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File:Carcharhinus galapagensis hooked.jpg

A Galapagos shark hooked by a fishing boat

Pain is defined by the International Association for the Study of Pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage."[1] However, for non-human animals, it is harder, if even possible, to know whether an emotional experience has occurred.[2] Therefore, this concept is often excluded in definitions of pain in animals, such as that provided by Zimmerman: "an aversive sensory experience caused by actual or potential injury that elicits protective motor and vegetative reactions, results in learned avoidance and may modify species-specific behaviour, including social behaviour."[3]

The standard measure of pain in a human is that person's testimony (see Pain scale), because only they can know the pain's quality and intensity, and the degree of suffering. Animals without human language cannot report their feelings, and whether they are conscious and capable of suffering has been a matter of some debate.

Physical pain is both an objective physiological process and a subjective conscious experience. The physiological component usually involves the transmission of a signal along a chain of nerve fibers from the site of a noxious stimulus at the periphery to the spinal cord and brain. This process may evoke a reflex response generated at the spinal cord and not involving the brain, such as flinching or withdrawal of a limb, and it may also involve brain activity, such as registering the location, intensity, quality and unpleasantness of the stimulus in various parts of the brain. This nervous activity is called nociception and it is found, in one form or another, across all major animal taxa.[4] Nociception can be observed using modern imaging techniques; and a physiological and behavioral response to nociception can be detected. The subjective component of pain involves conscious awareness of both the sensation (its location, intensity, quality, etc.) and the unpleasantness (the aversive, negative affect). The brain processes underlying conscious awareness of the unpleasantness (suffering), are not well understood.

To address the problem of assessing the capacity of other species to experience the affective state of pain (to suffer), we resort to argument-by-analogy. This is based on the principle that if an animal responds to a stimulus in a similar way to ourselves, it is likely to have had an analogous experience. If we stick a pin in a chimpanzee's finger and she rapidly withdraws her hand, we use argument-by-analogy and infer that like us, she felt pain. If we are consistent, we should also infer a cockroach experiences the same when it writhes after being stuck with a pin.[5][6] Analogous to humans, when given a choice of feeds, rats[7] and chickens[8] with clinical symptoms of pain will consume more of an analgesic-containing feed than animals not in pain. Additionally, the consumption of the analgesic carprofen in lame broiler chickens was positively correlated to the severity of lameness, and consumption resulted in an improved gait. Limitations of argument-by-analogy are that physical reactions may neither determine nor be motivated by mental states, and the approach is subject to criticism of anthropomorphic interpretation. For example, a single-celled organism such as an amoeba may writhe after being exposed to noxious stimuli despite the absence of nociception.


The idea that animals might not experience pain or suffering as humans do traces back at least to the 17th-century French philosopher, René Descartes, who argued that animals lack consciousness.[9][10][11]Researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain.[12] In his interactions with scientists and other veterinarians, Bernard Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain.[12] Some authors say that the view that animals feel pain differently is now a minority view.[9] Academic reviews of the topic are more equivocal, noting that, although it is likely that some animals have at least simple conscious thoughts and feelings,[13] some authors continue to question how reliably animal mental states can be determined.[10][14]

In different species[]

The ability to experience pain in an animal, or another human for that matter, cannot be determined directly but it may be inferred through analogous physiological and behavioral reactions.[15] Although many animals share similar mechanisms of pain detection to those of humans, have similar areas of the brain involved in processing pain, and show similar pain behaviours, it is notoriously difficult to assess how animals actually experience pain.[16]


Nociceptive nerves, which preferentially detect (potential) injury-causing stimuli, have been identified in a variety of animals, including invertebrates. The medicinal leech, Hirudo medicinalis, and sea slug are classic model systems for studying nociception.[16] Many other vertebrate and invertebrate animals also show nociceptive reflex responses similar to our own.


