­plain some anatom­i­cal, evolu­tion­ary, and be­hav­ioral fea­tures po­ten­tially in­dica­tive of the ca­pac­ity for con­scious ex­pe­rience in in­ver­te­brates. In the fourth post we ex­plain some drug re­sponses, mo­ti­va­tional trade­offs, and feats of cog­ni­tive so­phis­ti­ca­tion po­ten­tially in­dica­tive of the ca­pac­ity for con­scious ex­pe­rience in in­ver­te­brates. In the fifth post we ex­plain some learn­ing in­di­ca­tors, nav­i­ga­tional skills, and mood state be­hav­iors po­ten­tially in­dica­tive of the ca­pac­ity for con­scious ex­pe­rience in in­ver­te­brates. In the sixth, sev­enth, and eighth posts, we pre­sent our sum­mary of find­ings, both in nar­ra­tive form and as an in­ter­ac­tive database. In forth­com­ing work, to be pub­lished in late July, we an­a­lyze the ex­tent to which in­ver­te­brate welfare is a promis­ing cause area.

Anatom­i­cal and Evolu­tion­ary Features

Neu­ron count

Neu­rons are spe­cial­ized, elec­tri­cally ex­citable nerve cells that con­sti­tute the most im­por­tant part of the brain and ner­vous sys­tem.[3] Neu­rons com­mu­ni­cate with each other via synap­tic con­nec­tions to form densely in­ter­con­nected webs. The adult hu­man brain con­tains roughly 86 billion neu­rons, and each neu­ron av­er­ages around 7,000 synap­tic con­nec­tions.[4]

Neu­rons alone do not au­to­mat­i­cally gen­er­ate con­scious ex­pe­rience, and neu­rons are not them­selves in­trin­si­cally morally valuable. We can imag­ine a lump of billions of neu­rons swirling around a lab­o­ra­tory jar with ab­solutely no con­scious aware­ness. Con­versely, we can imag­ine an alien or a com­puter pro­gram with zero neu­rons which nonethe­less has the ca­pac­ity for con­scious ex­pe­rience. A high neu­ron count is thus nei­ther a nec­es­sary nor suffi­cient con­di­tion on con­scious ex­pe­rience.

Nev­er­the­less, neu­ron count might be able to provide some mod­est ev­i­dence for con­scious ex­pe­rience. Neu­ron count seems to be at least roughly cor­re­lated with cog­ni­tive abil­ity, and cog­ni­tive so­phis­ti­ca­tion of some de­gree is prob­a­bly a nec­es­sary con­di­tion on con­scious ex­pe­rience. Still, even grant­ing the im­por­tance of cog­ni­tive abil­ity to con­scious­ness, there are im­por­tant ex­cep­tions to the gen­eral rule that more neu­rons means more cog­ni­tive abil­ity. Larger an­i­mals need more neu­rons just to co­or­di­nate move­ment and au­to­nomic func­tions. Larger an­i­mals also re­quire more neu­rons to in­ner­vate their larger mus­cles,[5] and larger an­i­mals tend to pro­cess larger sen­sory fields to in­ter­act with their larger world, which re­quires a greater num­ber of neu­rons just to pro­cess the data at the same level of com­plex­ity as a smaller an­i­mal would. Th­ese ad­di­tional neu­rons do not nec­es­sar­ily al­low for a greater num­ber of dis­tinct be­hav­iors. At least by some meth­ods of count­ing, honey bees show more dis­tinct be­hav­iors then do some mam­mal species and perform bet­ter at color learn­ing than any stud­ied ver­te­brate species.[6]

Hu­mans are the most cog­ni­tively so­phis­ti­cated an­i­mal on the planet, but the Afri­can bush elephant has al­most three times as many neu­rons. The differ­ence be­tween hu­mans and elephants is that hu­mans have a much higher con­cen­tra­tion of neu­rons in the cere­bral cor­tex. Thus neu­ron count in spe­cific brain re­gions might be a more re­li­able guide to cog­ni­tive perfor­mance. (It is even some­times claimed that cor­ti­cal neu­ron num­ber matches in­tu­itive per­cep­tions of moral value across an­i­mals.[7]) Un­for­tu­nately, as­cer­tain­ing over­all neu­ron counts for in­ver­te­brate species is hard enough; de­ter­min­ing neu­ron count for spe­cific brain re­gions would be even more difficult.[8] In short, neu­ron count alone does not tell us how the neu­rons are or­ga­nized, how the neu­rons are used, or how many synap­tic con­nec­tions each neu­ron pos­sesses. Thus neu­ron count alone should not be con­sid­ered a par­tic­u­larly im­por­tant fea­ture for de­ter­min­ing ca­pac­ity for con­scious ex­pe­rience.

Fi­nally, it should be noted that brain size is im­por­tantly dis­tinct from neu­ron count and that there is no close cor­re­la­tion be­tween the two. Neu­ron den­si­ties vary dra­mat­i­cally across species. Grey par­rots (Psit­ta­cus er­itha­cus)[9] have roughly the same num­ber of neu­rons as owl mon­keys (Ao­tus trivir­ga­tus)[10] de­spite brains only half as large, as mea­sured by mass.[11] Hu­mans have more neu­rons per kilo­gram than com­pa­rably sized an­i­mals, but far fewer neu­rons per kilo­gram than small fish and ants. Neu­ron sizes also differ sig­nifi­cantly among an­i­mals. In gen­eral, smaller an­i­mals have smaller neu­rons, but there are no­table ex­cep­tions. The neu­rons of the sea hare Aplysia cal­ifor­nica, for in­stance, are so large they are visi­ble to the naked eye.[12] For these rea­sons, brain size should not be used as a proxy for neu­ron count, and neu­ron count should not be con­strued as a mea­sure of brain size.

Brain size

In­tu­itively, there is some plau­si­bil­ity to the claim that brain size is morally im­por­tant. In biolog­i­cal or­ganisms, the ca­pac­ity for valenced ex­pe­rience al­most cer­tainly su­per­venes on var­i­ous brain struc­tures and func­tions. De­stroy the brain, and you thereby de­stroy the ca­pac­ity for valenced ex­pe­rience. Be­cause the ca­pac­ity for valenced ex­pe­rience, and con­scious­ness more gen­er­ally, is a com­plex phe­nomenon, it plau­si­bly re­quires a com­plex brain. And, so it may seem, big­ger brains are at least in gen­eral more com­plex.

Hu­mans are the smartest an­i­mals on earth, but they do not have the biggest brains. Hu­man brains av­er­age around 1.35 kilo­grams, whereas Afri­can elephants sport brains at least three times as mas­sive (4.2 kilo­grams) and the brains of some large whales can reach an ex­traor­di­nary 9 kilo­grams.[13] Of course, it should be ex­pected that larger an­i­mals have larger brains. A bet­ter mea­sure might be brain mass as a pro­por­tion of over­all body mass. Here, again, though, hu­mans do not come out on top, even among mam­mals. Hu­man brains con­sti­tute roughly 2% of over­all body mass. Shrew brains con­sti­tute nearly 10% of over­all body mass.[14]

Another brain size met­ric of­ten dis­cussed in the liter­a­ture is en­cephal­iza­tion quo­tient (EQ). In lay terms, en­cephal­iza­tion quo­tient is a mea­sure of the de­vi­a­tion in brain size of a given species from a ‘stan­dard’ species of the same taxon. En­cephal­iza­tion quo­tient thus at­tempts to cor­rect for the effect of body size on brain size. Hu­mans have an en­cephal­iza­tion quo­tient be­tween 7.4 and 7.8, in­di­cat­ing that their brains are roughly seven and a half times big­ger than would be pre­dicted by body mass alone.[15] This is larger than any other mam­mal. (The next high­est is the bot­tlenose dolphin at 5.3.) So en­cephal­iza­tion quo­tient cor­rectly pre­dicts which an­i­mal is most in­tel­li­gent. Un­for­tu­nately, en­cephal­iza­tion quo­tient fares less well as a pre­dic­tor of rel­a­tive in­tel­li­gence across non­hu­man species. For ex­am­ple, New World ca­puchin mon­keys have a higher en­cephal­iza­tion quo­tient than chim­panzees and go­rillas de­spite their less so­phis­ti­cated cog­ni­tive abil­ities.[16] More gen­er­ally, for pre­dic­tions of in­tel­li­gence across non­hu­man pri­mates, Deaner et al. 2007 finds that ab­solute brain size does bet­ter than other mea­sures that cor­rect for body size, in­clud­ing en­cephal­iza­tion quo­tient.[17]

