Next Steps in Invertebrate Welfare, Part 1: Fundamental Research


Whether in­ver­te­brates have a ca­pac­ity for valenced ex­pe­rience is still un­cer­tain. Given that un­cer­tainty, we ar­gue that sup­port­ing the cause of in­ver­te­brate welfare means, at pre­sent, pro­mot­ing ad­di­tional re­search. To that end, we ex­plore and out­line key re­search ques­tions in two ar­eas: (i) in­ver­te­brate sen­tience and (ii) philo­soph­i­cal re­search into con­scious­ness. Re­gard­ing the first, we pro­pose fur­ther re­search on those fea­tures which, ac­cord­ing to ex­pert agree­ment, seem to be nec­es­sary for con­scious­ness (e.g., no­ci­cep­tors and cen­tral­ized in­for­ma­tion-pro­cess­ing struc­tures). We also sug­gest look­ing into the qual­ity of in­ver­te­brates’ lives. Fi­nally, con­cern­ing philo­soph­i­cal re­search into con­scious­ness, we sug­gest that the in­her­ent difficul­ties in the de­tec­tion of morally sig­nifi­cant pain and plea­sure in non­hu­mans should be fur­ther in­ves­ti­gated. We also high­light other more spe­cific prob­lems about phe­nom­e­nal con­scious­ness and its moral im­pli­ca­tions.


In­ver­te­brates are com­monly as­sumed to be in­ca­pable of ex­pe­rienc­ing pos­i­tive or nega­tive states. In a se­ries of pub­li­ca­tions by Re­think Pri­ori­ties, we have been ex­plor­ing sev­eral rele­vant ques­tions and sur­vey­ing the sci­en­tific ev­i­dence about this mat­ter. In this se­ries:

Here, in the eleventh post of this se­ries, we re­flect on pos­si­ble di­rec­tions for fu­ture work on in­ver­te­brate welfare.

A main challenge to con­sid­er­ing in­ver­te­brate welfare is the un­cer­tainty about whether (some) in­ver­te­brates have the ca­pac­ity to ex­pe­rience pain and plea­sure in a morally sig­nifi­cant way. Given this cur­rent epistemic state, sup­port­ing the cause of in­ver­te­brate welfare means, in gen­eral, pro­mot­ing ad­di­tional re­search. In this re­gard, two of the most press­ing ar­eas for fur­ther in­ves­ti­ga­tion are (i) in­ver­te­brate sen­tience and (ii) philo­soph­i­cal re­search into con­scious­ness. In what fol­lows, I will ex­plore these two ar­eas and out­line sev­eral key re­search ques­tions that should be tack­led in the fu­ture.

Re­search on in­ver­te­brate sen­tience and other re­lated issues

Re­gard­ing the ma­jor­ity of in­ver­te­brates, given the cur­rently available ev­i­dence, we are much less con­fi­dent about whether these an­i­mals are con­scious than we are about other non­hu­man in­di­vi­d­u­als, such as mam­mals and birds (see our sum­mary of find­ings, part 2). Fur­ther re­search is needed. In ad­di­tion, that might also con­tribute to a bet­ter un­der­stand­ing of non-hu­man sen­tience in gen­eral. Hence, this kind of knowl­edge may be es­pe­cially rele­vant in or­der to as­sess, for in­stance, ar­tifi­cial sen­tience in the fu­ture. In what fol­lows, I will men­tion some par­tic­u­lar is­sues which should be ad­dressed.

Fea­tures po­ten­tially in­dica­tive of consciousness

Fol­low­ing our dis­cus­sion on in­ver­te­brate sen­tience, we can af­firm that there are more un­knowns than cer­tain­ties about sev­eral in­di­ca­tors of con­scious­ness in these an­i­mals. In our In­ver­te­brate Sen­tience Table it can be ob­served that there is no ev­i­dence re­gard­ing 43.6% of the 53 fea­tures stud­ied across 12 in­ver­te­brate taxa. Given the cur­rent state of our knowl­edge, and those fea­tures which, ac­cord­ing to ex­pert agree­ment, seem to be nec­es­sary for con­scious­ness (Bate­son, 1991; Broom, 2013; EFSA, 2005; El­wood, 2011; Fiorito, 1986; Sned­don et al., 2014; Sned­don, 2017), ad­di­tional re­search on the fol­low­ing as­pects should be a pri­or­ity:

  • No­ci­cep­tors and cen­tral­ized in­for­ma­tion-pro­cess­ing struc­tures: Fur­ther ev­i­dence is needed about the struc­ture and func­tion­ing of the ner­vous sys­tem of var­i­ous in­ver­te­brate taxa, in par­tic­u­lar, about no­ci­cep­tors. Th­ese el­e­ments trans­duce nox­ious stim­uli into long-rang­ing elec­tri­cal sig­nals that are re­layed to higher brain cen­ters (Du­bin & Pat­apoutian, 2010). Thus, no­ci­cep­tors–when con­nected to cen­tral­ized in­for­ma­tion pro­cess­ing struc­tures–are a nec­es­sary con­di­tion for (biolog­i­cal) pain. Although ev­i­dence sup­ports the ex­is­tence of no­ci­cep­tion in some in­ver­te­brate an­i­mals, this con­clu­sion is mostly based on be­hav­ioral ob­ser­va­tions, rather than in the iden­ti­fi­ca­tion of no­ci­cep­tors. With some ex­cep­tions[1], it is not known whether or not an­i­mals of var­i­ous in­ver­te­brate species or taxa have no­ci­cep­tors, and if so, whether no­ci­cep­tors are con­nected to higher brain cen­ters or equiv­a­lent cen­tral­ized in­for­ma­tion-pro­cess­ing struc­tures.

