Pain is an aversive sensation and feeling associated with actual, or potential, tissue damage. It is widely accepted by a broad spectrum of scientists and philosophers that non-human animals can perceive pain, including pain in amphibians.
Pain is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in non-human animals cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness, which is deduced from comparative brain physiology as well as physical and behavioural reactions.
Amphibians, particularly anurans, fulfill several physiological and behavioural criteria proposed as indicating that non-human animals may experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors, opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements.
Pain in amphibians has societal implications including their exposure to pollutants, (preparation for) cuisine (e.g. frog legs) and amphibians used in scientific research.
Several scientists and scientific groups have expressed the belief that amphibians can feel pain, however, this remains somewhat controversial due to differences in brain structure and the nervous system compared with other vertebrates.
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Background The possibility that amphibians and other non-human animals may experience pain has a long history. Initially, pain in non-human animals was based around theoretical and philosophical argument, but more recently has turned to scientific investigation.
Philosophy
The idea that non-human animals might not feel pain goes back to the 17th-century French philosopher, René Descartes, who argued that animals do not experience pain and suffering because they lack consciousness. In 1789, the British philosopher and social reformist, Jeremy Bentham, addressed in his book An Introduction to the Principles of Morals and Legislation the issue of our treatment of animals with the following often quoted words: "The question is not, Can they reason? nor, can they talk? but, Can they suffer?"
Peter Singer, a bioethicist and author of Animal Liberation published in 1975, suggested that consciousness is not necessarily the key issue: just because animals have smaller brains, or are 'less conscious', this does not mean that they are not capable of feeling pain.
Bernard Rollin, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain. In his interactions with scientists and other veterinarians, Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain.
Continuing into the 1990s, discussions were further developed on the roles that philosophy and science had in understanding animal cognition and mentality. In subsequent years, it was argued there was strong support for the suggestion that some animals (most likely amniotes) have at least simple conscious thoughts and feelings and that the view animals feel pain differently to higher primates is now a minority view.
Scientific investigation
In the 20th- and 21st-century, there were many scientific investigations of pain in non-human animals.
Mammals
At the turn of the century, studies were published showing that arthritic rats self-select analgesic opiates. In 2014, the veterinary Journal of Small Animal Practice published an article on the recognition of pain which started "The ability to experience pain is universally shared by all mammals...", and in 2015, it was reported in the science journal Pain that several mammalian species (rat, mouse, rabbit, cat and horse) adopt a facial expression in response to a noxious stimulus that is consistent with the expression of pain.
Birds At the same time as the investigations using arthritic rats, studies were published showing that birds with gait abnormalities self-select for a diet that contains carprofen, an analgesic. In 2005, it was written "Avian pain is likely analogous to pain experienced by most mammals" and in 2014, "...it is accepted that birds perceive and respond to noxious stimuli and that birds feel pain."
Reptiles Veterinary articles have been published stating reptiles experience pain in a way analogous to mammals, and that analgesics are effective in this class of vertebrates.
Fish Several scientists or scientific groups have made statements indicating they believe fish can experience pain. For example, in 2004, Chandroo et al. wrote "Anatomical, pharmacological and behavioural data suggest that affective states of pain, fear and stress are likely to be experienced by fish in similar ways as in tetrapods". In 2009, the European Food Safety Authority published a document stating scientific opinion on the welfare of fish. The document contains many sections indicating that the scientific panel believe fish can experience pain, for example, "Fish that are simply immobilized or paralysed [before euthanasia] would experience pain and suffering..." In 2015, Brown wrote "A review of the evidence for pain perception strongly suggests that fish experience pain in a manner similar to the rest of the vertebrates."
Argument by analogy In 2012 the American philosopher Gary Varner reviewed the research literature on pain in animals. His findings are summarised in the following table.
Argument by analogy Property Fish Amphibians Reptiles Birds Mammals Has nociceptors Has brain Nociceptors and brain linked ? / ? / ? / Has endogenous opioids Analgesics affect responses ? ? Response to damaging stimuli similar to humans
Notes
Arguing by analogy, Varner claims that any animal which exhibits the properties listed in the table could be said to experience pain. On that basis, he concludes that all vertebrates, including amphibians, probably experience pain, but invertebrates apart from cephalopods probably do not experience pain.
Experiencing pain Although there are numerous definitions of pain, almost all involve two key components.
First, nociception is required. 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 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. 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; 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, it is likely to have had an analogous experience.
Nociception
Nociception 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 evokes a reflex arc response generated at the spinal cord and not involving the brain, such as flinching or withdrawal of a limb. Nociception is found, in one form or another, across all major animal taxa. Nociception can be observed using modern imaging techniques; and a physiological and behavioral response to nociception can be detected.
Emotional pain
Sometimes a distinction is made between "physical pain" and "emotional" or "psychological pain". Emotional pain is the pain experienced in the absence of physical trauma, e.g. the pain experienced after the loss of a loved one, or the break-up of a relationship. It has been argued that only primates can feel "emotional pain", because they are the only animals that have a neocortex – a part of the brain's cortex considered to be the "thinking area". However, research has provided evidence that monkeys, dogs, cats and birds can show signs of emotional pain and display behaviours associated with depression during painful experience, i.e. lack of motivation, lethargy, anorexia, unresponsiveness to other animals.
Physical pain
The nerve impulses of the nociception response may be conducted to the brain thereby registering the location, intensity, quality and unpleasantness of the stimulus. This subjective component of pain involves conscious awareness of both the sensation and the unpleasantness (the aversive, negative affect). The brain processes underlying conscious awareness of the unpleasantness (suffering), are not well understood.
There have been several published lists of criteria for establishing whether non-human animals experience pain, e.g. Some criteria that may indicate the potential of another species, including amphibians, to feel pain include: Has a suitable nervous system and sensory receptors Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics Physiological changes to noxious stimuli Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy Shows avoidance learning Shows trade-offs between noxious stimulus avoidance and other motivational requirements High cognitive ability and sentience
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.
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.
Research findings