# Pain in amphibians

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.

## 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

   The absence of a neocortex does not appear to preclude an organism from experiencing affective states. 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.
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 |
| Property | Fish | Amphibians | Reptiles | Birds | Mammals |
| Has nociceptors | Y | Y | Y | Y | Y |
| Has brain | Y | Y | Y | Y | Y |
| Nociceptors and brain linked | Y | ? / Y | ? / Y | ? / Y | Y |
| Has endogenous opioids | Y | Y | Y | Y | Y |
| Analgesics affect responses | Y | ? | ? | Y | Y |
| Response to damaging stimuli similar to humans | Y | Y | Y | Y | Y |