In this re­gard, even in the sce­nario that un­suc­cess­ful searches for no­ci­cep­tors had been con­ducted, it should be ex­am­ined whether other sen­sory neu­rons and/​or al­ter­na­tive (but un­known) mechanisms could sub­serve no­ci­cep­tion in in­ver­te­brates. This is a plau­si­ble hy­poth­e­sis in light of avoidance be­hav­iors and other re­sponses to han­dling or nox­ious stim­uli found in these an­i­mals.

Nev­er­the­less, the fact that in­ver­te­brates had no­ci­cep­tors would not im­ply per se that they could ex­pe­rience pain or plea­sure. As men­tioned, if no­ci­cep­tors are not con­nected to cen­tral­ized in­for­ma­tion-pro­cess­ing struc­tures, these neu­rons could trig­ger re­flex­ive re­ac­tions (i.e., similar to spinally me­di­ated re­sponses in mam­mals), but that would not im­ply that the no­ci­cep­tive in­put is con­sciously per­ceived (in hu­mans, see Becker et al., 2012; Du­bin & Pat­apoutian, 2010). If con­scious­ness is un­der­stood as suit­ably in­te­grated in­for­ma­tion (Oizumi et al., 2014), the pro­jec­tions of no­ci­cep­tors to in­te­gra­tive in­for­ma­tion-pro­cess­ing struc­tures is an ex­tremely im­por­tant as­pect to ex­am­ine when judg­ing the prob­a­bil­ity that a non­hu­man in­di­vi­d­ual is sen­tient.

  • The effects of anal­gesics: An im­por­tant part of the ex­ist­ing re­search on the effects of anal­gesics fo­cuses on the phys­iolog­i­cal con­se­quences of these sub­stances. Fur­ther re­search about their effects on pain-re­lated be­hav­iors–i.e., a re­duc­tion in no­ci­cep­tive re­flexes and avoidance be­hav­ior–may shed some light to the is­sue of whether in­ver­te­brates pos­sess the ca­pac­ity for valenced ex­pe­rience. Fur­ther­more, sup­pose anal­gesics had effects similar to those which these sub­stances cause in hu­mans. Ad­minis­tra­tion of anal­gesics should then be con­sid­ered a pos­si­ble in­ter­ven­tion that could re­lieve the suffer­ing of in­ver­te­brates, es­pe­cially of those used in var­i­ous hu­man ac­tivi­ties such as re­search. Similar ar­gu­ments can be ad­duced for fur­ther re­search on the effects of anx­iolyt­ics.

  • Are no­ci­cep­tive re­sponses in in­ver­te­brates mere re­flexes?: It is hy­poth­e­sized that one of the main func­tions of con­scious­ness is en­abling flex­ible, con­text-de­pen­dent be­hav­ior (Baars, 1993). In this re­gard, var­i­ous au­thors claim that in­sect be­hav­ior to­ward nox­ious stim­uli would not prove be­hav­ioral flex­i­bil­ity. In­stead, these re­sponses would largely obey to pre-pro­grammed pat­terns (e.g., Eise­mann et al., 1984; Gould & Gould, 1982). This seems to be the case of groom­ing be­hav­ior when in­sects–like fruit flies or cock­roaches–are in con­tact with a nox­ious sub­stance. Similarly, other au­thors de­bate whether au­to­tomy[2] in many in­ver­te­brate taxa (e.g., crus­taceans) is a re­flex ac­tion or if it in­volves a de­gree of vol­un­tary con­trol (Flem­ing et al., 2007).

In­ver­te­brates dis­play var­i­ous nox­ious stim­uli re­ac­tions but these are not, by them­selves, re­li­able signs of valenced ex­pe­rience. If a re­sponse is purely me­di­ated by no­ci­cep­tion, then it is a re­flex. In these cases, con­scious­ness is not nec­es­sary. In hu­man co­matose pa­tients, for ex­am­ple, nox­ious stim­u­la­tion of the lower ex­trem­ities can elicit a mere lo­cal with­drawal re­flex that is not nec­es­sar­ily a sig­nal of pain per­cep­tion (Gelb, 2010: 90).

In or­der to as­sess whether nox­ious stim­uli re­sponses in in­ver­te­brates are not mere au­toma­tisms, first, it should be ex­plored to what ex­tent be­hav­iors such as mov­ing away, es­cap­ing and avoidance ac­count for nox­ious stim­u­lus in­ten­sity and di­rec­tion. That is how it has been ob­served that, for ex­am­ple, parame­cia move away from nox­ious stim­uli (Boisseau et al., 2016; Jen­nings, 1906; Wood, 1969; Zu­panc, 2010: 99-101). There­fore, it could be ar­gued that they satisfy a loose defi­ni­tion of no­ci­cep­tive-type re­sponses (e.g., Smith, 1991). How­ever, this re­ac­tion is a ran­dom be­hav­ioral re­sponse that does not clearly in­te­grate the di­rec­tion of the stim­u­lus (Kava­liers, 1988), and hence, it is not solid ev­i­dence of valenced ex­pe­rience.

Se­cond, if most in­ver­te­brate be­hav­iors are rigid and pre-pro­grammed re­ac­tions, that would highly re­duce the need of learn­ing from nox­ious stim­uli and hence, one of the adap­tive val­ues of ex­pe­rienc­ing pain. In­stead, if (some) in­ver­te­brates are con­scious, they should be able to learn to avoid a similar nox­ious situ­a­tion or stim­u­lus later in the fu­ture. We know that at least some in­ver­te­brates are ca­pa­ble of re­mem­ber­ing and can show rel­a­tively com­plex forms of learn­ing. How­ever, long-term be­hav­ior al­ter­a­tions to avoid nox­ious stim­uli have been barely stud­ied in in­ver­te­brates. Fur­ther in­ves­ti­ga­tion in this re­gard could be fruit­ful and rele­vant for un­der­stand­ing the prob­a­bil­ities of in­ver­te­brates be­ing con­scious. Given the im­por­tance of mem­ory for en­hanc­ing fu­ture sur­vival (Triki & Bshary, 2019) and as­sess­ing con­scious­ness (Baars, 2003 in Osaka, 2003; Stein et al., 2016), we should also con­sider adding to our database other pos­si­ble proxy in­di­ca­tors of mem­ory, be­sides the spe­cific fea­ture of ‘long-term be­hav­ior al­ter­a­tion to avoid nox­ious stim­u­lus’.