Thus, neu­ro­scien­tific re­al­ity is more com­pli­cated than the in­tu­itive pic­ture painted at the out­set would sug­gest. Ab­solute brain size is prob­a­bly an in­ad­e­quate mea­sure of neu­ral com­plex­ity and a some­what poor pre­dic­tor of rel­a­tive in­tel­li­gence. Other brain size met­rics, such as brain-mass:body-mass ra­tio and en­cephal­iza­tion quo­tient, do slightly bet­ter, but the im­por­tance of these met­rics prob­a­bly pales in com­par­i­son to the im­por­tance of brain or­ga­ni­za­tion and ar­chi­tec­ture. Not all ar­eas of the brain are equally im­por­tant. Across species the size of cer­tain brain re­gions may be more in­for­ma­tive than over­all brain size. But even when com­par­ing the same com­pa­rably-sized brain re­gions across species, var­i­ous cy­toar­chi­tec­tural differ­ences, such as the ex­tent of cor­ti­cal fold­ing, in­terneu­ronal dis­tance, ax­onal con­duc­tion ve­loc­ity, de­gree of myeli­na­tion, and synap­tic trans­mis­sion speed, could plau­si­bly be more im­por­tant.


No­ci­cep­tion is the neu­ral pro­cess of en­cod­ing and pro­cess­ing nox­ious stim­uli. No­ci­cep­tion is ac­com­plished by no­ci­cep­tors, spe­cial­ized periph­eral sen­sory cells used by the body to de­tect po­ten­tially harm­ful stim­uli. No­ci­cep­tors are ac­ti­vated by ex­treme tem­per­a­tures, in­tense me­chan­i­cal pres­sure, and toxic chem­i­cals likely to cause phys­i­cal in­jury. Ac­ti­vated no­ci­cep­tors typ­i­cally trig­ger so-called “no­cif­en­sive be­hav­ior,” in­clud­ing re­flex­ive with­drawal, con­trac­tion, and con­di­tioned es­cape. The abil­ity to de­tect and re­spond to nox­ious stim­uli is a highly use­ful trait. Un­sur­pris­ingly, no­ci­cep­tion is widely con­served through­out the an­i­mal king­dom.[18] Even rel­a­tively sim­ple crea­tures, like the ne­ma­tode C. el­e­gans, pos­sess no­ci­cep­tors.[19]

In hu­mans, ac­ti­vated no­ci­cep­tors are also as­so­ci­ated with con­scious pain. For that rea­son no­ci­cep­tors are some­times mis­lead­ingly called “the sen­sors of the pain path­way.”[20] How­ever, con­scious pain must be care­fully dis­t­in­guished from mere no­ci­cep­tion.[21] No­ci­cep­tion op­er­ates be­low con­scious aware­ness and thus does not have an at­ten­dant phe­nomenol­ogy. No­ci­cep­tion is fast and re­flex­ive, but it is not nor­mally as­so­ci­ated with long-term mem­ory. Con­scious pain, on the other hand, with its at­ten­dant felt bad­ness, tends to leave a memo­rial im­print. Be­cause pain ex­pe­riences are of­ten stored in long-term mem­ory, pain tends to in­duce long-term be­hav­ioral and mo­ti­va­tional changes.[22] For long-lived an­i­mals in com­plex en­vi­ron­ments, pain is thus po­ten­tially more effec­tive at pro­tect­ing the an­i­mal from dam­age than mere no­ci­cep­tion.

For the pur­poses of this re­port, we have di­vided this fea­ture into two parts, a strict defi­ni­tion of no­ci­cep­tion and a loose defi­ni­tion of no­ci­cep­tion. An or­ganism satis­fies the strict defi­ni­tion of no­ci­cep­tion when and only when ac­tual no­ci­cep­tors have been iden­ti­fied in the an­i­mal. An or­ganism satis­fies the loose defi­ni­tion of no­ci­cep­tion when and only when it is ca­pa­ble of re­spond­ing to po­ten­tially dam­ag­ing stim­uli, even in the ab­sence of iden­ti­fi­able no­ci­cep­tors. The dis­tinc­tion is im­por­tant for two rea­sons. First, it’s pos­si­ble that an or­ganism could de­tect nox­ious stim­uli with cells which are dis­tinct from but ho­molo­gous to no­ci­cep­tors. Se­cond, iden­ti­fy­ing no­ci­cep­tors is a non­triv­ial sci­en­tific task. Scien­tists have yet to iden­tify no­ci­cep­tors even in some ex­tremely com­plex an­i­mals, like elas­mo­branch fish (i.e., car­tilag­i­nous fish, such as sharks) that ob­vi­ously have the ca­pac­ity to de­tect and re­spond to nox­ious stim­uli.[23]

Opi­oid-like receptors

Opi­oid re­cep­tors are a spe­cial­ized group of re­cep­tor sites that bond with molecules of opi­oid drugs and en­doge­nous opi­oid pep­tides.[24] In ver­te­brates there are four closely-re­lated va­ri­eties of opi­oid re­cep­tors, the mu (MOR), delta (DOR), kappa (KOR), and some­what more mys­te­ri­ous or­phanin re­cep­tor (ORL). MOR, DOR, KOR, and ORL evolved from a sin­gle opi­oid unire­cep­tor gene early in chor­date evolu­tion. This unire­cep­tor likely de­vel­oped from a proto-unire­cep­tor gene around the the time arthro­pods and chor­dates split.[25] In the liter­a­ture, func­tion­ally analo­gous but chem­i­cally differ­ent re­cep­tors in in­ver­te­brates are of­ten called “opi­oid-like re­cep­tors,” and we fol­low that ter­minol­ogy here.[26]

The pres­ence of opi­oid-like re­cep­tors is po­ten­tially im­por­tant be­cause opi­oids are anal­gesics (painkil­lers). Opi­oids are es­pe­cially sig­nifi­cant be­cause, un­like other anal­gesics, opi­oids can re­duce the af­fec­tive (valenced) com­po­nent of pain with­out a cor­re­spond­ing re­duc­tion in the sen­sory as­pect (e.g., the burn­ing/​throb­bing/​cut­ting/​sting­ing/​aching qual­ity) of pain.[27] The pres­ence of opi­oid-like re­cep­tors is thus mild ev­i­dence that an­i­mals which pos­sess them have the ca­pac­ity for valenced ex­pe­rience. Un­for­tu­nately, the ev­i­den­tial sta­tus of opi­oid-like re­cep­tors is com­pli­cated by the fact that opi­oid-like re­cep­tors play many biolog­i­cal roles be­sides pain mod­u­la­tion. Th­ese di­verse roles in­clude “reg­u­la­tion of mem­brane ionic home­osta­sis, cell pro­lifer­a­tion, emo­tional re­sponse, epilep­tic seizures, im­mune func­tion, feed­ing, obe­sity, res­pi­ra­tory and car­dio­vas­cu­lar con­trol.”[28] For this rea­son, the pres­ence of opi­oid-like re­cep­tors does not by it­self tell us much about an or­ganism’s ca­pac­ity for sub­jec­tive ex­pe­rience.

Cen­tral­ized in­for­ma­tion processing

This fea­ture refers to an en­tity’s abil­ity to in­te­grate dis­parate in­for­ma­tion from lo­cal sen­sory in­puts into a unified un­der­stand­ing[29] of the world. In ver­te­brates this cen­tral­ized pro­cess­ing oc­curs in the brain. En­tities with differ­ent neu­rolog­i­cal ar­chi­tec­tures may in­te­grate in­for­ma­tion differ­ently or not at all. Plants, for in­stance, re­act to the en­vi­ron­ment and reg­u­late their func­tion­ing through a net­work of cells, hor­mones and other growth reg­u­la­tors. Cru­cially, al­though the effects of these re­ac­tions may ra­di­ate out­ward to in­fluence the rest of the or­ganism, the re­ac­tions oc­cur and are ‘pro­cessed’ lo­cally. Hence, plants do not uti­lize cen­tral­ized in­for­ma­tion pro­cess­ing.

