känner maneter, insekter, sjöstjärnor m.fl. smärta? (samlingstråd)

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.

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.