The Senate Standing Committee on Legal and Constitutional Affairs
Invertebrates Feel Pain?
Invertebrates are classically defined as animals, which lack a’ backbone’ or dorsal nerve cord1, such as insects, crustacea (e.g. shrimp, lobster and crab), and molluscs (e.g. clams, snails, and squid). Traditionally, these animals have not been included in legislation concerning cruelty to animals2.
Pain is defined by the International Association for the Study of Pain (IASP) as “An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage”3. The subjective, emotional component of pain is considered its important aspect, not the activation of pain sensors (nociceptors) in the body. The IASP makes this clear “Activity induced in the nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may appreciate that pain most often has a proximate physical cause”3. In other words, the only animals capable of feeling pain are those that can feel fear, anxiety, distress and terror, similar to what humans feel when we receive noxious stimuli.
Almost all organisms, including bacteria, will attempt to escape from an aversive stimulus4. Because 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. 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 stimulus2. 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 pain2. 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 ago1.
Scientists have used three lines of reasoning to assess the likelihood that invertebrates are capable of feeling pain5.
- The evolutionary function of pain
- The neural capacity of invertebrates
- The behaviour of invertebrates
1. The evolutionary function of pain.
In vertebrates pain is thought to be an important educational tool6. Vertebrates are relatively long-lived creatures and learning shapes much of their behaviour. Learning from pain (and pleasure) plays a vital role in the development of their behaviour6.
Almost all invertebrates are short-lived and their behaviour is thought to be largely genetically determined7. Therefore, there is less evolutionary pressure selecting for the evolution of pain in this group of animals6.
2. The neural capacity of invertebrates.
Except for the cephalopods, invertebrates have small nervous systems, consisting of many small brains (ganglia). Because of the small number of neurons and the distributed organization of their nervous systems, invertebrates are thought to have limited cognitive capacity6. High cognitive capacity is thought to be a prerequisite for the development of an emotional response6.
3. The behaviour of invertebrates
Invertebrates show few, if any, of the behaviours that we would recognize as evidence of emotion6. Many invertebrates are cannibalistic, and many eat their young when given the chance. Most have no social behaviour. 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 mantid6.
Cephalopods are sometimes given special status by animal care committees (e.g. CCAC) because they have a large, vertebrate-like central nervous system, which is about the same size as that of a fish8. In the United Kingdom these animals have some legal protection, however in the United States they do not.
Although they have large brains, all the coleoid cephalopods (squid, octopus and cuttlefish) have short lifespans8. Most live less than one year. There is no parental care8. The absence of parental care suggests that most of their behaviour is genetically determined (i.e. they must be able to hunt, hide from predators, communicate etc. without instruction by others of their species). They are capable of learning, but their abilities are sometimes greater, sometimes less than that of fish8,9. Most are highly cannibalistic, even the schooling squid. We know nothing about their hormonal response to stress, and therefore we cannot determine whether they have a physiological response that resembles ours when confronted by aversive stimuli. We understand very little about their visual communication system and, therefore, we do not know whether they make any ‘pain-specific’ signals. Given our three criteria above, we have very little evidence that these animals feel pain. Nevertheless, it is possible that as we learn more about them, we may find evidence suggesting that they are capable of feeling pain.
Although it is impossible to know the subjective experience of another animal with certainty, the balance of the evidence suggests that most invertebrates do not feel pain. The evidence is most robust for insects, and, for these animals, the consensus is that they do not feel pain6.
1. Brusca R and Brusca G. 2002. The Invertebrates. 2nd edition. Sinauer.
2. Animal Behaviour Society, 2003. Anim. Behav. 65: 649-655
3. International Association for the Study of Pain. www.iasp-pain.org/terms-p.html
4. Berg, H 1975. Nature. 254: 389-392
5. Sherwin, C 2001. Anim. Welfare. 10: S103-S118
6. Eisemann C et al. 1984. Experientia 40: 164-167
7. Drickamer L et al. 2001. Animal Behavior: Mechanisms, Ecology and Evolution. 5th edition. McGraw-Hill.
8. Hanlon R and Messenger J 1996. Cephalopod Behaviour, Cambridge Univ. Press.
9. Boal J et al. 2000. Behav. Processes. 52: 141-153