Ner­vous sys­tem cen­tral­iza­tion is not an all-or-noth­ing fea­ture. Most in­ver­te­brates fall some­where be­tween plants and ver­te­brates. Take the com­mon oc­to­pus. Oc­to­puses are no­to­ri­ously in­tel­li­gent, and much of their be­hav­ior seems to re­quire or­ga­nized plan­ning and fore­sight. Yet fully two-thirds of their neu­rons are lo­cated in their arms, and the arms can move, taste, and touch in­de­pen­dently.[30] Nonethe­less, oc­to­puses do pos­sess cen­tral­ized brain struc­tures, with some ex­perts in the field go­ing so far as to call them “analo­gous to the hu­man cere­bral cor­tex.”[31]

In hu­mans, cen­tral­ized pro­cess­ing is as­so­ci­ated with con­scious ex­pe­rience while lo­cal­ized pro­cess­ing is not. Spinal cord re­flexes are sim­ple be­hav­iors (such as a knee jerk­ing when tapped) me­di­ated by cen­tral ner­vous sys­tem path­ways that lie en­tirely within the spinal cord. Our spinal re­flexes op­er­ate be­fore the sen­sa­tions are con­sciously per­ceived, though the sen­sory in­for­ma­tion is also usu­ally pro­cessed cen­trally in us. Spinal re­flexes can still oc­cur (with­out con­scious per­cep­tion) in peo­ple who are par­a­lyzed from the neck down.[32]

Ad­di­tion­ally, valenced ex­pe­rience al­lows for the pos­si­bil­ity of a gen­er­al­ized util­ity func­tion in which differ­ent pains and plea­sures are weighed against each other in or­der to reach an over­all de­ci­sion.[33] Without cen­tral­ized in­for­ma­tion pro­cess­ing, it’s not clear how such trade­offs could be made.

Fi­nally, ac­cord­ing to the In­te­grated In­for­ma­tion The­ory of con­scious­ness, con­scious­ness just is suit­ably in­te­grated in­for­ma­tion.[34] When the effec­tive in­for­ma­tional con­tent of a sys­tem, math­e­mat­i­cally defined in light of the sys­tem’s causal pro­file, is greater than the sum of the in­for­ma­tional con­tent of its parts, the sys­tem is said to carry in­te­grated in­for­ma­tion. In­te­grated in­for­ma­tion of the rele­vant source is con­scious, whether that in­te­gra­tion oc­curs in a hu­man brain or an in­sect gan­glion. In­te­grated In­for­ma­tion The­ory is of­ten con­sid­ered the lead­ing sci­en­tific the­ory of con­scious­ness. If a view like In­te­grated In­for­ma­tion The­ory seems to be well-jus­tified, then cen­tral­ized in­for­ma­tion pro­cess­ing is an ex­tremely im­por­tant fea­ture to ex­am­ine when judg­ing the prob­a­bil­ity that a non­hu­man an­i­mal is con­scious.

Ver­te­brate mid­brain-like function

The mid­brain[35] is the up­per­most re­gion of the brain­stem, situ­ated, as its name im­plies, be­tween the fore­brain and the hind­brain vesi­cles. In ver­te­brates the mid­brain helps reg­u­late alert­ness, body tem­per­a­ture, and sleep/​wake cy­cles. It also con­tributes to mo­tor con­trol, hear­ing, and vi­sion. Com­pared to more re­cent neu­ral de­vel­op­ments, such as the cere­bral cor­tex, which is found only in mam­mals, the mid­brain is phy­lo­ge­net­i­cally old. It is con­served across vir­tu­ally all ver­te­brate brain plans.

The mid­brain has tra­di­tion­ally re­ceived less at­ten­tion than other re­gions of the brain from re­searchers in­ves­ti­gat­ing the neu­ral un­der­pin­nings of con­scious­ness. For in­stance, the neo­cor­tex, the most re­cently evolved and (in hu­mans) largest re­gion of the cere­bral cor­tex, is some­times held to be, if not quite the seat of con­scious­ness, than at least a nec­es­sary con­di­tion on valenced ex­pe­rience.[36] This is not an ad hoc view. Func­tional imag­ing stud­ies show that, in hu­mans, there is a cor­re­la­tion be­tween the phe­nom­e­nal in­ten­sity of pain and ac­tivity in the an­te­rior cin­gu­late cor­tex and the so­matosen­sory cor­tex.[37] Hence, it was thought, any crea­ture which lacks a neo­cor­tex thereby lacks the ca­pac­ity for valenced ex­pe­rience. No mat­ter how similar the be­hav­ior, the ab­sence of a neo­cor­tex in a crea­ture served as a defeater for the view that that crea­ture ex­pe­riences pain and plea­sure.

To­day this view is in­creas­ingly challenged in both ex­per­i­men­tal psy­chol­ogy and com­par­a­tive neu­rol­ogy. For starters, ev­i­dence is emerg­ing that, even in hu­mans, a neo­cor­tex is not re­quired for con­scious ex­pe­rience.[38] More im­por­tantly, the ab­sence of a neo­cor­tex doesn’t im­ply that there aren’t ho­molo­gous brain re­gions perform­ing the same role in other crea­tures.[39] Thus, some re­searchers have be­gun to look at other neu­ral re­gions, such as the mid­brain.

Ac­cord­ing to An­drew Bar­ron and Colin Klein, a biol­o­gist and philoso­pher re­spec­tively at Mac­quarie Univer­sity in Syd­ney, the evolu­tion­ary func­tion of con­scious­ness is to pro­duce an in­te­grated and ego­cen­tric spa­tial model to guide an an­i­mal as it nav­i­gates a com­plex en­vi­ron­ment. Bar­ron and Klein ar­gue that in ver­te­brates this func­tion is me­di­ated by the mid­brain. Or­ganisms that have a spe­cial­ized brain re­gion for the pro­cess­ing of spa­tial in­for­ma­tion and or­ga­ni­za­tion of move­ment are said to pos­sess mid­brain-like func­tion­ing. Ac­cord­ing to Bar­ron and Klein, or­ganisms that pos­sess mid­brain-like func­tion­ing, in­clud­ing in­sects, plau­si­bly pos­sess the same ca­pac­ity for con­scious ex­pe­rience as ver­te­brates.[40]

Last com­mon an­ces­tor with humans

Un­der­stand­ing the evolu­tion­ary dis­tance[41] be­tween hu­mans and our tar­get taxa may help us bet­ter gauge the like­li­hood that mem­bers of those taxa are con­scious for two rea­sons. First, evolu­tion­ary dis­tance serves as a rough proxy for over­all similar­ity. Crea­tures more closely re­lated to hu­mans are more likely to share ma­jor anatom­i­cal struc­tures and func­tions, in­clud­ing im­por­tant neu­rolog­i­cal fea­tures which could un­der­pin con­scious ex­pe­rience. Se­cond, if we could es­ti­mate when con­scious­ness first emerged, know­ing when the an­ces­tors of hu­mans and our tar­get taxa di­verged could be sig­nifi­cant. If con­scious­ness evolved rel­a­tively early in our evolu­tion­ary his­tory, more crea­tures are likely to be con­scious to­day. On the other hand, if con­scious­ness is a rather re­cent evolu­tion­ary de­vel­op­ment, we would ex­pect the dis­tri­bu­tion of con­scious­ness to be more con­fined.

In­ves­ti­gat­ing the evolu­tion­ary ori­gins of con­scious ex­pe­rience is no easy task. Nonethe­less, the ques­tion has been ad­dressed from a va­ri­ety of per­spec­tives in the sci­en­tific and philo­soph­i­cal liter­a­ture.[42] Some re­searchers be­lieve that con­scious­ness evolved re­cently be­cause con­scious ex­pe­rience re­quires a par­tic­u­lar style of ad­vanc­ing men­tal pro­cess­ing that is only likely to be found in cog­ni­tively so­phis­ti­cated so­cial mam­mals.[43] Other re­searchers date the evolu­tion of con­scious­ness as far back as the Cam­brian Ex­plo­sion, ar­gu­ing that the com­plex preda­tor-prey re­la­tion­ships that de­vel­oped dur­ing this pe­riod led to a kind of per­cep­tual and cog­ni­tive arms-race, cul­mi­nat­ing in con­scious ex­pe­rience.[44]

Of course, draw­ing con­clu­sions about the char­ac­ter­is­tics of an­i­mals al­ive to­day based on the char­ac­ter­is­tics of an an­ces­tor that lived hun­dreds of mil­lions of years ago is prob­a­bly un­jus­tified. Even if the an­ces­tor was con­scious, it’s pos­si­ble that in cer­tain lineages con­scious­ness was se­lected against. (So even if all mol­lusks de­scended from a con­scious an­ces­tor, it’s pos­si­ble that to­day’s cephalo­pod mol­lusks are con­scious while to­day’s bi­valve mol­lusks are not.) And even if the an­ces­tor wasn’t con­scious, it’s still pos­si­ble that con­scious­ness arose in­de­pen­dently in differ­ent, later lineages.

