This video investigates the question of invertebrate sentience. Though the question is complicated, there is surprisingly strong evidence for the sentience of many invertebrates. We will see the evidence for their sentience ranges from very clear in the case of octopuses, compelling in the case of honeybees, and unclear in the case of snails.
Also available as a chapter of our companion ebook to the video course Introduction to wild animal suffering: A guide to the issues
We saw in the last section that being sentient means having felt experiences of the world — that is, there is something it is like to be a sentient animal. Experience is the important thing here. “Having experiences” has the same meaning as “being sentient.” We have also seen that, because we don’t know exactly what structures are necessary to give rise to consciousness, we can’t know for sure exactly which beings are sentient. However, there are some indicators for the presence of sentience that we can look for. Note that indicators of sentience are not proofs of sentience, and lack of them are not proof that sentience is not present. Indicators are simply different types of evidence that increase our confidence that sentience is present. An example is complex and diverse behaviors that seem to show learning and thinking. A weaker indicator would be the presence of complex features like eyes, which may suggest the capacity to have the experience of sight.
The question of sentience is more difficult when it comes to animals that are more numerous — that is, invertebrates. Invertebrates are animals that don’t have backbones and they are typically small. Invertebrates include arthropods (such as crustaceans and insects), mollusks (including cephalopods, snails, and bivalves), nematode worms, and many other types of animals. This is an especially important problem, not only because there’s a growing number of invertebrates being used for different human purposes, but also because their numbers in the wild are staggering.1 Getting a better idea of which of them might be sentient will help us to improve our estimates of how to best make a difference for them.2
To examine this question, we will now look at how different criteria for sentience are satisfied by different types of invertebrate animals.
Cephalopods are a unique class of mollusks with very complex nervous systems. Octopuses have up to 500 million neurons. Electroencephalography recordings of octopuses and cuttlefish showed that electrical activity varied with brain states in similar ways to mammals; this is considered an indicator of consciousness.3 They also display very complex behaviors. The evidence is therefore quite powerful that they are sentient. They were explicitly mentioned as an example of conscious beings in the Cambridge Declaration of Consciousness in 2012 by a prominent group of scientists.4 Due to this, we do not need to discuss the case of these animals in detail, and can instead consider other cases where sentience is less clear. An example is arthropods.
Arthropods are invertebrates with hard external skeletons, and many limbs that have multiple joints. Examples include insects (such as bees and flies), crustaceans (such as crabs and lobsters), and spiders. Arthropods generally live in complex, demanding worlds, where it seems like consciousness would have adaptive value. A key function of consciousness might be to create a picture of the world where potential options can be traded off against each other and actions can be selected.
The scientific evidence on whether insects and other arthropods meet the criteria for consciousness isn’t complete. However, the evidence that does exist tends to show that they do satisfy these criteria.5 Many people think insects aren’t conscious, maybe because they are so small and people know little about them. But, in fact, some insects are capable of some pretty complex behaviors and traits, many of which most people are unaware of. We’ll see some examples of behaviors that, if we saw them in larger animals, most of us would think provided good evidence of consciousness.
There is more evidence that fruit flies are conscious than there is for many other invertebrates. Fruit flies have smaller brains than honeybees, and their minds may be closer to that of the average insect, so evidence of sentience in them would make the case stronger for other insects. There is evidence that fruit flies respond in a non-reflexive way that is reminiscent of anxiety. When there is a shadow overhead (a possible predator), they will often stop eating and fly away, but when they are very hungry, they sometimes decide to stay and eat.6 This suggests that they take various positive and negative factors into consideration and come to an overall decision, which seems like a key function that consciousness plays. It also suggests fear in them.
For insects with simpler behaviors and nervous systems, we might not know if their behavior and physiology is sufficient for sentience. But we can consider an argument by analogy: other insects have quite complex behaviors. A clear example of this is bees. Their behavior, including their famous waggle dance, which is used to communicate with other bees, leads us to think that they are conscious. Because of the similarity of their nervous systems, if bees are conscious, then it could follow that other insects are conscious, too. There is also evidence of various types of complex behavior in ants, including some level of flexible tool use.7
Less is known about arachnids than insects. However, the evidence that does exist indicates that their nervous systems are of similar size, complexity, and centralization to those of insects. Therefore, it might make sense to infer that if insects are sentient, then arachnids are sentient too.