Many animals also exhibit more complex behavioural and physiological changes indicative of the ability to experience pain: they eat less food, their normal behaviour is disrupted, their social behaviour is suppressed, they may adopt unusual behaviour patterns, they may emit characteristic distress calls, experience respiratory and cardiovascular changes, as well as inflammation and release of stress hormones.[16]

Some criteria that may indicate the potential of another species to feel pain include:[17]

  1. Has a suitable nervous system and sensory receptors
  2. Physiological changes to noxious stimuli
  3. Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy
  4. Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics
  5. Shows trade-offs between stimulus avoidance and other motivational requirements
  6. Shows avoidance learning
  7. High cognitive ability and sentience



Main article: Pain in fish

Fish have been shown to have sensory neurons that are sensitive to damaging stimuli and are physiologically identical to human nociceptors.[18] Behavioural and physiological responses to a painful event appear comparable to those seen in amphibians, birds, and mammals, and administration of an analgesic drug reduces these responses in fish.[19]

Animal protection advocates have raised concerns about the possible suffering of fish caused by angling. In light of recent research, some countries, like Germany, have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish.[20]


Main article: Pain in invertebrates
Further information: Pain in crustaceans

Though it has been argued that most invertebrates do not feel pain,[21][22][23] there is some evidence that invertebrates, especially the decapod crustaceans (e.g. crabs and lobsters) and cephalopods (e.g. octopuses), exhibit behavioural and physiological reactions indicating they may have the capacity for this experience.[24][5][6] Nociceptors have been found in nematodes, annelids and molluscs.[25] Most insects do not possess nociceptors,[26][27][28] one known exception being the fruit fly.[29] In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opiate receptors. Opioid peptides and opiate receptors occur naturally in nematodes,[30][31] molluscs,[32][33] insects[34][35] and crustaceans.[36][37] The presence of opioids in crustaceans has been interpreted as an indication that lobsters may be able to experience pain,[38][39] although it has been claimed "at present no certain conclusion can be drawn".[38]

One suggested reason for rejecting a pain experience in invertebrates is that invertebrate brains are too small. However, brain size does not necessarily equate to complexity of function.[40] Moreover, weight for body-weight, the cephalopod brain is in the same size bracket as the vertebrate brain, smaller than that of birds and mammals, but as big as or bigger than most fish brains.[41][42]

In medicine and research[]

Veterinary medicine[]

Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.[43]


Dolorimetry (dolor: Latin: pain, grief) is the measurement of the pain response in animals, including humans. It is practiced occasionally in medicine, as a diagnostic tool, and is regularly used in research into the basic science of pain, and in testing the efficacy of analgesics. Non-human animal pain measurement techniques include the paw pressure test, tail flick test and hot plate test.

Laboratory animals[]

Animals are kept in laboratories for a wide range of reasons, some of which may involve pain, suffering or distress, whilst others (e.g. many of those involved in breeding) will not. The extent to which animal testing causes pain and suffering in laboratory animals is the subject of much debate.[44] Marian Stamp Dawkins defines "suffering" in laboratory animals as the experience of one of "a wide range of extremely unpleasant subjective (mental) states."[45] The U.S. National Research Council has published guidelines on the care and use of laboratory animals,[46] as well as a report on recognizing and alleviating pain in vertebrates.[47] The United States Department of Agriculture defines a "painful procedure" in an animal study as one that would "reasonably be expected to cause more than slight or momentary pain or distress in a human being to which that procedure was applied."[48] Some critics argue that, paradoxically, researchers raised in the era of increased awareness of animal welfare may be inclined to deny that animals are in pain simply because they do not want to see themselves as people who inflict it.[49] Animal research with the potential to cause pain is regulated by the Animal Welfare Act of 1966 in the US, and research likely to cause "pain, suffering, distress or lasting harm" is regulated by the Animals (Scientific Procedures) Act 1986 in the UK.

Severity scales[]

To date (2011), eleven countries have national severity classification systems relating to pain and suffering experienced by animals used in research: Australia, Canada, Finland, Germany, The Republic of Ireland, The Netherlands, New Zealand, Poland, Sweden, Switzerland, and the UK. The US also has a mandated national scientific animal-use classification system, but it is markedly different from other countries in that it reports on whether pain-relieving drugs were required and/or used.[50] The first severity scales were implemented in 1986 by Finland and the UK. The number of severity categories ranges between 3 (Sweden and Finland) and 9 (Australia). In the UK, research projects are classified as "mild", "moderate", and "substantial" in terms of the suffering the researchers conducting the study say they may cause; a fourth category of "unclassified" means the animal was anesthetized and killed without recovering consciousness. It should be remembered that in the UK system, many research projects (e.g. transgenic breeding, feeding distasteful food) will require a license under the Animals (Scientific Procedures) Act 1986, but may cause little or no pain or suffering. In December 2001, 39 percent (1,296) of project licenses in force were classified as "mild", 55 percent (1,811) as "moderate", two percent (63) as "substantial", and 4 percent (139) as "unclassified".[51] In 2009, of the project licenses issued, 35 percent (187) were classified as "mild", 61 percent (330) as "moderate", 2 percent (13) as "severe" and 2 percent (11) as unclassified.[52]