Third, if a re­ac­tion is a pure re­flex, it is au­to­matic and rigid. Hence, it should not change re­gard­less of other mo­ti­va­tional pri­ori­ties. That is, we would ex­pect an in­di­vi­d­ual to avoid a nox­ious stim­u­lus re­gard­less of whether she is hun­gry or sa­ti­ated, even if food is pre­sent (El­wood et al., 2009). On the con­trary, if the in­di­vi­d­ual shows differ­ent re­ac­tions to nox­ious stim­uli, de­pend­ing on ex­oge­nous or en­doge­nous changes, it is con­ceiv­able that there ex­ists some form of pro­cess­ing sys­tem in which en­vi­ron­men­tal changes or com­pet­ing needs of the in­di­vi­d­ual are weighed (McFar­land & Sibly, 1975). In other words, if a re­sponse to nox­ious stim­uli varies ac­cord­ing to other mo­ti­va­tional re­quire­ments, then this be­hav­ior is more likely to in­volve some sort of cen­tral pro­cess­ing rather than be­ing purely a re­flex re­sponse. That is to say, for ex­am­ple, if the es­cape re­sponse of an an­i­mal varies based on the pres­ence or ab­sence of food that would mean it is less likely to be a purely re­flex­ive be­hav­ior, par­tic­u­larly if that re­sponse changes de­pend­ing on time since their last meal or the qual­ity and quan­tity of food (Ap­pel & El­wood, 2009). Un­for­tu­nately, of all the in­di­ca­tors that we con­sid­ered, mo­ti­va­tional trade­offs are the most ne­glected re­search area re­gard­ing con­scious­ness in in­ver­te­brates.

In gen­eral, if an or­ganism is able to show non-pre-pro­grammed be­hav­ior, then she should ex­hibit flex­ible re­sponse pat­terns. There­fore, more re­search is needed to as­sess be­hav­ioral flex­i­bil­ity in in­ver­te­brates. Fur­ther em­piri­cal ev­i­dence about the fol­low­ing will con­tribute to this end: (i) whether no­ci­cep­tive re­sponses ac­count for nox­ious stim­u­lus in­ten­sity and di­rec­tion, (ii) in­di­ca­tors of ‘long-term’ learn­ing and mem­ory, and (iii) mo­ti­va­tional trade­offs in in­ver­te­brates.

  • Study­ing the neu­ro­phys­iolog­i­cal bases of con­scious­ness: In­ver­te­brates pos­sess sim­ple cen­tral ner­vous sys­tems and fewer neu­rons com­pared to cen­tral ner­vous sys­tems in mam­mals (Hoyle, 1970 in Eise­mann et al., 1984). Ac­cord­ing to Eise­mann et al. (1984), “this at least raises the ques­tion of whether any ex­pe­rience akin to hu­man pain could be gen­er­ated” (166). Fur­ther­more, it is hy­poth­e­sized that their mi­ni­a­tur­ized ner­vous sys­tems would limit their ca­pac­ity of in­for­ma­tion pro­cess­ing (Niven & Far­ris, 2012). The above could im­ply that in­ver­te­brates have less need for sen­sory in­for­ma­tion and hence, in many situ­a­tions, the con­scious ex­pe­rience of pain would not be adap­tive.

Still, there are many un­knowns about the pos­si­ble anatom­i­cal and neu­ro­phys­iolog­i­cal sub­strate of in­ver­te­brate con­scious­ness. Whilst the in­ver­te­brate cen­tral ner­vous sys­tem gen­er­ally has a rel­a­tively sim­ple or­ga­ni­za­tion and fewer neu­rons, re­gard­ing en­ergy use and in­for­ma­tion pro­cess­ing small brains seem to be more effi­cient (Niven et al., 2007; Niven & Far­ris, 2012). Fur­ther­more, al­though in­ver­te­brate cen­tral ner­vous sys­tems usu­ally in­volve fewer neu­rons than a ver­te­brate ner­vous sys­tem (Eise­mann et al., 1984), when it comes to the ra­tio of brain weight to body weight, some in­ver­te­brate brains are sur­pris­ingly large. In fact, in­sects have higher brain:body-mass ra­tios than any ver­te­brate we know of (Ray, 2019). In spi­ders, for in­stance, re­searchers have found that the smaller the spi­der, the big­ger its brain rel­a­tive to its body size (Que­sada et al., 2011). By the same to­ken, in some small ants, the brain ac­counts for 17 of their body mass (Seid et al., 2010). In hu­mans, this ra­tio is only 140, which–in com­par­i­son with other mam­mals–is still im­pres­sive (1/​100 in cats, 1125 in dogs, 1/​2800 in hip­popotami) (Godz­ińska, 2017). Th­ese find­ings do not en­tail, per se, that an­i­mals of cer­tain taxa are more in­tel­li­gent or con­scious than oth­ers. As Broom (2013) states, “there are many anoma­lies in re­la­tion­ships be­tween abil­ity and brain size so no com­par­a­tive con­clu­sions can be reached (...)”. How­ever, these find­ings high­light that ner­vous sys­tems in in­ver­te­brates are com­monly con­strained to fit within tiny vol­umes. In that sce­nario, some el­e­ments can be­come sim­plified due to loss or fu­sion of com­po­nents. At the same time, given the need to max­i­mize func­tional ca­pac­ity within a small space, in­ver­te­brate neu­rons may also be mul­ti­func­tional, op­er­at­ing in mul­ti­ple cir­cuits and con­tribut­ing to mul­ti­ple be­hav­iors (Niven & Far­ris, 2012; Niven & Chit­tka, 2010; see the case of neu­ral or­gani­sa­tion and cog­ni­tive abil­ities of oc­to­puses in Zullo & Hochner, 2011; and of mul­ti­func­tional neu­rons in the ner­vous sys­tem of C. el­e­gans in Hall et al., 2005)[3]