In the case of crustaceans, the available evidence suggests that they are conscious. Some of these animals show a deliberate and non-reflexive response to noxious stimuli, which is suggestive of consciousness.8 For example, crabs show evidence of nursing, rubbing, and guarding wounds. This appears to be a long term, non-reflexive response to injury that is plausibly a key reason why suffering evolved. Another example is hermit crabs. Hermit crabs must find new shells to live in as they grow. When they choose a shell that injures them, they don’t automatically give up that shell, but they will change shells as soon as they have another option.9 In crayfish, there is evidence for a behavioral state that looks like anxiety as it is expressed in conscious animals.
One common argument against arthropods being sentient is that their brains might be too small to be able to support sentience, which seems like a complex thing.10 However, we don’t have a good understanding of how complex the physiological basis of consciousness is. The basic experiences of pain and pleasure might be quite simple, as they don’t require complex thoughts. Also, insects can perform some complex behaviors with such small brains, so it’s not clear why they couldn’t also be conscious.
Some people believe that arthropods — insects, for example — have very inflexible behavior. They believe that insects only have preset and rigid responses to stimuli. If this were true, arthropods would probably have little need for consciousness. However, while their behavior is less flexible than vertebrate behavior, it is still flexible.11 An example of this is bees’ waggle dance that was mentioned above. Bees communicate to other bees in their colony about the location of food by moving in ways that vary depending on different factors. They agitate their bodies according to how much food they found, they move in a certain direction to signal the direction of the food, and they move for a certain length of time to indicate how far away the food is.12 On the other hand, as of yet, little evidence of some behaviors of sentience such as wound guarding or limping has been observed in insects. There also isn’t much evidence that insects will selectively prioritize noxious stimuli (for example, by stopping all other activities to respond to a threat). But this is more a lack of available evidence, rather than positive evidence that insects do not do these things. Also, recall that these things are indicators of sentience, not requirements for it. When we say “indicators of consciousness,” we don’t mean things that are necessary for consciousness. They just indicate a certain likelihood of sentience, and some indicators are stronger evidence than others.
The problem becomes more complex if we consider other animals with a simpler structure — without a brain, but only some central nervous ganglia. This is the structure of many invertebrates, for example, bivalve mollusks (such as mussels and oysters) and gastropods (such as snails). The behavior that many of these animals display is very simple. It could be performed without requiring that the animals that display it be conscious. This may be the case with animals that stay attached to rocks or other surfaces without moving, such as bivalves or animals such as barnacles.
Bivalves can perform some movements, such as opening and closing their shells. But that doesn’t necessarily indicate sentience — these movements could be triggered in a more economical way in terms of energy by a stimulus-response mechanism. In fact, their behavior is not more complex than that of other beings without a centralized nervous system, such as carnivorous plants. However, many bivalves are mobile when they are young, and some, such as European fingernail clams, are more active, climbing on weeds to find a feeding spot. Some can swim and have image-forming eyes. An animal that has eyes might also have the experience of sight. And being able to experience something is what it means to be sentient. Some fingernail clams react with increased heart rates when under attack. Behind these movements, there may be more than simply stimulus-response, though their physiology leaves the question open. But we should keep in mind that these animals are much more closely related to sentient animals than, for example, plants are.
Snails have a slightly larger number of neurons and they are more active than bivalves. There is also more available evidence about whether snails are conscious, but this is mostly because bivalves have been less studied, and not because we have conclusive evidence that bivalves are not conscious. The differences between their nervous systems are small.
Overall, it seems clear that the evidence for arthropod consciousness is stronger, but snail and bivalve consciousness are possibilities that should not be dismissed.
Nematode worms are a possible edge case of consciousness. It is unclear whether they could be conscious. They have a small number of neurons — only around 300−400. Yet, they have what is known as a circumoral brain, which is a nerve ring, though it is not known whether this is enough for sentience. In addition, there are some indications that they are conscious, including evidence that they go into a fear-like state when they smell the odor of a predator.13
It is sometimes claimed that invertebrates could not experience conscious pain because they do not have nociception, which is the ability to detect damaging stimuli. Yet, specialized nociceptors have been found in a number of invertebrates. Though nociception alone does not determine whether an animal can feel pain, it plays a key role in the experience of pain in many animals. There are also invertebrates where nociceptors have not been found, but they still show the ability to detect noxious stimuli by other mechanisms. If they can detect it, it could potentially be translated into an experience of pain.14
There are strong reasons to give animals of uncertain sentience the benefit of the doubt. If we treat them as though they are sentient when they are not, we might waste some resources, but nothing too great. On the other hand, if they are sentient but we treat them as though they are not, then we might cause or permit great harm.15
All this is relevant because human beings often harm not only large animals, but especially small ones, such as many invertebrates, in very high numbers. When it comes to caring about wild animals, it is not only large, emblematic animals that we should be concerned about, but also small animals like invertebrates, which make up the majority of animals, and tend to have shorter and more precarious lives.
