Today is the 10th anniversary of the Cambridge Declaration on Consciousness. On July 7, 2012, a group of prominent scientists from around the world came together to sign the Declaration, in which they affirmed that the evidence indicates that many types of nonhuman animals have the capacity for consciousness.
In the Declaration, they state:
“Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.”1
What do they mean by “neurological substrates?” It refers to foundational structures in a nervous system that makes it possible to generate consciousness. There is no single substrate or structure that is necessary for consciousness across all types of animals. The structures that can give rise to consciousness are different in different animals.2 Human-like brain structure is not required for consciousness.3 One requirement is a certain degree of centralization of the nervous system,4 but this can take many forms, not all of which are known.5
Scientists agree that many animals, including many invertebrates, have the capacity for consciousness. Consciousness means having subjective experiences or awareness.6 Consciousness makes it possible to have feelings that we can experience as good or bad. People often equate consciousness with self-awareness or the ability to think and reason, but these things are not a necessary part of consciousness.7 A being is conscious, also known as sentient, if they are able to experience things in their environment as positive or negative. Behaviors that involve attention, integrating, and storing information to apply it to future scenarios are hard to explain without consciousness, so when we see these things, we attribute consciousness to the animals exhibiting these behaviors.8 However, at its most basic level, consciousness just means being able to be aware of anything at all without further reflection on that awareness,9 whether or not one acts on it or even has a concept of self.
It may seem obvious to us that mammals, birds, and octopuses are conscious because of the way they act and the way they react to pleasant and unpleasant things. When he heard about the Cambridge Declaration, ethologist Marc Bekoff said he thought it was a joke because animal consciousness is something so obvious to anyone who works with or lives with nonhuman animals.10
So why did it take so long for scientists to declare this, and why was their wording so careful? Instead of directly claiming that the nonhuman animals they mentioned are conscious, they said that other animals have “the substrates of conscious states along with the capacity to exhibit intentional behaviors.”
Actually, at this point we cannot say anything definitive about consciousness and who is conscious until we understand how consciousness arises physically. We still don’t know the exact mechanisms that generate consciousness in humans or in any other animals.11 This is called the hard problem of consciousness. Technically, we can’t even be sure that anyone else besides ourselves is truly conscious, because we have no way to actually prove it. However, we do have evidence and indicators, like brain structure, physiology, and behavior. If we see a brain structure that allows for centralized processing, that is a good indicator, because in order to feel anything, nerve impulses must be able to travel back and forth from the brain to other parts of the body. If we see nociceptors, which help register pain, and receptors for opioids, which reduce pain, those also indicate consciousness because only a conscious being can feel pain.12 Behaviors that look like pain reactions or focused attention to a novel task also indicate consciousness.13 Note that an “indicator” of consciousness should not be mistaken for “proof” of consciousness. Indicators are evidence of consciousness being present, but not proof.
Are all animals conscious? We can’t be certain of this until we understand exactly what gives rise to consciousness or all the ways it can be manifested. But we can be pretty confident in saying that sponges are not conscious. They are the simplest of animals and do not have any nervous system or organs with which to perceive the world. They have specialized cells that allow them to react to stimuli, but that is all. Other animals, like sea stars, have nervous systems but not a central brain so they can gather information about their environment but it is not clear if they can feel pain. Octopuses have central brains but also have distributed nervous systems with the majority of their neurons in their body and tentacles, rather than in their brains.14 Scientists are uncertain the degree to which the tentacles act independently and the ability of their brains to control them, but it is clear that octopuses have a brain that does some centralized processing of the information received from the tentacles.15
Many other animals have complex and centralized nervous systems that process information centrally. This demonstrated capacity does not, however, provide proof, but provides us with a strong indication of consciousness. Proof that anyone is conscious, including us, will not be available until we solve the hard problem of consciousness.
Given the information that we have, we should give nonhuman animals moral consideration and give them the benefit of the doubt when we are uncertain.
Criteria for recognizing sentience
Invertebrate sentience: A review of the neuroscientific literature
An illustrated physiology of nervous systems in invertebrates
1 Low, P.; Panksepp, J.; Reiss, D.; Edelman, D.; Van Swinderen, B. & Koch, C. (2012) “The Cambridge Declaration on Consciousness”, Francis Crick Memorial Conference [accessed on 7 July 2022].
2 Smith, J. A. (1991) “A question of pain in invertebrates”, ILAR Journal, 33, pp. 25-31 [accessed on 7 July 2022]. Mather, J. A. (2001) “Animal suffering: An invertebrate perspective”, Journal of Applied Animal Welfare Science, 4, pp. 151-156. Mather, J. A. & Anderson, R. C. (2007) “Ethics and invertebrates: A cephalopod perspective”, Diseases of Aquatic Organisms, 75, pp. 119-129 [accessed on 7 July 2022]. Crook, R. J. & Walters, E. T. (2011) “Nociceptive behavior and physiology of molluscs: Animal welfare implications”, ILAR Journal, 52, pp. 185-195. Wigglesworth, V. B. (1980) “Do insects feel pain?”, Antenna, 4, pp. 8-9. Allen-Hermanson, S. (2008) “Insects and the problem of simple minds: Are bees natural zombies?”, Journal of Philosophy, 105, pp. 389-415.