Nev­er­the­less, all of the above are mere hy­pothe­ses that should be em­piri­cally tested. Cur­rently, we lack de­tailed neu­ral con­nec­tivity maps in all but a few in­ver­te­brate species (Albert Ein­stein Col­lege of Medicine, 2019; Boly et al., 2013). How­ever, through our liter­a­ture re­view, we found that some in­ver­te­brates such as cephalopods (i.e., oc­to­puses), crus­taceans (i.e., crabs and crayfish) and in­sects (i.e., fruit flies, honey bees) show rich be­hav­ioral reper­toires sug­ges­tive of high-or­der neu­ral func­tion. In this sense, the next steps will be to iden­tify their neu­roanatom­i­cal and neu­ro­phys­iolog­i­cal prop­er­ties (e.g., char­ac­ter­ize the elec­tro­mag­netic waves in the por­tions of the ner­vous sys­tem that par­ti­ci­pate in these be­hav­iors), their molec­u­lar play­ers and the synap­tic mechanisms that mod­u­late these re­sponses and sen­sory pro­cess­ing. Re­cent neu­ro­scien­tific re­search about oc­to­pus be­hav­ior (i.e., Sivi­tilli & Gire, 2019 in AGU, 2019) sug­gests that this could be a fer­tile area of re­search, at least as far as these an­i­mals are con­cerned. How­ever, in or­der to in­ves­ti­gate less com­plex in­ver­te­brates and for the sake of em­piri­cal rigor, new rele­vant neu­ro­scien­tific find­ings will prob­a­bly re­quire si­mul­ta­neous method­olog­i­cal ad­vances. Thus, the de­vel­op­ment and ap­pli­ca­tion of more elab­o­rate neu­roimag­ing and elec­tro­phys­iolog­i­cal tech­niques ap­pli­ca­ble to a wide va­ri­ety of small in­ver­te­brates are needed.

In sum, we can con­clude that in­te­grat­ing be­hav­ioral re­sponses in the char­ac­ter­i­za­tion of neu­ral ar­chi­tec­tures and in­ver­te­brate phys­iol­ogy would con­sti­tute a qual­i­ta­tive step for­ward in the study of con­scious­ness in in­ver­te­brates.

Study­ing con­scious­ness in other species

Re­search into other in­ver­te­brate species

Re­cent years have wit­nessed in­creased in­ter­est in the study of be­hav­ioral and neu­ro­phys­iolog­i­cal ev­i­dence sug­ges­tive of con­scious­ness in some an­i­mals, es­pe­cially mam­mals (Boly et al., 2013; Edel­man & Seth, 2009). How­ever, the study of con­scious­ness in in­ver­te­brates is at a very early stage. In­ver­te­brates are phy­lo­ge­net­i­cally dis­tant from hu­mans and so it should come as no sur­prise that the or­ga­ni­za­tion of their ner­vous sys­tems di­verges so greatly from those of mam­mals and even of other non-mam­malian ver­te­brates. Thus, un­til re­cently, learn­ing and cog­ni­tive skills in in­ver­te­brates have been rarely con­sid­ered as ob­jects of sci­en­tific re­search.

In our study, we ex­am­ined sci­en­tific ev­i­dence about 18 rep­re­sen­ta­tive biolog­i­cal taxa, of which 12 were in­ver­te­brates. In a pre­vi­ous post, we pro­posed sev­eral in­ver­te­brate taxa that we would like to see in­cluded in this database. Some of them are:

  • Oys­ters, clams, mus­sels and scal­lops (phy­lum Mol­lusca, class Bi­valvia): Mol­lusca is a highly broad phy­lum with over 100,000 species, di­vided into seven classes. The phy­lum en­com­passes or­ganisms of enor­mous di­ver­sity, rang­ing from oc­to­puses to sea cu­cum­bers, and in­clud­ing mol­lusks. There is a sig­nifi­cant in­t­ra­phy­lum vari­a­tion in neu­ral and sen­sory com­plex­ity, char­ac­ter­is­tics that are prob­a­bly rele­vant for the abil­ity to feel pain or ex­pe­rience dis­tress. Hence, al­though oc­to­puses are likely to be con­scious, there is lit­tle ev­i­dence that bi­valves are (Crook & Walters, 2011). Study­ing and com­piling ex­tant ev­i­dence about bi­valves will be of in­ter­est to bet­ter un­der­stand the dis­tri­bu­tion of sen­tience within Mol­lusca, and be­cause of the ex­ten­sive use of these an­i­mals for hu­man con­sump­tion.

  • Snails (phy­lum Mol­lusca, class Gas­tropoda): Snails are part of the Mol­lusca phy­lum, albeit, un­like bi­valves, they are typ­i­cally motile and ac­tive for­agers (Crook & Walters, 2011). More­over, they are con­sumed by hu­mans in many cul­tures, as it will be de­scribed in an up­com­ing post. Thus, for rea­sons similar to those men­tioned above, we con­sider it rele­vant to study what fea­tures po­ten­tially in­dica­tive of con­scious­ness are ob­served in these an­i­mals.

  • Shrimps and prawns (sub­or­der Den­dro­branchi­ata): Fish­count (2019) es­ti­mates that 190-470 billion shrimps and prawns were kil­led in aqua­cul­ture pro­duc­tion in 2015. Be­fore propos­ing pos­si­ble welfare mea­sures for these an­i­mals, it is nec­es­sary to dis­cern whether they are con­scious or not.

