Changing priorities about protective shelters: a review of a key method to investigate possible pain in crustaceans.

Testing if non-human taxa experience pain is difficult because we need to exclude the possibility that responses are nociceptive reflexes. One approach is to identify an essential, high priority, resource and then ask if the animal will abandon and subsequently avoid that resource if it is paired with a noxious stimulus. This approach has been used with crustaceans that hide in dark shelters and electric shocks have been used as noxious stimuli. A range of species show escape responses and avoid shelters if the shock is presented within, and these responses increase with increasing voltage or repetition of shocks. Crustaceans also switch to using alternative shelters and appear to dramatically alter their behavioural priorities. Animals shocked outside of a shelter, however, subsequently increase their use of shelters and benefit from reduced predation. These changes in priorities cannot be due only to nociceptive reflexes because they persist long after the cessation of the stimulus. Increasing the apparent costs of leaving a shelter decreases the probability of leaving, indicating that, by taking into account costs, they are responding via behavioural decisions and not reflexes. This provides a method to determine how much the animal will pay to avoid the shocks and similar techniques should provide powerful ways to examine potential pain in different taxa.