In the US, the Guide for the Care and Use of Laboratory Animals defines the parameters for animal testing regulations. It states, "The ability to experience and respond to pain is widespread in the animal kingdom...Pain is a stressor and, if not relieved, can lead to unacceptable levels of stress and distress in animals."[53] The Guide states that the ability to recognize the symptoms of pain in different species is essential for the people caring for and using animals. Accordingly, all issues of animal pain and distress, and their potential treatment with analgesia and anesthesia, are required regulatory issues for animal protocol approval.

See also[]


  1. IASP Pain Terminology
  2. Andrew right, A Criticism of the IASP's Definition of Pain,
  3. Zimmerman M., (1986). Physiological mechanisms of pain and its treatment. Klinische Anaesthesiol Intensivether, 32:1–19
  4. Sneddon, L.U., (2004). Evolution of nociception in vertebrates: comparative analysis of lower vertebrates. Brain Research Reviews, 46: 123–130
  5. 5.0 5.1 Sherwin, C.M., (2001). Can invertebrates suffer? Or, how robust is argument-by-analogy? Animal Welfare, 10 (supplement): S103-S118 Cite error: Invalid <ref> tag; name "Sherwin, 2001" defined multiple times with different content
  6. 6.0 6.1 Elwood, R.W., (2011). Pain and suffering in invertebrates? Institute of Laboratory Animal Resources Journal, 52(2): 175-84 [1] Cite error: Invalid <ref> tag; name "Elwood, 2011" defined multiple times with different content
  7. Colpaert, F.C., Tarayre, J.P., Alliaga, M., Slot, L.A.B., Attal N. and Koek, W. (2001). Opiate self-administration as a measure of chronic nociceptive pain in arthritic rats. Pain, 91: 33-45
  8. Danbury, T.C., Weeks, C.A. Chambers, J.P., Waterman-Pearson, A.E. and Kestin. S.C. (2000). Self-selection of the analgesic drug carprofen by lame broiler chickens. Veterinary Record, 14:307-311
  9. 9.0 9.1 Carbone, Larry. '"What Animal Want: Expertise and Advocacy in Laboratory Animal Welfare Policy. Oxford University Press, 2004, p. 149.
  10. 10.0 10.1 The Ethics of research involving animals Nuffield Council on Bioethics, Accessed 27 February 2008 Template:Wayback
  11. Talking Point on the use of animals in scientific research, EMBO reports 8, 6, 2007, pp. 521–525
  12. 12.0 12.1 Rollin, Bernard. The Unheeded Cry: Animal Consciousness, Animal Pain, and Science. New York: Oxford University Press, 1989, pp. xii, 117-118, cited in Carbone 2004, p. 150.
  13. (2004). New evidence of animal consciousness.. Animal cognition 7 (1): 5–18.
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  15. Abbott FV, Franklin KB, Westbrook RF (January 1995). The formalin test: scoring properties of the first and second phases of the pain response in rats. Pain 60 (1): 91–102.
  16. 16.0 16.1 16.2 Sneddon, Lynne Can animals feel pain?. PAIN. URL accessed on 18 March 2012.
  17. Elwood, R.W., Barr, S. and Patterson, L., (2009) Pain and stress in crustaceans? Applied Animal Behaviour Science, 118 (3): 128–136.
  18. L.U. Sneddon et al. Do fishes have nociceptors? Evidence for the evolution of a vertebrate sensory system.. National Center for Biotechnology Information. URL accessed on 18 March 2012.
  19. Sneddon L.U. 2009 Pain and Distress in Fish. ILAR J. 50 (4), 338-342.
  20. Leake, J. "Anglers to Face RSPCA Check," The Sunday Times – Britain, 14 March 2004
  21. Eisemann C.H., Jorgensen W.K., Merritt, D.J. Rice, M.J. Cribb, B.W. Webb P.D. and Zalucki M.P. (1984). Do insects feel pain? - A biological view. Experentia, 40:164-167
  22. "Do Invertebrates Feel Pain?", The Senate Standing Committee on Legal and Constitutional Affairs, The Parliament of Canada Web Site, accessed 11 June 2008.
  23. Jane A. Smith (1991). A question of pain in invertebrates. ILAR Journal 33 (1–2).
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  25. St John Smith, E. and Lewin, G.R., (2009). Nociceptors: a phylogenetic view. Journal of Comparative Physiology A Neuroethology Sensory Neural and Behavioral Physiology, 195: 1089-1106