  • In­sects used as food: Ac­cord­ing to FAO (Van Huis et al., 2013), bee­tles (or­der Coleoptera), but­terflies and moths (or­der Lepi­doptera), grasshop­pers and crick­ets (or­der Orthoptera), ci­cadas and leafhop­pers (or­der Hemiptera), are some of the top in­sect or­ders con­sumed by hu­mans wor­ld­wide. If in­sects will play a big­ger role in the fu­ture of agri­cul­ture, it is worth con­sid­er­ing how likely these an­i­mals are to be con­scious (see our up­com­ing post about re­search on pos­si­ble in­ter­ven­tions).

  • Other in­ver­te­brates ex­ploited by hu­mans, like silk­worms (Bom­byx mori, genus Bom­byx), cochineal (genus Dacty­lopius), meal­worms (genus Tene­brio), lac scales (Ker­ria lacca, fam­ily Ker­rii­dae), and krill (or­der Euphau­si­acea) merit fur­ther in­ves­ti­ga­tion (see our up­com­ing post about re­search on pos­si­ble in­ter­ven­tions).

Re­search into ver­te­brate species

There is a grow­ing in­ter­est in, and an in­creas­ing knowl­edge about, the neu­ro­phys­iol­ogy and neu­roanatomy of non-mam­mal ver­te­brates such as birds, rep­tiles, am­phibi­ans, and fishes (Boly et al., 2013). Although this kind of re­search can­not di­rectly an­swer our ques­tions about in­ver­te­brate con­scious­ness, the study of com­par­a­tive phys­iol­ogy can shed some light on com­mon phyletic con­di­tions and neu­ral sub­strates un­der­ly­ing the emer­gence of an­i­mal con­scious­ness. Broadly, these find­ings could en­able us to draw a frame­work for as­sess­ing con­scious­ness in non-hu­man in­di­vi­d­u­als. Two con­crete re­sults of such an ap­proach would be: (i) iden­ti­fy­ing the con­scious­ness-re­lated fea­tures that have greater pre­dic­tive strength, or at least, which are given greater weight in the on­go­ing ex­pert de­bate about these fea­tures’ roles, and (ii) re­fin­ing our method­olog­i­cal ap­proach for the as­sess­ment of con­scious­ness in an­i­mals.

As men­tioned in a pre­vi­ous post, in or­der to strengthen pos­si­ble analog­i­cal ar­gu­ments and other similar­ity-driven con­sid­er­a­tions, it would be helpful to add to our database ev­i­dence about ver­te­brate con­scious­ness re­gard­ing:

  • A rep­re­sen­ta­tive rep­tile (class Rep­tilia),

  • A rep­re­sen­ta­tive bony fish (su­per­class Oste­ichthyes), and

  • A rep­re­sen­ta­tive car­tilag­i­nous fish (class Chon­drichthyes).

Ad­di­tion­ally, progress in our knowl­edge about the ori­gins and evolu­tion of con­scious­ness can con­tribute to dis­cern­ing to what ex­tent find­ings for a species or taxa are gen­er­al­iz­able to a higher tax­o­nomic rank. Since, for in­stance, in­sect brains have re­mained rel­a­tively similar since the Devo­nian (Straus­feld, 2012; Straus­feld et al., 2016), and most flight con­trol be­hav­iors found in Drosophila are also ob­served in other fly­ing in­sects, it is pos­si­ble to think that fly­ing in­sects are likely to share a similar set of ba­sic be­hav­ioral mod­ules for flight con­trol (the so-called ‘Devo­nian toolkit’; see Dick­in­son, 2014). Hence, available find­ings on nav­i­ga­tion in Drosophila could be gen­er­al­ized to other fly­ing in­sects–at least un­til new ev­i­dence arises[4]. This kind of po­ten­tial con­tri­bu­tion of evolu­tion­ary stud­ies is of spe­cial in­ter­est given that ex­ist­ing knowl­edge about con­scious­ness-re­lated fea­tures in in­ver­te­brates is scarce, and com­monly, the more ro­bust sci­en­tific ev­i­dence is cir­cum­scribed to only a few par­tic­u­lar species.

Re­cent ad­vances in func­tional imag­ing and elec­tro­phys­iolog­i­cal tech­niques ap­plied to hu­mans have sig­nifi­cantly ex­panded our knowl­edge of the neu­ral cor­re­lates of con­scious­ness in our species (Boly et al., 2013; Chen, 2001; Gosseries et al., 2014). This ap­proach aims to iden­tify rel­a­tively limited parts of the brain (or rel­a­tively spe­cific fea­tures of neu­ral pro­cess­ing) that cor­re­late di­rectly with sub­jec­tive ex­pe­rience. Re­cently, some of the great­est progress in this field has been made on the study of the neu­ral cor­re­lates for vi­sion. Through differ­ent tech­niques, re­searchers have dis­rupted what seemed to be the un­equiv­o­cal re­la­tion­ship be­tween a vi­sual stim­u­lus and its as­so­ci­ated sub­jec­tive per­cep­tion. In this man­ner, the neu­ral mechanisms that re­spond to the sub­jec­tive per­cept, rather than the phys­i­cal stim­u­lus, can be iso­lated, al­low­ing vi­sual con­scious­ness to be tracked in the hu­man brain (Chalmers, 2004; Kim & Blake, 2005).

Fur­ther re­search on the neu­ral cor­re­lates of con­scious­ness may be a step to­ward an ex­plana­tory frame­work of con­scious­ness. If these find­ings are in­te­grated within a the­ory, we can gen­er­ate pre­dic­tions about con­scious­ness that could be ac­com­mo­dated when as­sess­ing this phe­nomenon in other non-hu­man in­di­vi­d­u­als. Ul­ti­mately, these pre­dic­tions should be em­piri­cally as­sessed in in­ver­te­brates.

