page_title: Pain in invertebrates
section_title: Trade-offs between stimulus avoidance and other motivational requirements
hierarchical_section_title: Pain in invertebrates / Trade-offs between stimulus avoidance and other motivational requirements
caption_attribution_description: English: A hermit crab in unusual shell fighting other hermit crab for a new home.
context_page_description: Pain in invertebrates is a contentious issue. 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 evokes a reflex response that 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. The second component is the experience of "pain" itself, or suffering—i.e., the internal, emotional interpretation of the nociceptive experience. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; 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 a non-human animal's responses to stimuli are similar to those of humans, it is likely to have had an analogous experience.
context_section_description: This is a particularly important criterion for assessing whether an animal has the capacity to experience pain rather than only nociception. Nociceptive responses do not require consciousness or higher neural processing; this results in relatively fixed, reflexive actions. However, the experience of pain does involve higher neural centres which also take into account other factors of relevance to the animal, i.e. competing motivations. This means that a response to the experience of pain is likely to be more plastic than a nociceptive response when there are competing factors for the animal to consider.
  Robert Elwood and Mirjam Appel at the Queen's University of Belfast argue that pain may be inferred when the responses to a noxious stimulus are not reflexive but are traded off against other motivational requirements, the experience is remembered and the situation is avoided in the future. They investigated this by giving hermit crabs small electric shocks within their shells. Only crabs given shocks evacuated their shells indicating the aversive nature of the stimulus, but fewer crabs evacuated from a preferred species of shell demonstrating a motivational trade-off. Most crabs, however, did not evacuate at the shock level used, but when these shocked crabs were subsequently offered a new shell, they were more likely to approach and enter the new shell. They approached the new shell more quickly, investigated it for a shorter time and used fewer cheliped probes within the aperture prior to moving in. This demonstrates the experience of the electric shock altered future behaviour in a manner consistent with a marked shift in motivation to get a new shell to replace the one previously occupied.
caption: Hermit crabs fighting over a shell
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