3 Allen-Hermanson, S. (2016) “Is cortex necessary?”, Animal Sentience, 1 (9) [accessed on 7 July 2022]. Damasio, A. & Carvalho, G. B. (2013) “The nature of feelings: Evolutionary and neurobiological origins”, Nature Reviews Neuroscience, 14, pp. 143-152. 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 [accessed on 7 July 2022].
4 Barr, M. M. & Garcia, L. R. (2006) “Male mating behavior”, in The C. Elegans Research Community, Wormbook (ed.) Wormbook, Pasadena: California Institute of Technology [accessed on 7 July 2022]. Feinberg, T. E. & Mallatt, J. (2013) “The evolutionary and genetic origins of consciousness in the Cambrian Period over 500 million years ago”, Frontiers in Psychology, 04 October [accessed on 7 July 2022]. Merker, B. (2005) “The liabilities of mobility: A selection pressure for the transition to consciousness in animal evolution”, Consciousness and Cognition, 14, pp. 89-114. Merker, B. (2007) “Consciousness without a cerebral cortex: A challenge for neuroscience and medicine”, Behavioral and Brain Sciences, 30, pp. 63-81. Morsella, E. (2005) “The function of phenomenal states: Supramodular interaction theory”, Psychological Review, 112, pp. 1000-1021.
5 Chittka, L. & Niven, J. (2009) “Are bigger brains better?”, Current Biology, 19, pp. R995-R1008 [accessed on 7 July 2022]. Klein, C. & Barron, A. B. (2016) “Insects have the capacity for subjective experience”, Animal Sentience, 1 (9) [accessed on 7 July 2022]. Gelperin, A. & Tank, D. W. (1990) “Odour-modulated collective network oscillations of olfactory interneurons in a terrestrial mollusc”, Nature, 345, pp. 437-440.
6 Nagel, T. (1974) “What is it like to be a bat?”, Philosophical Review, 83, pp. 435-450.
7 See Antony, M. V. (2002) “Concepts of consciousness, kinds of consciousness, meanings of ‘consciousness’”, Philosophical Studies, 109, pp. 1-16. Ben-Artzi, E.; Mikulincer, M. & Glaubman, H. (1995) “The multifaceted nature of self-consciousness: Conceptualization, measurement, and consequences”, Imagination, Cognition and Personality, 15, pp. 17-43.
8 Menzel, R. & Mercer. A (eds.) Neurobiology and behavior of honeybees, Berlin: Springer, p. 127. Núñez, J.; Almeida, L.; Balderrama, N. & Giurfa, M. (1997) “Alarm pheromone induces stress analgesia via an opioid system in the honeybee”, Physiology & Behaviour, 63, p. 78. 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.
9 Klein, C. & Barron, A. B. (2016) “Insects have the capacity for subjective experience”, op. cit.
10 Bekoff, M. (2013) “After 2,500 studies, it’s time to declare animal sentience proven”, Live Science, September 06 [accessed on 3 July 2022].
11 Chalmers, D. J. (1996) The conscious mind: In search of a fundamental theory, Oxford: Oxford University Press.
12 Sneddon, L. U. (2004) “Evolution of nociception in vertebrates: Comparative analysis of lower vertebrates”, Brain Research Reviews, 46, pp. 123-130. Jones, R. C. (2013) “Science, sentience, and animal welfare”, Biology and Philosophy, 28, pp. 1-30. Kavaliers, M.; Hirst, M. & Tesky, G. C. (1983) “A functional role for an opiate system in snail thermal behaviour”, Science, 220, pp. 99-101. Wilson, C. D.; Arnott, G. & Elwood, R. W. (2012) “Freshwater pearl mussels show plasticity of responses to different predation risks but also show consistent individual differences in responsiveness”, Behavioural Processes, 89, pp. 299-303.
13 Klein, C. & Barron, A. B. (2016) “Insects have the capacity for subjective experience”, op. cit.
14 See Sumbre, G.; Gutfreund, Y.; Fiorito, G.; Flash, T. & Hochner, B. (2001) “Control of octopus arm extension by a peripheral motor program”, Science, 293, pp. 1845-1848.
15 Gutnick, T.; Zullo, Letizia; Hochner, Binyamin; Kuba, Michael J. (2020) “Use of peripheral sensory information for central nervous control of arm movement by Octopus vulgaris”, Current Biology, 30 (21) [accessed on 7 July 2022].