In sum, some ques­tions that we would like to see an­swered are:

  • What is the likely dis­tri­bu­tion of phe­nom­e­nal con­scious­ness across differ­ent (in­ver­te­brate) taxa?

  • Can we iden­tify com­mon phyletic con­di­tions and neu­ral sub­strates of con­scious­ness?

  • What in­ver­te­brate neu­ral struc­tures and mechanisms may be analo­gous in func­tion to hu­man or mam­malian cor­ti­cal sys­tems?

  • Are there spe­cific anatom­i­cal fea­tures and/​or func­tional ac­ti­va­tion of ner­vous sys­tem ar­eas that are nec­es­sary (al­though not suffi­cient) for con­scious­ness? Are they ob­served in in­ver­te­brates?

  • Ac­cord­ing to ex­perts, which are the con­scious­ness-re­lated fea­tures that have greater pre­dic­tive strength? What is the on­go­ing ex­pert de­bate about these fea­tures?

  • What method­olog­i­cal con­sid­er­a­tions should be es­pe­cially taken into ac­count when as­sess­ing con­scious­ness in non-hu­man in­di­vi­d­u­als?

  • What func­tion does con­scious­ness serve in adap­tive be­hav­ior?

  • How might an in­te­gra­tive frame­work fa­cil­i­tate the study of con­scious­ness in differ­ent (ver­te­brate and in­ver­te­brate) taxa?

Pre­sum­ably, aca­demics in ethol­ogy, biol­ogy, neu­ro­science, and other rele­vant fields are in the best po­si­tion to an­swer these ques­tions. There­fore, con­sult­ing ex­perts about their views on these is­sues–in­clud­ing their agree­ments, dis­agree­ments, and method­olog­i­cal con­cerns–would be es­pe­cially helpful. In ad­di­tion, ex­pert in­ter­pre­ta­tions about whether spe­cific in­ver­te­brate taxa are con­scious or not should also be ex­plored. All the above could be done through in­ter­views, sur­veys, or, if the ap­pro­pri­ate means are available, by ap­ply­ing a Delphi method (an in­ter­ac­tive fore­cast­ing tech­nique that re­lies on a panel of ex­perts). Lastly, ex­pert opinions may con­tribute to get­ting a bet­ter grasp of fur­ther rele­vant re­search ques­tions on these top­ics.

Un­der­stand­ing in­ver­te­brate lives

The lives of in­ver­te­brates in nature

Even if we have some con­fi­dence that in­di­vi­d­u­als of a given in­ver­te­brate species are con­scious, we still have to gain deeper knowl­edge about the qual­ity of those an­i­mals’ lives in the wild. While some writ­ers have con­fi­dently claimed that suffer­ing is more com­mon than plea­sure in na­ture (Horta, 2010; Faria, 2016; Paez, 2015), a re­cent pa­per by Groff and Ng (2019) finds that at least one math­e­mat­i­cal ar­gu­ment for this was mis­taken and that the situ­a­tion is, at pre­sent, the­o­ret­i­cally am­bigu­ous.

A rea­son why it is not nec­es­sar­ily true that there is net suffer­ing in na­ture is the hy­poth­e­sis that small in­di­vi­d­u­als–as in­ver­te­brates–may have less in­tense sen­tient ex­pe­riences. In that sce­nario, small an­i­mals would ex­pe­rience rel­a­tively less suffer­ing and more en­joy­ment than larger ones. In par­tic­u­lar, for an­i­mals of small species, like in­sects, where the ma­jor­ity of in­di­vi­d­u­als fail to re­pro­duce, there would be net en­joy­ment (Groff & Ng, 2019). How­ever, the nu­mer­ous in­ver­te­brate species, their di­ver­sity, and the high lev­els of un­cer­tainty about the func­tion­ing of in­ver­te­brates’ ner­vous sys­tems im­pose se­vere con­straints to this hy­poth­e­sis and its im­pli­ca­tions[5]. At pre­sent, we do not know whether the lives of in­ver­te­brates are net-pos­i­tive or net-nega­tive.

Hence, sev­eral ques­tions re­main unan­swered: What is, as a rule, the gen­eral health sta­tus of in­ver­te­brates? What threats do these an­i­mals face? What are the main causes of their suffer­ing? Are they one-off or chronic threats? How com­monly do they face these threats? What pro­por­tion of those an­i­mals reach adult­hood? Like­wise, what are their main causes of death? What are these deaths like? Are there ways to die that are es­pe­cially slow and painful? At what ages do their mor­tal­ity rates in­crease? How many of their offspring are sen­tient be­fore they die? Our anal­y­sis fo­cused on adult in­di­vi­d­u­als, but be­cause many in­ver­te­brates have rad­i­cally differ­ent life stages, it could be the case that ju­ve­niles (i.e., lar­vae and pu­pae) do not have the ca­pac­ity to ex­pe­rience pain and plea­sure in a morally sig­nifi­cant way. In that case, im­ma­ture in­di­vi­d­u­als would have a differ­ent moral sta­tus than adults.

On the other hand, what are their pos­i­tive ex­pe­riences like? What are their main sources of plea­sure/​hap­piness? Which are their prefer­ences? What is their re­pro­duc­tive strat­egy? In gen­eral, to what ex­tent life-his­tory strate­gies are re­lated to welfare? A life his­tory re­port on her­bivorous in­sects, re­cently pub­lished by Re­think Pri­ori­ties, con­sti­tutes some progress in this di­rec­tion. Even so, fur­ther re­search, con­ducted by ex­perts, is needed.

The lives of farmed invertebrates

Similar ques­tions arise re­gard­ing in­ver­te­brates un­der hu­man con­trol. To these, we must add ques­tions about their con­di­tions in cap­tivity. Some as­pects that should be in­ves­ti­gated in the fu­ture are:

  • Meth­ods and con­di­tions of re­pro­duc­tion or cap­ture.