Aver­age lifespan

It seems that one of the most im­por­tant evolu­tion­ary func­tions of valenced ex­pe­rience is to fa­cil­i­tate learn­ing. Be­hav­iors which im­prove evolu­tion­ary fit­ness, such as mat­ing, tend to have a pos­i­tive valence, while be­hav­iors which re­duce evolu­tion­ary fit­ness, such as eat­ing spoiled meat, tend to have a nega­tive valence. When these valences are paired with the ap­pro­pri­ate be­hav­iors of­ten enough, an an­i­mal learns to pur­sue fit­ness-boost­ing be­hav­iors and avoid fit­ness-re­duc­ing be­hav­iors.

Short-lived crea­tures may be less likely to pos­sess the ca­pac­ity for valenced ex­pe­rience for two rea­sons. First, short-lived crea­tures may not have enough time to learn from plea­sure and pain. Se­cond, short-lived crea­tures are less likely to en­counter novel situ­a­tions, and thus pre-pro­grammed, in­nate be­hav­ior may be a bet­ter strat­egy for max­i­miz­ing fit­ness. Short-lived crea­tures need to start act­ing right away in life in or­der to sur­vive.[45]

Two fac­tors com­pli­cate the im­por­tance of this fea­ture. First, lifes­pan is a mea­sure of how long an or­ganism is al­ive. How­ever, life af­ter fer­til­ity ces­sa­tion (the generic term for what in hu­mans is called ‘menopause’) doesn’t con­tribute to fit­ness. From an evolu­tion­ary per­spec­tive, the value of learn­ing is that it helps an or­ganism pass on its genes. Time to sex­ual ma­tu­rity may thus seem like bet­ter ev­i­dence re­gard­ing the ca­pac­ity for valenced ex­pe­rience. Un­for­tu­nately, time to sex­ual ma­tu­rity doesn’t cap­ture the huge var­i­ance among an­i­mals in num­ber of re­pro­duc­tive cy­cles. Two crea­tures might both reach sex­ual ma­tu­rity in 30 days, but if one crea­ture dies af­ter a sin­gle re­pro­duc­tive cy­cle while the other goes on to suc­cess­fully re­pro­duce for months af­ter­ward, the former will have less need for valenced ex­pe­rience than the later. Thus, the ideal met­ric is re­ally some­thing like ‘time to fer­til­ity ces­sa­tion.’ For­tu­nately, av­er­age lifes­pan is an ex­cel­lent proxy for this met­ric. The vast ma­jor­ity of an­i­mals die shortly af­ter fer­til­ity ces­sa­tion.[46]

Se­cond, it is un­clear at just what point av­er­age lifes­pan be­comes ev­i­dence against the ca­pac­ity for valenced ex­pe­rience. Oc­to­puses don’t nor­mally live more than a year or two, a far shorter lifes­pan than com­pa­rably in­tel­li­gent ver­te­brates. Still, two years seems like plenty of time to en­counter un­ex­pected situ­a­tions, and it is con­sid­er­ably longer than the mere month that typ­i­cal fruit flies live. On the other hand, the prob­a­bil­ity of en­coun­ter­ing novel ex­pe­riences also de­pends on the type of en­vi­ron­ment in which an an­i­mal lives and the way in which the an­i­mal in­ter­acts with that en­vi­ron­ment.[47] An­i­mals in trop­i­cal re­gions, which are highly vari­able, prob­a­bly en­counter more novel stim­uli per unit of time than com­pa­rable an­i­mals in tem­per­ate zones.[48] Similarly, many mi­gra­tory species (like monarch but­terflies) must con­tend with a va­ri­ety of en­vi­ron­ments and hence prob­a­bly un­dergo a larger num­ber of novel ex­pe­riences than com­pa­rable non-mi­gra­tory species. Thus, al­though the gen­eral point that short-lived crea­tures have less of a need for valenced ex­pe­rience may be sound, the im­por­tance of lifes­pan length for a given species can­not be as­sessed with­out ap­pro­pri­ate back­ground knowl­edge and con­text. And with­out at least a rough grasp of the cut­off where, ce­teris paribus, valenced ex­pe­rience be­comes more of a bur­den than a benefit, in­for­ma­tion about lifes­pan may not be par­tic­u­larly use­ful.[49]


Motility is the abil­ity to move spon­ta­neously and ac­tively via the con­sump­tion of metabolic en­ergy.[50] For our pur­poses, strict motility should be dis­t­in­guished from pas­sive lo­co­mo­tion, in which an or­ganism uti­lizes the en­vi­ron­ment (e.g., wind or wa­ter cur­rents) for trans­porta­tion. Strict motility should also be dis­t­in­guished from adap­ta­tions such as pos­i­tive pho­totropism, in which the shoots of plants ‘move’ by grow­ing to­ward light.

For motile or­ganisms, the abil­ity to spa­tially model the world (in some loose sense) and then in­cor­po­rate one’s own move­ments into that model would seem to con­fer a fit­ness ad­van­tage. Rep­re­sent­ing the world and or­ga­niz­ing sen­sa­tions and self-move­ment into a spe­cial model are things that con­scious­ness ap­pears to fa­cil­i­tate in hu­mans. In­deed, ac­cord­ing to some re­searchers, the evolu­tion­ary func­tion of con­scious­ness is to cre­ate an in­te­grated and ego­cen­tric spa­tial model to guide an or­ganism as it moves through a com­plex en­vi­ron­ment.[51][52] Thus, there is some rea­son to think that motile or­ganisms are more likely to be con­scious than ses­sile (non-motile) or­ganisms.

Distinct sleep-wake states

Adap­tive in­ac­tivity is found through­out the biolog­i­cal world. In the plant king­dom, for in­stance, seeds will stay dor­mant un­til soil con­di­tions are op­ti­mal for sur­vival. Many uni­cel­lu­lar or­ganisms en­ter sea­sonal dor­mant states due to changes in tem­per­a­ture or mois­ture level. In re­sponse to pre­dictable, un­fa­vor­able en­vi­ron­men­tal con­di­tions, many in­sect species un­dergo a pe­riod of di­a­pause dur­ing some stage of de­vel­op­ment, which can last for weeks, even months. Many mam­mals, in­clud­ing some bats and many species of ro­dents, hi­ber­nate dur­ing the win­ter.[53]

Daily sleep cy­cles are one form of adap­tive in­ac­tivity. Sleep re­duces brain and body en­ergy con­sump­tion, it in­creases be­hav­ioral effi­ciency, and it is thought to play a role in sup­port­ing brain plas­tic­ity, learn­ing, and mem­ory.[54] Sleep de­pri­va­tion has a num­ber of ad­verse effects. In hu­mans it is as­so­ci­ated with re­duced cog­ni­tive abil­ities, and stud­ies sug­gest it im­pairs both the im­mune sys­tem and the body’s abil­ity to heal wounds.[55]

There is a great di­ver­sity of sleep pat­terns in the an­i­mal king­dom. Sleep length ranges from 2 to 20 hours a day. In gen­eral, car­nivores tend to sleep longer than om­nivores, who in turn sleep longer than her­bivores. There is also great di­ver­sity re­gard­ing depth of sleep. In gen­eral, an­i­mals more at risk of pre­da­tion awaken more eas­ily than those an­i­mals not at risk of pre­da­tion.[56]