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  27. Lockwood JA (1987). The moral standing of insects and the ethics of extinction. The Florida Entomologist 70 (1): 70–89.
  28. 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. Cellular and Molecular Life Sciences 40: 1420–1423.
  29. Tracey, J., W. Daniel, R. I. Wilson, G. Laurent, and S. Benzer. 2003. painless, a Drosophila gene essential for nociception. Cell 113: 261-273.
  30. Wittenburg, N. and Baumeister, R., (1999). Thermal avoidance in Caenorhabditis elegans: an approach to the study of nociception. Proceedings of the National Academy of Sciences USA, 96: 10477–10482
  31. Pryor, S.C., Nieto, F., Henry, S. and Sarfo, J., (2007). The effect of opiates and opiate antagonists on heat latency response in the parasitic nematode Ascaris suum. Life Sciences, 80: 1650–1655
  32. Dalton, L.M. and Widdowson, P.S., (1989). The involvement of opioid peptides in stress-induced analgesia in the slug Arion ater. Peptides:, 10:9-13
  33. Kavaliers, M. and Ossenkopp, K.-P., (1991). Opioid systems and magnetic field effects in the land snail, Cepaea nemoralis. Biological Bulletin, 180: 301-309
  34. Dyakonova, V.E., Schurmann, F. and Sakharov, D.A., (1999) Effects of serotonergic and opioidergic drugs on escape behaviors and social status of male crickets. Naturwissenschaften, 86: 435–437
  35. Zabala, N. and Gomez, M., (1991). Morphine analgesia, tolerance and addiction in the cricket, Pteronemobius. Pharmacology, Biochemistry and Behaviour, 40: 887-891
  36. Lozada, M., Romano, A. and Maldonado, H., (1988). Effect of morphine and naloxone on a defensive response of the crab Chasmagnathus granulatus. Pharmacology, Biochemistry and Behavior, 30: 635–640
  37. Maldonado, H. and Miralto, A., (1982). Effects of morphine and naloxone on a defensive response of the mantis shrimp (Squilla mantis). Journal of Comparative Physiology, A, 147: 455–459
  38. 38.0 38.1 L. Sømme (2005). Sentience and pain in invertebrates: Report to Norwegian Scientific Committee for Food Safety. Norwegian University of Life Sciences, Oslo.
  39. (2005) Cephalopods and decapod crustaceans: their capacity to experience pain and suffering, Advocates for Animals.Template:Dead url
  40. DOI:10.1016/j.cub.2009.08.023
    This citation will be automatically completed in the next few minutes. You can jump the queue or expand by hand
  41. Cephalopod brain size
  42. Packard A (1972) Cephalopods and fish: the limits of convergence" pp.266-7 Biological Reviews, 47: 241–307.
  43. Viñuela-Fernández I, Jones E, Welsh EM, Fleetwood-Walker SM (September 2007). Pain mechanisms and their implication for the management of pain in farm and companion animals. Vet. J. 174 (2): 227–39.
  44. Duncan IJ, Petherick JC. "The implications of cognitive processes for animal welfare", J. Anim. Sci., volume 69, issue 12, 1991, pp. 5017–22. pmid 1808195; Curtis SE, Stricklin WR. "The importance of animal cognition in agricultural animal production systems: an overview", J. Anim. Sci.. volume 69, issue 12, 1991, pp. 5001–7. pmid 1808193
  45. Stamp Dawkins, Marian. "Scientific Basis for Assessing Suffering in Animals," in Singer, Peter. In Defense of Animals: The Second Wave. Blackwell, 2006. p. 28.
  46. Template:Cite report
  47. Template:Cite report
  48. Animal Welfare; Definitions for and Reporting of Pain and Distress", Animal Welfare Information Center Bulletin, Summer 2000, Vol. 11 No. 1-2, United States Department of Agriculture.
  49. Carbone 2004, p. 151.
  50. Fenwick, N., Ormandy, E., Gauthier, C. and Griffin, G. (2011). Classifying the severity of scientific animal use: a review of international systems. Animal Welfare, 20: 281-301
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  52. Home Office Statistics:accessdate=31 October 2011.
  53. Guide for the Care and Use of Laboratory Animals, ILAR, National Research Council, 1996 copyright, pg 64

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