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Pain in animals

From Wikipedia, the free encyclopedia

 A Galapagos shark hooked by a fishing boat
A Galapagos shark hooked by a fishing boat

In humans, pain is a distressing feeling often caused by intense or damaging stimuli. Whether animals apart from humans also experience pain is often contentious despite being scientifically verifiable. The standard measure of pain in humans is how a person reports that pain, (for example, on a pain scale). "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] Only the person experiencing the pain can know the pain's quality and intensity, and the degree of suffering. 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] Non-human animals cannot report their feelings to language-using humans in the same manner as human communication, but observation of their behaviour provides a reasonable indication as to the extent of their pain. Just as with doctors and medics who sometimes share no common language with their patients, the indicators of pain can still be understood.

According to the U.S. National Research Council Committee on Recognition and Alleviation of Pain in Laboratory Animals, pain is experienced by many animal species, including mammals and possibly all vertebrates.[4]

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Humans know the surprising prick of a needle, the searing pain of a stubbed toe, and the throbbing of a toothache. We can identify many types of pain and have multiple ways of treating it. But what about other species? How do the animals all around us experience pain? It's important that we find out. We keep animals as pets, they enrich our environment, we farm many species for food, and we use them in experiments to advance science and human health. Animals are clearly important to us, so it's equally important that we avoid causing them unnecessary pain. For animals that are similar to us, like mammals, it's often obvious when they're hurting. But there's a lot that isn't obvious, like whether pain relievers that work on us also help them. And the more different an animal is from us, the harder it is to understand their experience. How do you tell whether a shrimp is in pain? A snake? A snail? In vertebrates, including humans, pain can be split into two distinct processes. In first, nerves and the skin sense something harmful and communicate that information to the spinal cord. There, motor neurons activate movements that make us rapidly jerk away from the threat. This is the physical recognition of harm called nociception, and nearly all animals, even those with very simple nervous systems, experience it. Without this ability, animals would be unable to avoid harm and their survival would be threatened. The second part is the conscious recognition of harm. In humans, this occurs when the sensory neurons in our skin make a second round of connections via the spinal cord to the brain. There, millions of neurons in multiple regions create the sensations of pain. For us, this is a very complex experience associated with emotions like fear, panic, and stress, which we can communicate to others. But it's harder to know exactly how animals experience this part of the process because most them can't show us what they feel. However, we get clues from observing how animals behave. Wild, hurt animals are known to nurse their wounds, make noises to show their distress, and become reclusive. In the lab, scientists have discovered that animals like chickens and rats will self-administer pain-reducing drugs if they're hurting. Animals also avoid situations where they've been hurt before, which suggests awareness of threats. We've reached the point that research has made us so sure that vertebrates recognize pain that it's illegal in many countries to needlessly harm these animals. But what about other types of animals like invertebrates? These animals aren't legally protected, partly because their behaviors are harder to read. We can make good guesses about some of them, like oysters, worms, and jellyfish. These are examples of animals that either lack a brain or have a very simple one. So an oyster may recoil when squirted with lemon juice, for instance, because of nociception. But with such a simple nervous system, it's unlikely to experience the conscious part of pain. Other invertebrate animals are more complicated, though, like the octopus, which has a sophisticated brain and is thought to be one of the most intelligent invertebrate animals. Yet, in many countries, people continue the practice of eating live octopus. We also boil live crawfish, shrimp, and crabs even though we don't really know how they're affected either. This poses an ethical problem because we may be causing these animals unnecessary suffering. Scientific experimentation, though controversial, gives us some clues. Tests on hermit crabs show that they'll leave an undesirable shell if they're zapped with electricity but stay if it's a good shell. And octopi that may originally curl up an injured arm to protect it will risk using it to catch prey. That suggests that these animals make value judgements around sensory input instead of just reacting reflexively to harm. Meanwhile, crabs have been known to repeatedly rub a spot on their bodies where they've received an electric shock. And even sea slugs flinch when they know they're about to receive a noxious stimulus. That means they have some memory of physical sensations. We still have a lot to learn about animal pain. As our knowledge grows, it may one day allow us to live in a world where we don't cause pain needlessly.