  • Con­fine­ment con­di­tions:

  • Den­sity and re­stric­tion of move­ments.

  • En­rich­ment: phys­i­cal, sen­sory, oc­cu­pa­tional, and/​or so­cial (if rele­vant, for so­cial species).

  • Water or air qual­ity.

  • Pro­vi­sion of ad­e­quate light.

  • Feed­ing: e.g. ra­tio of an­i­mals and ac­cess to feed­ers.

  • Health in­di­ca­tors:

  • Pre-slaugh­ter mor­tal­ity rates.

  • Per­centage of dis­eased or in­jured an­i­mals.

  • Others.

  • Be­hav­ior-re­lated in­di­ca­tors:

  • Fre­quency of de­vi­a­tions from nor­mal be­hav­ior ac­cord­ing to the species (e.g. au­to­tomy in oc­to­puses as a sign of dis­tress, stereo­typ­ies, undis­turbed rest­ing).

  • Even­ness of us­ing the space.

  • Trans­port con­di­tions, in­clud­ing:

  • Con­fine­ment con­di­tions and den­si­ties.

  • Han­dling pro­ce­dures.

  • Trans­porta­tion time limit.

  • Slaugh­ter con­di­tions.

In gen­eral, many farmed in­ver­te­brates are main­tained in con­di­tions that of­ten ap­pear to be detri­men­tal to their welfare, with min­i­mal care and over­sight (Carere et al., 2011; Hor­vath et al., 2013). To as­sess their qual­ity of life, it would be use­ful to mea­sure the to­tal cu­mu­la­tive welfare effect of differ­ent farm­ing con­di­tions on in­di­vi­d­u­als of a given in­ver­te­brate species. That would also con­tribute to iden­tify the most de­ter­min­ing fac­tors in their qual­ity of life and pri­ori­tize forms of in­ter­ven­tion. In this re­gard, re­cent find­ings sug­gest that mea­sur­ing biolog­i­cal ag­ing through bio­mark­ers could provide a highly promis­ing al­ter­na­tive mea­sure of cu­mu­la­tive welfare. That is nec­es­sary “to de­ter­mine which con­di­tions provide the best over­all qual­ity of life for non­hu­man an­i­mals,” ac­cord­ing to Will Brad­shaw (2019) from Wild An­i­mal Ini­ti­a­tive. Although the use of bio­mark­ers of ag­ing has its limi­ta­tions and spe­cific challenges arise if it is to be ap­plied to in­ver­te­brates, this method seems to be a promis­ing path to ob­jec­tively mea­sure cu­mu­la­tive welfare among an­i­mals.

Over­all, an­swer­ing the ques­tions men­tioned above can in­form high-pri­or­ity welfare case stud­ies and sug­gest con­crete ways of im­prov­ing the lives of farmed in­ver­te­brates or those liv­ing in na­ture. At the very least, it can sug­gest novel ap­plied re­search in this re­gard.

Philo­soph­i­cal re­search into consciousness

While the study of an­i­mal con­scious­ness is largely an em­piri­cal work, it is im­pos­si­ble not to rely on cer­tain philo­soph­i­cal as­sump­tions—for ex­am­ple, on the na­ture of mind, of pain, or of agency. Hence, ques­tions about in­ver­te­brate con­scious­ness are not only sci­en­tific, but also philo­soph­i­cal. They in­volve an episte­molog­i­cal, a meta­phys­i­cal, and a phe­nomenolog­i­cal di­men­sion.

In our first post of this se­ries, we dis­cussed the philo­soph­i­cal difficul­ties in­her­ent in the de­tec­tion of morally sig­nifi­cant pain and plea­sure in non­hu­mans. We iden­ti­fied eight con­cep­tu­ally se­quen­tial steps needed to as­sess this is­sue. Th­ese steps are:

  1. Deter­mine that other minds ex­ist.

  2. Check to see if the non­hu­man en­tity in ques­tion en­gages in pain be­hav­ior. If so, check to see if there are any defeaters for the ex­pla­na­tion that the en­tity in ques­tion feels pain.

  3. Ap­ply one’s best the­ory of con­scious­ness to see what it says about the like­li­hood that the en­tity in ques­tion feels pain.

  4. As­sum­ing that the en­tity feels pain, check to see if it ex­pe­riences the felt bad­ness of pain.

  5. Deter­mine the phe­nom­e­nal in­ten­sity and phe­nom­e­nal ex­ten­sion of the pain.

  6. Deter­mine the de­gree to which the en­tity is aware of the pain.

  7. Deter­mine the en­tity’s moral stand­ing rel­a­tive to other en­tities which ex­pe­rience pain.

  8. Check to see if your fi­nal re­sult con­sti­tutes a re­duc­tio on the whole pro­cess.

Th­ese steps and their difficul­ties are de­scribed in greater de­tail in our first post. Fur­ther re­search into these ar­eas can con­tribute to de­ter­mine whether phe­nom­e­nal con­scious­ness is rare out­side hu­mans or if it is more widely dis­tributed, as well as how morally sig­nifi­cant it is.

Sev­eral other more spe­cific prob­lems should be ad­di­tion­ally con­sid­ered:

  • Which kinds of pro­cesses re­lated to con­scious­ness are suffi­cient for moral pa­tient­hood?

  • How to weigh the in­ter­ests of differ­ent taxa that are po­ten­tially con­scious against each other? That is, how to ad­dress the ques­tion of “moral weight”?

  • How plau­si­ble are ‘hid­den qualia’ (con­scious ex­pe­riences that we can­not in­tro­spect)? What are their im­pli­ca­tions for ex­ist­ing find­ings and the­o­riza­tions on con­scious­ness?