Be­cause sleep is one type of adap­tive in­ac­tivity, and adap­tive in­ac­tivity comes in many forms and is ex­hibited to var­i­ous de­grees, there is no sharp cut­off be­tween be­hav­ior that qual­ifies as sleep and be­hav­ior that does not. Thus, ev­i­dence for sleep in in­ver­te­brates is open, to some ex­tent, to in­ter­pre­ta­tion. In ver­te­brates sleep is char­ac­ter­ized by cer­tain elec­tro­phys­iolog­i­cal, en­er­getic, and be­hav­ioral changes, such as “de­creases in mus­cle tone, heart rate, breath­ing, blood pres­sure, and metabolic rate.”[57] In­so­far as in­ver­te­brates dis­play similar changes, the more likely it is that they ex­pe­rience rec­og­niz­able sleep/​wake cy­cles. Some of the po­ten­tial points of similar­ity are be­hav­ioral sleep, detri­men­tal health effects from sleep de­pri­va­tion, re­lated genes, and similar neu­rolog­i­cal fea­tures.[58]

Some have ar­gued that sleep cy­cles are an in­di­ca­tion of con­scious­ness be­cause there is a time when the crea­ture is aware and a time when it is not.[59] Surely, how­ever, this move is too quick. Jen­nifer Mather takes a more mea­sured ap­proach. She writes, “Sleep is an­other area in which the link­age of brain or­ga­ni­za­tion to be­hav­ior is ob­vi­ous… How­ever, link­age of sleep to con­scious­ness must be more about the de­tails of sleep than the pos­ses­sion of sleep it­self. As is true for pain, the un­der­ly­ing phys­iol­ogy may be par­allel but the be­hav­ioral man­i­fes­ta­tions more com­plex and in­di­cat­ing a higher or­der of con­trol.”[60]

Nox­ious Stim­uli Reactions

Phys­iolog­i­cal re­sponses to no­ci­cep­tion or handling

In hu­mans, con­scious pain states are as­so­ci­ated with a num­ber of au­to­nomic phys­iolog­i­cal changes, such as ele­vated heart rate, pupil di­la­tion, in­creased blood pres­sure, ele­vated res­pi­ra­tory rate, and in­creased body tem­per­a­ture. If similar changes are de­tected in non­hu­mans af­ter ex­po­sure to nox­ious stim­uli, that is mod­est ev­i­dence that those crea­tures also ex­pe­rience con­scious pain states. Two stan­dard caveats ap­ply. First, in gen­eral, the more phy­lo­ge­net­i­cally dis­tant an or­ganism is from hu­mans, the less similar we should ex­pect its phys­iolog­i­cal re­sponses to con­scious pain to be.[61] (For ex­am­ple, among earth­worms, one com­mon re­ac­tion to nox­ious stim­uli is the se­cre­tion of co­pi­ous amounts of ni­troge­nous mu­cus.[62]) Se­cond, even in hu­mans it is not always clear whether au­to­nomic changes are a re­sponse to con­scious pain or merely to no­ci­cep­tion. It thus seems likely that or­ganisms which do not ex­pe­rience con­scious pain nev­er­the­less un­dergo phys­iolog­i­cal changes in re­sponse to nox­ious stim­uli. Hence, this fea­ture is best used in the nega­tive: if a biolog­i­cal or­ganism musters no phys­iolog­i­cal re­sponse to nox­ious stim­uli, that is strong ev­i­dence that the or­ganism does not feel con­scious pain.

Pro­tec­tive behavior

Pro­tec­tive be­hav­ior is a type of non-re­flex­ive re­ac­tion to in­jury in which an in­jured an­i­mal at­tempts to guard, groom, or oth­er­wise tend to the in­jured body part. Ex­am­ples in­clude limp­ing, wound rub­bing, wound lick­ing, and wound guard­ing. Th­ese re­ac­tions are typ­i­cally part of a long-term re­sponse to in­jury, mea­sured in hours and days rather than sec­onds and min­utes. In gen­eral, long-term re­sponses to pu­ta­tively painful ex­pe­riences are bet­ter ev­i­dence for con­scious ex­pe­rience than acute re­sponses. Pro­tec­tive be­hav­ior, in our sense, must be care­fully dis­t­in­guished from re­flex­ive re­ac­tions known (in hu­mans) to op­er­ate sub­con­sciously, such as gri­mac­ing, rapid with­drawal, pos­tu­ral ad­just­ments, and some par­al­in­guis­tic fea­tures of vo­cal­iza­tion.[63]

In hu­mans pro­tec­tive be­hav­ior is of­ten me­di­ated and con­trol­led by the con­scious sen­sa­tion of pain. For ex­am­ple, a hu­man will typ­i­cally avoid us­ing an in­jured limb for cer­tain ac­tivi­ties or types of move­ment that would ag­gra­vate the limb, but not other ac­tivi­ties or types of move­ment that would not do so. Hu­mans also mod­ify the mo­tion in­volved in those ac­tivi­ties some­what in or­der to en­gage in them with­out ag­gra­vat­ing the limb. This is con­trol­led in a nu­anced way through the sen­sa­tion of pain. In the ab­sence of defeaters, similar be­hav­ior among non­hu­man an­i­mals is ev­i­dence they too feel con­scious pain.

Ev­i­dence for pro­tec­tive be­hav­ior among ver­te­brates is fairly ex­ten­sive, and for some time it was thought that this type of be­hav­ior is re­stricted to ver­te­brates. For in­stance, Eise­mann et al. (1984) write, “No ex­am­ple is known to us of an in­sect show­ing pro­tec­tive be­hav­ior to­wards in­jured body parts, such as by limp­ing af­ter leg in­jury or de­clin­ing to feed or mate be­cause of gen­eral ab­dom­i­nal in­juries. On the con­trary, our ex­pe­rience has been that in­sects will con­tinue with nor­mal ac­tivi­ties even af­ter se­vere in­jury or re­moval of body parts” (166).[64] The Eise­mann re­view is still some­times cited as ev­i­dence that in­sects (or in­ver­te­brates more gen­er­ally) don’t ex­hibit pro­tec­tive be­hav­ior, for ex­am­ple in Tye 2016.[65] How­ever, new ev­i­dence is be­gin­ning to un­der­mine the old con­sen­sus. Co­or­di­nated groom­ing be­hav­ior in re­sponse to nox­ious stim­uli has re­cently been re­ported in hon­ey­bees,[66] cock­roaches,[67] and fruit flies.[68] It is as yet un­clear whether these re­ac­tions con­sti­tute gen­uine pro­tec­tive be­hav­ior.[69]

Defen­sive be­hav­ior/​fight­ing back

In the wild many an­i­mals face near-con­stant risk of at­tack from preda­tors and/​or con­speci­fics. An­i­mals have evolved a di­verse and com­plex reper­toire of pro­cesses to re­spond to im­me­di­ate and po­ten­tial threats of this kind. Sea hares re­lease a nox­ious mix­ture of ink and opal­ine[70] when at­tacked.[71] Crabs will self-am­pu­tate their claws if firmly grasped by preda­tors.[72] Jel­lyfish de­ploy cnido­cytes, ex­plo­sive cells con­tain­ing a par­a­lyz­ing toxin, to pro­tect them­selves from pre­da­tion. Col­lec­tively, these re­sponses are known as defen­sive be­hav­ior.

Defen­sive be­hav­ior is mod­est ev­i­dence of nega­tively valenced emo­tional states like fear and anger. Felt fear may be an adap­tive tool to teach an­i­mals to avoid situ­a­tions that are dan­ger­ous, and felt anger may be an adap­tive state when con­fronta­tion is in­evitable.[73] On the other hand, defen­sive be­hav­ior is “phy­lo­ge­net­i­cally old and ex­hibited by or­ganisms through­out the an­i­mal king­dom” and thus may be me­di­ated by mechanisms much sim­pler than con­scious emo­tional states.[74]

Defen­sive be­hav­iors should be dis­t­in­guished from more pas­sive adap­ta­tions, such as spines, ar­mor, cam­ou­flage, mimicry, and pre­emp­tive avoidance (e.g., noc­tur­nal­ity), that help an an­i­mal re­duce the chance of an at­tack. Th­ese defen­sive struc­tures are ac­tive all or most of an an­i­mal’s life, and thus do not provide spe­cific ev­i­dence for oc­cur­rent nega­tively valenced emo­tional states.