1 Knutsson, S. (2015) The moral importance of small animals, master’s thesis, Gothenburg: University of Gothenburg.
2 Carere, C. & Mather, J. (eds.) (2019) The welfare of invertebrate animals, Dordrecht: Springer. See also Mather, J. A. (2001) “Animal suffering: An invertebrate perspective”, Journal of Applied Animal Welfare Science, 4, pp. 151-156; Horvath, K.; Angeletti, D.; Nascetti, G. & Carere, C. (2013) “Invertebrate welfare: An overlooked issue”, Annali dell´Istituto superiore di sanità, 49, pp. 9-17.
3 Hochner, B.; Shomrat, T. & Fiorito, G. (2006) “The octopus: A model for a comparative analysis of the evolution of learning and memory mechanisms”, The Biological Bulletin, 210, pp. 308-317. See also Godfrey-Smith, P. (2016) Other minds: The octopus, the sea, and the deep origins of consciousness, New York: Farrar, Straus and Giroux.
4 Low, P.; Panksepp, J.; Reiss, D.; Edelman, D.; Van Swinderen, B. & Koch, C. (2012) The Cambridge Declaration on Consciousness, http://fcmconference.org/img/ CambridgeDeclarationOnConsciousness.pdf [accessed on 14 August 2019].
5 Gherardi, F. (2009) “Behavioural indicators of pain in crustacean decapods”, Annali dell’Istituto Superiore di Sanità, 45, pp. 432-438; Barron, A. B. & Klein, C. (2016) “What insects can tell us about the origins of consciousness”, Proceedings of the National Academy of Sciences, 113, pp. 4900-4908.
6 Gibson, W. T.; Gonzalez, C. R.; Fernandez, C.; Ramasamy, L.; Tabachnik, T.; Du, R. R.; Felsen P. D.; Maire, M. R.; Perona, P. & Anderson, D. J. (2015) “Behavioral responses to a repetitive visual threat stimulus express a persistent state of defensive arousal in Drosophila”, Current Biology, 25, pp. 1401-1415.
7 Maák, I.; Lőrinczi, G.; Le Quinquis, P.; Módra, G.; Bovet, D.; Call, J. & d’Ettorre, P. (2017) “Tool selection during foraging in two species of funnel ants”, Animal Behaviour, 123, pp. 207-216.
8 McCambridge, C.; Dick, J. T. & Elwood, R. W. (2016) “Effects of autotomy compared to manual declawing on contests between males for females in the edible crab cancer pagurus: implications for fishery practice and animal welfare”, Journal of Shellfish Research, 35, pp. 1037-1044.
9 Elwood, R. W. & Appel, M. (2009) “Pain experience in hermit crabs?”, Animal Behaviour, 77, pp. 1243-1246.
10 Adamo, S. A. (2016) “Do insects feel pain? A question at the intersection of animal behaviour, philosophy and robotics”, Animal Behaviour, 118, pp. 75-79.
11 See regarding this Keijzer, F. (2013) “The Sphex story: How the cognitive sciences kept repeating an old and questionable anecdote”, Philosophical Psychology, 26, pp. 502-519.
12 Griffin, D. R. & Speck, G. B. (2004) “New evidence of animal consciousness”, Animal cognition, 7, pp. 5-18.
13 Liu, Z.; Kariya, M. J.; Chute, C. D.; Pribadi, A. K.; Leinwand, S. G.; Tong, A.; Curran, K. P.; Bose, N.; Schroeder, F. C.; Srinivasan, J. & Chalasani, S. H. (2018) “Predator-secreted sulfolipids induce defensive responses in C. elegans”, Nature Communications, 9, a. 1128.
14 Eisemann, C. H.; Jorgensen, W. K.; Merritt, D. J.; Rice, M. J.; Cribb, B. W.; Webb, P. D. & Zalucki, M. P. (1984) “Do insects feel pain?—A biological view”, Experientia, 40, pp. 164-167.
15 Birch, J. (2017) “Animal sentience and the precautionary principle”, Animal Sentience: An Interdisciplinary Journal on Animal Feeling, 2/16, a. 1.