The experience of pain

Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required.[5] This is the ability to detect noxious stimuli which evoke a reflex response that rapidly moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not imply any adverse, subjective "feeling" – it is a reflex action. An example in humans would be the rapid withdrawal of a finger that has touched something hot – the withdrawal occurs before any sensation of pain is actually experienced.

The second component is the experience of "pain" itself, or suffering – the internal, emotional interpretation of the nociceptive experience. Again in humans, this is when the withdrawn finger begins to hurt, moments after the withdrawal. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. 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.

Reflex response to painful stimuli

 Reflex arc of a dog when its paw is stuck with a pin. The spinal cord responds to signals from receptors in the paw, producing a reflex withdrawal of the paw. This localized response does not involve brain processes that might mediate a consciousness of pain, though these might also occur.
Reflex arc of a dog when its paw is stuck with a pin. The spinal cord responds to signals from receptors in the paw, producing a reflex withdrawal of the paw. This localized response does not involve brain processes that might mediate a consciousness of pain, though these might also occur.

Nociception usually involves the transmission of a signal along nerve fibers from the site of a noxious stimulus at the periphery to the spinal cord. Although this signal is also transmitted on to the brain, a reflex response, such as flinching or withdrawal of a limb, is produced by return signals originating in the spinal cord. Thus, both physiological and behavioral responses to nociception can be detected, and no reference need be made to a conscious experience of pain. Based such criteria nociception has been observed in all major animal taxa.[5]

Awareness of pain

Nerve impulses from nociceptors may reach the brain, where information about the stimulus (e.g. quality, location, and intensity), and affect (unpleasantness) are registered. Though the brain activity involved has been studied, the brain processes underlying conscious awareness are not well understood.[citation needed]

Adaptive value

The adaptive value of nociception is obvious; an organism detecting a noxious stimulus immediately withdraws the limb, appendage or entire body from the noxious stimulus and thereby avoids further (potential) injury. However, a characteristic of pain (in mammals at least) is that pain can result in hyperalgesia (a heightened sensitivity to noxious stimuli) and allodynia (a heightened sensitivity to non-noxious stimuli). When this heightened sensitisation occurs, the adaptive value is less clear. First, the pain arising from the heightened sensitisation can be disproportionate to the actual tissue damage caused. Second, the heightened sensitisation may also become chronic, persisting well beyond the tissues healing. This can mean that rather than the actual tissue damage causing pain, it is the pain due to the heightened sensitisation that becomes the concern. This means the sensitisation process is sometimes termed maladaptive. It is often suggested hyperalgesia and allodynia assist organisms to protect themselves during healing, but experimental evidence to support this has been lacking.[6][7]

In 2014, the adaptive value of sensitisation due to injury was tested using the predatory interactions between longfin inshore squid (Doryteuthis pealeii) and black sea bass (Centropristis striata) which are natural predators of this squid. If injured squid are targeted by a bass, they began their defensive behaviours sooner (indicated by greater alert distances and longer flight initiation distances) than uninjured squid. If anaesthetic (1% ethanol and MgCl2) is administered prior to the injury, this prevents the sensitisation and blocks the behavioural effect. The authors claim this study is the first experimental evidence to support the argument that nociceptive sensitisation is actually an adaptive response to injuries.[8]