When a hu­man is in pain, she of­ten cries out. Many ver­te­brates do the same when ex­posed to pu­ta­tively painful stim­uli. Hence, groan­ing, whin­ing, whim­per­ing, yelping, scream­ing and other such vo­cal­iza­tions[75] might plau­si­bly be con­sid­ered mod­est ev­i­dence of con­scious pain. In fact, vo­cal­iza­tion has re­cently been taken to be a good met­ric of an­i­mal welfare in farmed pigs, cows, and chick­ens.[76]

We ought to view this fea­ture with cau­tion, how­ever, for two rea­sons. First, ow­ing to anatom­i­cal differ­ences, we should not ex­pect in­ver­te­brates to vo­cal­ize in the same man­ner as ver­te­brates, if at all. More im­por­tantly, al­though vo­cal­iza­tion is as­so­ci­ated with con­scious pain in hu­mans, it’s not clear that the vo­cal­iza­tion is always caused by the con­scious sen­sa­tion of pain. At least some re­searchers re­gard pain-re­lated vo­cal­iza­tion in hu­mans as re­flex­ive and au­to­matic, akin to one’s with­draw­ing her hand from a hot stove be­fore she even re­al­izes she is touch­ing it.[77] Thus, nox­ious stim­uli re­lated vo­cal­iza­tions may be me­di­ated by mechanisms much sim­pler than con­scious ex­pe­rience.

Move­ment away from nox­ious stimuli

The abil­ity to with­draw from po­ten­tially dam­ag­ing stim­uli is a ba­sic evolu­tion­ary fea­ture and is ex­tremely well-con­served among motile or­ganisms. This abil­ity was prob­a­bly one of the pri­mary early ad­van­tages of motile or­ganisms over ses­sile (non-motile) or­ganisms.[78] Even many uni­cel­lu­lar or­ganisms are ca­pa­ble of mov­ing un­der their own power to­ward at­trac­tants such as en­ergy sources and away from re­pel­lents such as tox­ins. (This abil­ity of­ten takes the form of chemo­taxis, move­ment un­der the in­fluence of a chem­i­cal gra­di­ent.[79]) In hu­mans, the with­drawal re­flex can oc­cur un­con­sciously and even in co­matose pa­tients. Be­cause this fea­ture is so eas­ily satis­fied, it prob­a­bly re­veals lit­tle about an or­ganism’s ca­pac­ity for valenced ex­pe­rience.


This es­say is a pro­ject of Re­think Pri­ori­ties. It was writ­ten by Ja­son Schukraft with con­tri­bu­tions from Max Carpen­dale. Thanks to Kim Cud­ding­ton, Mar­cus A. Davis, Peter Hur­ford, Te­gan McCaslin, Daniela Wald­horn, and Rachael Woodard for helpful feed­back. If you like our work, please con­sider sub­scribing to our newslet­ter. You can see all our work to date here.


  1. Ver­te­brates con­sti­tute a sub­phy­lum in the phy­lum Chor­data. Cladis­ti­cally, it would be more pre­cise to speak of ‘chor­dates’ and ‘non-chor­dates.’ In us­ing the terms ‘ver­te­brates’ and ‘in­ver­te­brates’ we defer to com­mon us­age. How­ever, the num­ber of in­ver­te­brates in the phy­lum Chor­data is triv­ial com­pared to the num­ber of in­ver­te­brates out­side Chor­data, so com­mon us­age is not wholly in­ac­cu­rate. ↩︎

  2. We use the terms ‘sen­tience,’ ‘phe­nom­e­nal con­scious­ness,’ and ‘sub­jec­tive ex­pe­rience’ in­ter­change­ably. An or­ganism is sen­tient just in case there is some­thing it is like to be that or­ganism. ‘Valenced ex­pe­rience’ de­notes a proper sub­set of con­scious ex­pe­rience in which ex­pe­riences take on a pos­i­tive or nega­tive af­fect. All crea­tures with the ca­pac­ity for valenced ex­pe­rience are nec­es­sar­ily sen­tient, but not all sen­tient crea­tures nec­es­sar­ily have the ca­pac­ity for valenced ex­pe­rience. Note: ‘sen­tience’ gets used in differ­ent ways by differ­ent philo­soph­i­cal com­mu­ni­ties. In philos­o­phy of mind, the term is nor­mally used in its broad sense, to mean ‘phe­nom­e­nal con­scious­ness.’ (See, in­ter alia, this SEP ar­ti­cle on an­i­mal con­scious­ness.) In moral philos­o­phy, the term is nor­mally used in its nar­row sense, to mean ‘valenced ex­pe­rience.’ (See, in­ter alia, this SEP ar­ti­cle on the grounds of moral sta­tus.) We have adopted the philos­o­phy of mind us­age. ↩︎

  3. It is im­por­tant to re­mem­ber that, even in hu­mans, neu­rons are not re­stricted to the brain. The hu­man en­teric ner­vous sys­tem stretches across the gas­troin­testi­nal tract and is ca­pa­ble of me­di­at­ing re­flex be­hav­ior in­de­pen­dent of the brain or spinal cord. It con­tains about 100 mil­lion neu­rons. (It’s es­pe­cially im­por­tant to re­mem­ber that brain neu­rons are a proper sub­set of over­all neu­rons when con­sid­er­ing in­ver­te­brates, which tend to have less cen­tral­ized ner­vous sys­tems. Two-thirds of an oc­to­pus’s neu­rons, for in­stance, are lo­cated in its ten­ta­cles.) ↩︎

  4. David A. Drach­man. 2005. “Do We Have Brain to Spare?Neu­rol­ogy 64 (12). ↩︎

  5. This does not nec­es­sar­ily give them greater pre­ci­sion in move­ment; in­sects on av­er­age have similar num­bers of dis­tinct mus­cles in to­tal. ↩︎

  6. Lars Chit­tka and Jeremy Niven. 2009. “Are Big­ger Brains Bet­ter?Cur­rent Biol­ogy 19: R995–1008. ↩︎

  7. Those claims are some­times par­tially re­tracted. And some­times effec­tive al­tru­ist or­ga­ni­za­tions re­solve some of the dis­crep­an­cies be­tween the origi­nal post and the data that led to the re­trac­tion. ↩︎

  8. This difficulty is com­pounded by the fact that in­ver­te­brate brains are struc­tured differ­ently than ver­te­brate brains. It’s not always clear which re­gions are ho­molo­gous to which. ↩︎

  9. Sew­eryn Olkow­icz, Martin Ko­courek, Radek K. Lučan, Michal Porteš, W. Te­cum­seh Fitch, Suzana Her­cu­lano-Houzel, and Pavel Němec. 2016. “Birds have pri­mate-like num­bers of neu­rons in the fore­brain.” Pro­ceed­ings of the Na­tional Academy of Sciences 113, no. 26: 7255-7260. ↩︎

  10. Suzana Her­cu­lano-Houzel, Chris­tine E. Col­lins, Peiyan Wong, and Jon H. Kaas. 2007. “Cel­lu­lar scal­ing rules for pri­mate brains.” Pro­ceed­ings of the Na­tional Academy of Sciences 104, no. 9: 3562-3567. ↩︎

  11. I learned this fact (and many oth­ers) from Te­gan McCaslin’s ex­cel­lent (2019) “In­ves­ti­ga­tion into the re­la­tion­ship be­tween neu­ron count and in­tel­li­gence across differ­ing cor­ti­cal ar­chi­tec­tures” for AI Im­pacts. ↩︎

  12. They mea­sure roughly 1 mm in di­ame­ter. Leonid L. Moroz. 2011. “Aplysia.” Cur­rent Biol­ogy 21: PR60-R61. ↩︎

  13. See Table 1 in Ger­hard Roth and Ur­sula Dicke. 2005. “Evolu­tion of the Brain and In­tel­li­gence.” Trends in Cog­ni­tive Science 9: P250-257. ↩︎

  14. See Figure 2 in Roth and Dicke 2005. ↩︎

  15. En­cephal­iza­tion quo­tient de­pends on which species is taken as the “stan­dard” for the taxon. The 7.4-7.8 figure uses cats as the stan­dard (EQ=1) an­i­mal for mam­mals. ↩︎

  16. Roth and Dcike 2005: 252. ↩︎

  17. Robert O. Deaner, Karin Isler, Ju­dith Burkart, and Carel Van Schaik. 2007. “Over­all brain size, and not en­cephal­iza­tion quo­tient, best pre­dicts cog­ni­tive abil­ity across non-hu­man pri­mates.” Brain, Be­hav­ior and Evolu­tion 70, no. 2: 115-124. ↩︎

