Species variant pain

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Nociception is born in the dorsal horn of the spinal cord, but we don’t call it pain till it reaches the brain.[1]

Pain refers to the subjective, unpleasant sensation that accompanies damage or near-damage to tissues. It is an ‘experience’, which is the product of the parts of the brain responsible for mental processing of the noxious stimulus. Generally speaking, pain is perceived in the brain. Cognition is involved in the formulation of this perception. There are emotional consequences, and behavioral responses to the cognitive and emotional aspects of pain. Pain can be either:

  • Acute - produced by transient stimuli and produces a stress response, but usually does not lead to distress, because the pain is short-lived. In the face of such phasic tissue-damaging stimuli, animals are generally able to adapt their behavior and accept their discomfort.
  • Chronic - animals in chronic pain can be divided into three broad categories: those with a known pathologic condition (e.g., arthritis, cancer, or injury), those in which an organic cause of the pain can be inferred from the results of the clinical examination and history (e.g., pain of musculoskeletal origin, peripheral nerve damage, or disease of the central nervous system), and those with signs that resemble signs in one of the other categories but without obvious cause. Animals in all three categories can exhibit signs of psychologic or psychosocial dysfunction.

Almost all organisms, including bacteria and plants, will attempt to escape from an aversive stimulus[2]. Though pain is a universal phenomenon with most living organisms, a cognitive interpretation of this pain is what concerns a veterinarian when it abiding by a code of veterinary ethics.

Attitude in the past to Animals and Pain;

- Rene Descartes (1596-1650): Animals do have no capacity for reasoning, therefore no perception of pain.
- Jeremy Bentham (1748-1832): The question is not, can they reason? Nor, can the talk? But, can they suffer?

A role for genetics in the perception of pain is suggested by the genetic factors inherent in gender, and more controversially, ethnicity, that give rise to differences in pain sensitivity in humans and animals. Heritability is essentially the proportion of variance due to all genetic factors[3]. A wide range of heritability estimates have been obtained for different types of clinical pain. In one carefully controlled animal study, 24% of heat pain sensitivity could be attributed to heritability[4]. In humans, over 40 cases of congenital insensitivity to pain (CIP) with preservation of all other sensory modalities have been reported since the original description of a carnival performer known as 'The Human Pincushion'. Recently, the genetic basis of this neuropathy (CIP with anhidrosis; hereditary sensory and autonomic neuropathy type IV) was elucidated[5].

Even though animal studies have provided a list of candidate ‘pain genes’, only a few genes have been identified that are associated with the perception of pain in humans. Therefore, the majority consensus is to observe that the majority of animals do in fact feel varying degrees of pain (nociceptive receptors are present in all animals), albeit their individual interpretations of that pain may vary.


Generally, bacteria are not thought to be capable of feeling pain (e.g. they lack a nervous system), possessing an escape response to an aversive stimulus is not enough evidence to demonstrate that a species is capable of feeling pain.


Invertebrates show few, if any, of the behaviors that humans would recognize as evidence of emotion. Many invertebrates are cannibalistic, and many eat their young when given the chance. Most have no social behavior. Although they can respond vigorously to noxious stimuli, even this response is inconsistent. Insects, for example, will continue with normal activity even after severe injury. An insect walking with a crushed tarsus (lower leg) will continue applying it to the ground with undiminished force. Locusts will writhe when sprayed with DDT. However, they will also continue feeding while being eaten by a praying mantis.


To infer that a non-human vertebrate (mammals, birds and reptiles) is in pain, researchers rely on the vocalizations and physiological responses (e.g. the release of stress hormones) that an animal produces when faced with an aversive stimulus. Because these responses are similar to our own when we are in pain, researchers argue that, by analogy, animals showing these responses are also in pain. This technique cannot be used with invertebrates. Invertebrate physiology is different from our own1. The invertebrates diverged from that of vertebrates hundreds of millions of years ago[6].

Although it is impossible to know the subjective experience of another animal with certainty, the balance of the evidence suggests that most invertebrates do' feel pain. The evidence is less clear for insects[7].


Various fish have been shown to feel pain, and the consensus must hold that most do feel pain, although their capacity to rationally interpret it may be diminished[8]. However, their ability to perceive a future noxious stimuli via memory is startling evident and cannot be misinterpreted as asensory[9].


There appears to be a general uniformity amongst vertebrate animals with respect to their capacity to feel both physical and emotional pain.


In veterinary medicine, interpreting pain can sometimes be complicated by an anthropomorphic interpretation of an animal's response to stimuli.


Dr Russell Brain, an eminent neurologist remarked:

Every particle of factual evidence supports the contention that the higher mammalian vertebrates experience pain sensations at least as acute as our own. To say that they feel less because they are lower animals is an absurdity; it can easily be shown that many of their senses are far more acute that ours--visual acuity in certain birds, hearing in most wild animals, and touch in others; these animals depend more than we do today on the sharpest possible awareness of a hostile environment. Apart from the complexity of the cerebral cortex (which does not directly perceive pain) their nervous systems are almost identical to ours and their reactions to pain remarkably similar, though lacking (so far as we know) the philosophical and moral overtones. The emotional element is all too evident, mainly in the form of fear and anger.

An objective analysis of an animal's condition must be based on physical parameters in conjunction with an accurate record of past experiences.


  1. [http://www.euprim-net.eu/network/courses/downloads/presentations/course4/3_braskamp.pdf euprim.net (2007)
  2. Berg, HC (1975) Bacterial behaviour. Nature 254(5499):389-92
  3. MacGregor AJ, et al (2002) BMJ 324:1100-1101
  4. Chesler EJ, et al (2002) Nat Neurosci 5:1101-1102
  5. Thrush, DC (1973) Congenital insensitivity to pain. Brain 96:369-386
  6. Brusca R and Brusca G. (2002) The Invertebrates. 2nd edition. Sinauer
  7. Drickamer L et al (2001) Animal Behavior: Mechanisms, Ecology and Evolution. 5th edition. McGraw-Hill
  8. Rose, JD (2007) Anthropomorphism and 'mental welfare' of fishes. Dis Aquat Organ 75(2):139-54
  9. [1]