To assess the capacity of other species to consciously suffer pain we resort to argument-by-analogy. That is, if an animal responds to a stimulus the way a human does, 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. It might be argued that consistency requires us infer, also, that a cockroach experiences conscious pain when it writhes after being stuck with a pin. The usual counter-argument is that although the physiology of consciousness is not understood, it clearly involves complex brain processes not present in relatively simple organisms.[9] Other analogies have been pointed out. For example, when given a choice of foods, rats[10] and chickens[11] with clinical symptoms of pain will consume more of an analgesic-containing food than animals not in pain. Additionally, the consumption of the analgesic carprofen in lame chickens was positively correlated to the severity of lameness, and consumption resulted in an improved gait. Such anthropomorphic arguments face the criticism that physical reactions indicating pain may be neither the cause nor result of conscious 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.[12][13][14] 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.[15] 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.[15] Some authors say that the view that animals feel pain differently is now a minority view.[12] 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,[16] some authors continue to question how reliably animal mental states can be determined.[13][17]

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.[18] 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.[19]


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.[19] 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.[19]

Some criteria that may indicate the potential of another species to feel pain include:[20]

  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



A typical human cutaneous nerve contains 83% C type trauma receptors (the type responsible for transmitting signals described by humans as excruciating pain); the same nerves in humans with congenital insensitivity to pain have only 24-28% C type receptors.[21] The rainbow trout has about 5% C type fibres, while sharks and rays have 0%.[22] Nevertheless, fish have been shown to have sensory neurons that are sensitive to damaging stimuli and are physiologically identical to human nociceptors.[23] 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.[24]

Animal welfare advocates have raised concerns about the possible suffering of fish caused by angling. Some countries, e.g. Germany, have banned specific types of fishing, and the British RSPCA now formally prosecutes individuals who are cruel to fish.[25]


Though it has been argued that most invertebrates do not feel pain,[26][27][28] 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.[9][29][30] Nociceptors have been found in nematodes, annelids and molluscs.[31] Most insects do not possess nociceptors,[32][33][34] one known exception being the fruit fly.[35] In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opiate receptors. Opioid peptides and opiate receptors occur naturally in nematodes,[36][37] molluscs,[38][39] insects[40][41] and crustaceans.[42][43] The presence of opioids in crustaceans has been interpreted as an indication that lobsters may be able to experience pain, although it has been claimed "at present no certain conclusion can be drawn".[44]

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.[45] 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.[46][47]

Since September 2010, all cephalopods being used for scientific purposes in the EU are protected by EU Directive 2010/63/EU which states "...there is scientific evidence of their [cephalopods] ability to experience pain, suffering, distress and lasting harm.[48] In the UK, animal protection legislation[49] means that cephalopods used for scientific purposes must be killed humanely, according to prescribed methods (known as "Schedule 1 methods of euthanasia") known to minimise suffering.[50]

In medicine and research

Veterinary medicine

Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.[51]


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, hot plate test and grimace scales.

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.[52] Marian Stamp Dawkins defines "suffering" in laboratory animals as the experience of one of "a wide range of extremely unpleasant subjective (mental) states."[53] The U.S. National Research Council has published guidelines on the care and use of laboratory animals,[54] as well as a report on recognizing and alleviating pain in vertebrates.[55] 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."[56] 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.[57] PETA however argues that there is no doubt about animals in laboratories being inflicted with pain.[58] In the UK, animal research likely to cause "pain, suffering, distress or lasting harm" is regulated by the Animals (Scientific Procedures) Act 1986 and research with the potential to cause pain is regulated by the Animal Welfare Act of 1966 in the US.

In the U.S., researchers are not required to provide laboratory animals with pain relief if the administration of such drugs would interfere with their experiment. Laboratory animal veterinarian Larry Carbone writes, “Without question, present public policy allows humans to cause laboratory animals unalleviated pain. The AWA, the Guide for the Care and Use of Laboratory Animals, and current Public Health Service policy all allow for the conduct of what are often called “Category E” studies – experiments in which animals are expected to undergo significant pain or distress that will be left untreated because treatments for pain would be expected to interfere with the experiment.”[59]

Severity scales

Eleven countries have national classification systems of 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.[60] 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".[61] 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.[62]

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. "[63] 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


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