  18. Ewan Smith and Gary Lewin. 2009. “No­ci­cep­tors: A Phy­lo­ge­netic View.” Jour­nal of Com­par­a­tive Phys­iol­ogy A 195: 1096. ↩︎

  19. Lynne U. Sned­don. 2017. “Com­par­a­tive Phys­iol­ogy of No­ci­cep­tion and Pain.” Phys­iol­ogy 33: 63-73. ↩︎

  20. Adrienne E. Du­bin and Ar­dem Pat­apoutian. 2010. “No­ci­cep­tors: the Sen­sors of the Pain Path­way.” The Jour­nal of Clini­cal In­ves­ti­ga­tion 120: 3760-3772. ↩︎

  21. It is clear that no­ci­cep­tion is not a suffi­cient con­di­tion for con­scious suffer­ing. Nei­ther is it a nec­es­sary con­di­tion. Hu­mans are able to suffer emo­tion­ally from stim­uli that are not de­tected by no­ci­cep­tors. ↩︎

  22. Reto Bisaz, Ales­sio Travaglia, and Cristina M. Alber­ini. 2014. “The neu­ro­biolog­i­cal bases of mem­ory for­ma­tion: from phys­iolog­i­cal con­di­tions to psy­chopathol­ogy.” Psy­chopathol­ogy 47, no. 6: 347-356. ↩︎

  23. Sned­don 2017: 67 ↩︎

  24. The term opi­ate refers to drugs de­rived from opium poppy. Opi­oid is broader, en­com­pass­ing both nat­u­rally-oc­cur­ring and syn­thetic sub­stances. ↩︎

  25. Craig W. Stevens. 2009. “The evolu­tion of ver­te­brate opi­oid re­cep­tors.” Fron­tiers in bio­science: a jour­nal and vir­tual library 14 (2009): 1247-1269. ↩︎

  26. With the un­der­stand­ing that opi­oid re­cep­tors in ver­te­brates triv­ially qual­ify as opi­oid-like. ↩︎

  27. See Adam Shriver. 2006. “Mind­ing Mam­mals.” Philo­soph­i­cal Psy­chol­ogy 19: 433-442 (es­pe­cially §2) for an ac­cessible overview. ↩︎

  28. Yuan Feng, Xiaozhou He, Yilin Yang, Dong­man Chao, Lawrence H. Lazarus, and Ying Xia. 2012. “Cur­rent Re­search on Opi­oid Re­cep­tor Func­tion.” Cur­rent Drug Tar­gets 13 (2). ↩︎

  29. Here ‘un­der­stand­ing’ is not meant to be con­strued in a phe­nom­e­nal, con­scious sense. Non-con­scious robots can un­der­stand their sur­round­ings in this sense. ↩︎

  30. Peter God­frey-Smith. 2016. “The Mind of an Oc­to­pus.” Scien­tific Amer­i­can Mind, 28: 62–69 ↩︎

  31. Grazi­ano Fiorito et al. 2015. “Guidelines for the Care and Welfare of Cephalopods in Re­search–A con­sen­sus based on an ini­ti­a­tive by CephRes, FELASA and the Boyd Group.” Lab­o­ra­tory An­i­mals 49(S2): 1-90. ↩︎

  32. Todd E. Fein­berg and Jon M. Mal­latt. 2017. The An­cient Ori­gins of Con­scious­ness. MIT Press: 25. ↩︎

  33. Mir­jam Ap­pel and Robert W. El­wood. 2009. “Mo­ti­va­tional Trade-Offs and Po­ten­tial Pain Ex­pe­rience in Her­mit Crabs.” Ap­plied An­i­mal Be­havi­our Science 119: 120–24. ↩︎

  34. Masafumi Oizumi, Larissa Alban­takis, Giulio Tononi. 2014. “From the Phenomenol­ogy to the Mechanisms of Con­scious­ness: In­te­grated In­for­ma­tion The­ory 3.0.” PLOS Com­pu­ta­tional Biol­ogy 10(5): e1003588. ↩︎

  35. More for­mally known as the mes­en­cephalon ↩︎

  36. See, in­ter alia, §4 “The Ca­pac­ity for Con­scious­ness Depends on Func­tions of the Neo­cor­tex, a Brain Struc­ture Unique to Mam­mals” in James D. Rose. 2002. “The Neu­robe­hav­ioral Na­ture of Fishes and the Ques­tion of Aware­ness and Pain.” Re­views in Fish­eries Science 10 (2). ↩︎

  37. Or­rin Dev­in­sky, Martha J. Mor­rell, and Brent A. Vogt. 1995. “Con­tri­bu­tions of An­te­rior Cin­gu­late Cor­tex to Be­havi­our.”Brain: A Jour­nal of Neu­rol­ogy, 118(1), 279-306. ↩︎

  38. Bjorn Merker. 2007. “Con­scious­ness with­out a Cere­bral Cor­tex: A challenge for Neu­ro­science and Medicine.” Be­hav­ioral and Brain Sciences, 30(1), 63-81. It should be noted that this claim only ap­plies to chil­dren born with­out a neo­cor­tex. Adults with dam­aged neo­cor­tices re­main com­pletely veg­e­ta­tive. ↩︎

  39. Erich D. Jarvis et al. 2005. “Avian brains and a new un­der­stand­ing of ver­te­brate brain evolu­tion.” Na­ture Re­views. Neu­ro­science. 6 (2): 151–9. ↩︎

  40. An­drew B. Bar­ron and Colin Klein. 2016. “What In­sects Can Tell Us about the Ori­gins of Con­scious­ness.” Pro­ceed­ings of the Na­tional Academy of Sciences of the United States of Amer­ica 113 (18): 4900-4908. ↩︎

  41. We used TimeTree to de­ter­mine the dates for last com­mon an­ces­tor. TimeTree is a free in­ter­ac­tive tool from Tem­ple Univer­sity’s Cen­ter for Bio­di­ver­sity. It re­lies on the pub­lished data from over 3,000 stud­ies, cov­er­ing nearly 100,000 species. ↩︎

  42. See Peter God­frey-Smith. 2016. Other Minds: The Oc­to­pus, The Sea, and the Deep Ori­gins of Con­scious­ness. New York: Far­rar, Straus and Giroux: 87-97 for an ac­cessible overview. ↩︎

  43. See, in­ter alia, Stanis­las De­haene. 2014. Con­scious­ness and the Brain: De­ci­pher­ing How the Brain Codes Our Thoughts. Pen­guin Books. ↩︎

  44. See, in­ter alia, Michael Trest­man. 2013. “The Cam­brian Ex­plo­sion and the Ori­gins of Em­bod­ied Cog­ni­tion.” Biolog­i­cal The­ory 8:80-92. ↩︎

  45. C. H. Eise­mann, W. K. Jor­gensen, D. J. Mer­ritt, M. J. Rice, B. W. Cribb, P. D. Webb and M. P. Zalucki. 1984. “Do In­sects Feel Pain? - A Biolog­i­cal View.” Ex­per­en­tia 40: 164-167. ↩︎

  46. There are liter­ally only three ex­cep­tions: kil­ler whales, short-finned pi­lot whales, and hu­mans. Dar­ren P. Croft, Lau­ren JN Brent, Daniel W. Franks, and Michael A. Cant. 2015. “The evolu­tion of pro­longed life af­ter re­pro­duc­tion.” Trends in Ecol­ogy & Evolu­tion 30, no. 7: 407-416. ↩︎

  47. Thanks to Kim Cud­ding­ton for rais­ing this point. ↩︎

  48. Fer­ran Sayol, Joan Maspons, Oriol Lapiedra, An­drew N. Iwa­niuk, Tamás Székely, and Daniel Sol. 2016. “En­vi­ron­men­tal vari­a­tion and the evolu­tion of large brains in birds.” Na­ture com­mu­ni­ca­tions 7: 13971. ↩︎

  49. Note also that an­i­mals with faster metabolisms and smaller body sizes tend, ac­cord­ing to some met­rics, to pro­cess in­for­ma­tion faster. Thus, there is some rea­son to think that smaller an­i­mals have, in gen­eral, faster sub­jec­tive ex­pe­riences. Th­ese con­sid­er­a­tions could mean that small, short-lived crea­tures have pro­por­tion­ally greater need for con­scious pain and this would weigh against the con­sid­er­a­tions de­scribed in this sec­tion. ↩︎