  • As Muehlhauser (2017[2018]) won­ders, what are the im­pli­ca­tions of illu­sion­ism (i.e., treat­ing phe­nom­e­nal prop­er­ties as illu­sory) for moral pa­tient­hood?

  • What eth­i­cal im­pli­ca­tions would emerge if con­vinc­ing ev­i­dence is ob­tained for wide­spread in­ver­te­brate con­scious­ness?

  • Is the case for in­ver­te­brate con­scious­ness a weak­ened form of Pas­cal’s mug­ging[6]?

  • In gen­eral, progress on a well-jus­tified gen­eral the­ory of con­scious­ness is needed. Such a the­ory can bet­ter con­tribute to mak­ing ap­pro­pri­ate at­tri­bu­tions of con­scious­ness to non­hu­man in­di­vi­d­u­als. How­ever, in the mean­time, what can be our best guesses about the dis­tri­bu­tion of phe­nom­e­nal con­scious­ness across differ­ent in­ver­te­brate taxa?

De­spite un­cer­tainty, there is an in­creas­ing in­ter­est in an­i­mal con­scious­ness from a range of philo­soph­i­cal per­spec­tives. A di­alogue be­tween philoso­phers of mind, ethi­cists, and philoso­phers of sci­ence can en­rich fu­ture work on this com­plex prob­lem. In this sense, out­reach­ing aca­demics of these ar­eas can con­tribute to get a bet­ter grasp of the cur­rent de­bate about phe­nom­e­nal con­scious­ness, con­scious­ness-de­rived moral pa­tient­hood, and the moral im­pli­ca­tions of the above.


Given our cur­rent un­cer­tainty about whether in­ver­te­brates are sen­tient or not, sup­port­ing the cause of in­ver­te­brate welfare means, at pre­sent, pro­mot­ing ad­di­tional re­search. In this re­gard, we ex­plored two ar­eas that de­serve fur­ther in­ves­ti­ga­tion: (i) in­ver­te­brate sen­tience and (ii) philo­soph­i­cal re­search into con­scious­ness. Mak­ing progress in these fields is es­sen­tial for ad­dress­ing key ques­tions about in­ver­te­brate and non­hu­man con­scious­ness in an em­piri­cal-sci­en­tific, yet philo­soph­i­cally so­phis­ti­cated, way.

If re­search in those ar­eas al­lows us to de­ter­mine that there is an im­por­tant prob­a­bil­ity that in­ver­te­brates of cer­tain species are con­scious, this work will di­rectly in­form fur­ther re­search about new pos­si­ble in­ter­ven­tions and sug­gest which available in­ter­ven­tions are the most press­ing. This is­sue will be ad­dressed in an up­com­ing post.


This es­say is a pro­ject of Re­think Pri­ori­ties.

It was writ­ten by Daniela R. Wald­horn. Thanks to Eze Paez, Gavin Tay­lor, Ja­son Schukraft, Mar­cus A. Davis, Ma­tias Vasquez, Peter Hur­ford, Si­mon Lied­holm, and Zach Fre­itas-Groff for their con­tri­bu­tion.

If you like our work, please con­sider sub­scribing to our newslet­ter. You can see all our work to date here.


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  1. E.g., fruit flies, C. el­e­gans, crayfish, sea hares and oc­to­puses. See our In­ver­te­brate Sen­tience Table and In­ver­te­brate Sen­tience: Sum­mary of find­ings, Part 2. ↩︎

  2. Au­to­tomy or self-am­pu­ta­tion is the be­hav­ior whereby an an­i­mal sheds or dis­cards a part of the body (e.g. the tail of a lizard), usu­ally as a self-defense mechanism to elude a preda­tor’s grasp or to dis­tract the preda­tor and thereby al­low es­cape. ↩︎

  3. Ad­di­tion­ally, it should be con­sid­ered that “analo­gous yet dis­parate struc­tures have evolved through­out the an­i­mal king­dom. For ex­am­ple, the com­pound eye of some in­ver­te­brates is strik­ingly differ­ent in form from the mam­malian eye, yet they both achieve the same func­tion–they al­low the an­i­mal to per­ceive light. Parts of the ner­vous sys­tem of in­ver­te­brates that are not the an­te­rior brain are ca­pa­ble of con­trol­ling breath­ing, move­ment and learn­ing (e.g. oc­to­puses, cock­roaches)” (EFSA, 2005: 33). ↩︎

  4. I thank Gavin Tay­lor for these points. ↩︎

  5. It could be ar­gued, for in­stance, that sim­pler ner­vous sys­tems could provide more limited re­sources for cop­ing with nox­ious stim­uli and nega­tive ex­pe­riences such as pain. Hu­mans, for ex­am­ple, are able to ra­tio­nal­ise painful ex­pe­riences or to an­ti­ci­pate if they will not last long. It is likely that sim­pler or­ganisms do not have such mechanisms to deal with nega­tive ex­pe­riences, and thus, they might suffer more than, for in­stance, hu­mans. Like­wise, fear is likely to have a greater im­pact if the con­text and risk can­not be an­a­lyzed, as it may hap­pen in or­ganisms with sim­pler brains (Broom, 2013). As a con­se­quence, nox­ious stim­uli may cause greater nega­tive ex­pe­riences in sim­pler an­i­mals. ↩︎

  6. Pas­cal’s mug­ging is a thought-ex­per­i­ment demon­strat­ing a prob­lem in ex­pected util­ity max­i­miza­tion. A ra­tio­nal agent should choose ac­tions whose out­comes, when weighed by their prob­a­bil­ity, have higher util­ity. But some very un­likely out­comes may have very great util­ities, and these util­ities can grow faster than the prob­a­bil­ity diminishes. Hence the agent should fo­cus more on vastly im­prob­a­ble cases with im­plau­si­bly high re­wards. How­ever, the above leads to counter-in­tu­itive choices, and in­co­her­ences as the util­ity of ev­ery choice be­comes un­bounded. ↩︎