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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 1/7 of their body mass (Seid et al., 2010). In humans, this ratio is only 1/40, which–in comparison with other mammals–is still impressive (1/100 in cats, 1/125 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.

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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. ↩︎

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Great work - thanks for writing this up!

The question of invertebrate sentience is surely important, but I'm not sure if further research on this is a top priority. Some relevant uncertainties:

  • Would further research significantly reduce uncertainty about invertebrate sentience? It seems that most people who thought about this have settled on something like "there is a significant chance that many invertebrate taxa are sentient, but we don't know for sure".
  • To what is society's lack of moral concern for invertebrates due to the belief that invertebrates are not sentient, rather than other factors (e.g. disgust reaction towards many invertebrates, or the difficulty of avoiding harm to insects in everyday life)?

Hi Tobias!

Thanks again for sharing your views. Regarding the role of further research, first, we should keep in mind that the scientific literature in invertebrate sentience is still scarce, and the extent to which invertebrates have been investigated varies. Thus, there are some particular species about which there is a comparatively great deal of knowledge (e.g., fruit flies). But for several other taxa, potentially consciousness-indicating features have not been investigated at all. It is unknown if many invertebrate taxa display particular anatomical, physiological, or behavioral functions that seem to be necessary for consciousness (e.g., nociceptors in honey bees). Especially in these cases, I think further research may significantly reduce current uncertainty.

Additionally, we should consider that the scientific community does not agree that many invertebrate taxa are sentient. Probably, the only exception are cephalopods. In this case, we have seen how further research has led to relevant ethical discussions and specific welfare measures on the use of live cephalopods in science (at least, in Europe). Some researchers believe that in the future, other invertebrate taxa will face similar challenges, and additional research will promote further discussions on invertebrate welfare. Similarly, new research would also contribute to identify the most determining factors in invertebrates' quality of life and prioritize forms of intervention.

Nevertheless, lack of scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent potential suffering –especially in cases of potential threats of serious or irreversible harms. Do you have other ideas about this?

Your question about society's lack of moral concern for invertebrates is very interesting. In general, existing knowledge about our attitudes towards invertebrates mostly looks into occurrent, spontaneous, or superficial responses that people tend to experience in the course of their everyday life (e.g., disgust, as you say). But, importantly, those attitudes do not necessarily reflect people's views on the moral consideration of invertebrates, or their actual behavior towards these animals. For example, even if someone feels disgust towards insects, that does not necessarily entail that she believes they are not sentient, that their pain does not matter or that using insecticide is morally innocuous.  Apparently, most people have not even thought about these issues as moral problems and, in turn, may hold a collection of unreflected ideas. If you're interested in this, I've just published a new post about our attitudes towards invertebrates and challenges.

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.

I don't understand how this follows. Wouldn't less intense experiences affect both suffering and pleasure equally?

Hi Tobias!

Thanks for commenting. You're right. All else equal, smaller organisms or those with less intense experiences may have relatively less positive and negative experiences. However, since smaller individuals are typically r-selected, while they can suffer in large numbers, they usually live relatively short lives. Thus, two shifts happen –one of which drives the net balance of suffering up, and the other which drives the net balance of suffering down:

  • First, as the more organisms in a species fail, the more cases of suffering there are relative to enjoyment.
  • Second, as the number of organisms which fail increases, based on an evolutionary constrained optimization, the degree each organism suffers may decrease. Evolution may put relatively less weight on suffering since suffering becomes more costly for genes in expectation the more organisms experience it. In the case of r-selected individuals (who are typically smaller), if indeed evolution is optimizing, then their shorter lifespan may serve to attenuate their suffering.

Because these two effects go in opposite directions, as the number of failing organisms rises, total suffering may come to exceed overall enjoyment, or the opposite may occur instead. In other words, while greater intensity of experience would lead to more suffering relative to enjoyment, a higher rate of evolutionary failure relative to success may have a surprisingly ambiguous effect.

For the time being, given this trade-off between number and degree of suffering, it is not possible to conclude whether suffering or enjoyment predominates in nature. Additionally, it should be noted that the question of how evolution produces positive and negative experiences is highly uncertain.

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