  50. ‘Motility’ should not be con­fused with ‘mo­bil­ity.’ ‘Mo­bil­ity’ is merely the abil­ity of an ob­ject to be moved. A bas­ket­ball is mo­bile, but it is not motile. ↩︎

  51. Colin Klein and An­drew B Bar­ron. 2016. “In­sects Have the Ca­pac­ity for Sub­jec­tive Ex­pe­rience.” An­i­mal Sen­tience 9: 1–19. ↩︎

  52. Michael Trest­man. 2013. “The Cam­brian Ex­plo­sion and the Ori­gins of Em­bod­ied Cog­ni­tion.” Biolog­i­cal The­ory 8:80-92. ↩︎

  53. Jerome M Siegel. 2011. “Sleep in An­i­mals: A State of Adap­tive In­ac­tivity.” Prin­ci­ples and Prac­tice of Sleep Medicine. ↩︎

  54. Ste­fa­nia Pis­copo. 2009. “Sleep and Its Pos­si­ble Role in Learn­ing: A Phy­lo­ge­netic View.” Fron­tiers in Bio­science S1: 437-447. ↩︎

  55. Gümüştekín K, Seven B, Karab­u­lut N, Ak­taş O, Gür­san N, As­lan S, Keleş M, Varoglu E, Dane S. 2004. “Effects of Sleep Depri­va­tion, Ni­co­tine, and Se­le­nium on Wound Heal­ing in Rats.” In­ter­na­tional Jour­nal of Neu­ro­science. 114: 1433–1442. ↩︎

  56. Siegel 2011. ↩︎

  57. Th­ese changes de­scribe non-REM sleep. REM sleep is only found in mam­mals and some ju­ve­nile birds. D. Purves, G. J. Au­gus­tine, D. Fitz­patrick, L. C. Katz, A. S. LaMan­tia, J. O. McNa­mara, and S. M. Willi­ams. 2001. “Phys­iolog­i­cal changes in sleep states.” Neu­ro­science. 2nd edn. Sun­der­land, MA: Si­nauer As­so­ci­ates. ↩︎

  58. Jerome M.Siegel. “Do All An­i­mals Sleep?.” Trends in Neu­ro­sciences 31: 208-213. ↩︎

  59. David Pap­ineau and Howard Selina. 2000. In­tro­duc­ing Con­scious­ness. New York: Totem. ↩︎

  60. Jen­nifer Mather. 2008. “Cephalo­pod Con­scious­ness: Be­havi­oural Ev­i­dence.” Con­scious­ness and Cog­ni­tion 17: 37-48. ↩︎

  61. Of course, phy­lo­ge­netic dis­tance is an im­perfect proxy for phys­iolog­i­cal similar­ity. Phys­iolo­gies across very dis­tantly re­lated taxa can con­verge due to the fit­ness land­scape for a given mor­pholog­i­cal space, and rel­a­tively close taxa can have im­por­tant differ­ences in phys­iol­ogy. Thanks to Te­gan McCaslin for bring­ing this point to my at­ten­tion. ↩︎

  62. Robert H. Ressler, Robert B. Cial­dini, Mitchell L. Ghoca, and Suzanne M. Kleist. 1968. “Alarm Pheromone in the Earth­worm Lum­bri­cus ter­restris.” Science 161: 597-599. ↩︎

  63. Ka­mal Kaur Sekhon, Sa­man­tha R. Fash­ler, Ju­dith Ver­sloot, Spencer Lee, and Ken­neth D. Craig. 2017. “Chil­dren’s be­hav­ioral pain cues: Im­plicit au­to­mat­ic­ity and con­trol di­men­sions in ob­ser­va­tional mea­sures.” Pain Re­search and Man­age­ment 2017: 3017837. ↩︎

  64. C. H. Eise­mann, W. K. Jor­gensen, D. J. Mer­ritt, M. J. Rice, B. W. Cribb, P. D. Webb and M. P. Zalucki. 1984. “Do In­sects Feel Pain? - A Biolog­i­cal View.” Ex­per­en­tia 40: 164-167. ↩︎

  65. Michael Tye. 2016. “Are In­sects Sen­tient?”. An­i­mal Sen­tience 9 (5): 3. ↩︎

  66. Vic­to­ria Hurst, Philip C. Steven­son, and Geral­dine A. Wright. 2014. “Tox­ins nduce ‘malaise’ be­havi­our in the hon­ey­bee (Apis mel­lifera).” Jour­nal of Com­par­a­tive Phys­iol­ogy A 200: 881-890. ↩︎

  67. Mar­i­anna I. Zhukovskaya. 2014. “Groom­ing Be­hav­ior in Amer­i­can Cock­roach is Affected by Novelty and Odor.” The Scien­tific World Jour­nal, ar­ti­cle ID 329514. ↩︎

  68. Ti­mothy Mur­phy et al. 2015. “A Be­hav­ioral As­say for Mechanosen­sa­tion of MARCM-based Clones in Drosophila melanogaster.” Jour­nal of Vi­su­al­ized Ex­per­i­ments 106: 53537. ↩︎

  69. Many in­ver­te­brates, such as crus­taceans and spi­ders, en­gage in au­to­tomy (self-am­pu­ta­tion un­der threat, e.g., a lizard cast­ing off its tail when grasped by a preda­tor), but we have clas­sified au­to­tomy as a defen­sive be­hav­ior rather than a pro­tec­tive be­hav­ior. Au­to­tomy is a re­sponse to dan­ger, not a re­sponse to in­jury. ↩︎

  70. Opal­ine is a whitish fluid se­creted by sea hares that be­comes vis­cous upon con­tact with wa­ter. ↩︎

  71. Cyn­thia E. Kick­lighter, Markus Ger­mann, Michiya Kamio, and Charles D. Derby. 2007. “Molec­u­lar iden­ti­fi­ca­tion of alarm cues in the defen­sive se­cre­tions of the sea hare Aplysia cal­ifor­nica.” An­i­mal Be­havi­our 74, no. 5: 1481-1492. ↩︎

  72. Michael H. Robin­son, Lawrence G. Abele, Bar­bara Robin­son. 1970. “At­tack Au­to­tomy: A Defense against Preda­tors.” Science 169: 300-301. ↩︎

  73. Robert J.Blan­chard, D. Caroline Blan­chard, Guy Griebel, David Nutt. 2008. “In­tro­duc­tion to the Hand­book on Fear and Anx­iety.” Hand­book of Be­hav­ioral Neu­ro­science 17: 3-7. ↩︎

  74. Christoph P. Wie­den­mayer. 2009. “Plas­tic­ity of Defen­sive Be­hav­ior and Fear in Early Devel­op­ment.” Neu­ro­science and Biobe­hav­ioral Re­views 33: 432-441. ↩︎

  75. Here we use “vo­cal­iza­tion” in a broad and not ex­clu­sively pho­netic sense to mean more than sounds that liter­ally em­anate from vo­cal chords. ↩︎

  76. Ger­hard Man­teuffel, Birger Puppe, and Peter C. Schön. 2004. “Vo­cal­iza­tion of farm an­i­mals as a mea­sure of welfare.” Ap­plied An­i­mal Be­havi­our Science 88, no. 1-2: 163-182. ↩︎

  77. Bo Kar­ls­son, Gu­nilla Burell, Ulla-Maria An­der­berg, and Kurt Svärd­sudd. 2015. “Cog­ni­tive be­havi­our ther­apy in women with fibromyal­gia: A ran­dom­ized clini­cal trial.” Scan­d­i­na­vian Jour­nal of Pain 9: 11-21. ↩︎

  78. Ses­sile or­ganisms such as plants have nec­es­sar­ily evolved differ­ent strate­gies to pro­tect them­selves from nox­ious stim­uli. ↩︎

  79. Gun­jan Pandey and Rakesh K. Jain. 2002. “Bac­te­rial Che­mo­taxis to­ward En­vi­ron­men­tal Pol­lu­tants: Role in Biore­me­di­a­tion.” Ap­plied and En­vi­ron­men­tal Micro­biol­ogy 68 (12): 5789–5795. ↩︎