Abstract
ABSTRACTInsights into the potential for pain may be obtained from examination of behavioural responses to noxious stimuli. In particular, prolonged responses coupled with long-term motivational change and avoidance learning cannot be explained by nociceptive reflex but are consistent with the idea of pain. Here, we placed shore crabs alternately in two halves of a test area divided by an opaque partition. Each area had a dark shelter and in one repeated small electric shocks were delivered in an experimental but not in a control group. Crabs showed no specific avoidance of the shock shelter either during these trials or in a subsequent test in which both were offered simultaneously; however they often emerged from the shock shelter during a trial and thus avoided further shock. More crabs emerged in later trials and took less time to emerge than in early trials. Thus, despite the lack of discrimination learning between the two shelters they used other tactics to markedly reduce the amount of shock received. We note that a previous experiment using simultaneous presentation of two shelters demonstrated rapid discrimination and avoidance learning but the paradigm of sequential presentation appears to prevent this. Nevertheless, the data show clearly that the shock is aversive and tactics, other than discrimination learning, are used to avoid it. Thus, the behaviour is only partially consistent with the idea of pain.
Highlights
Animal pain is difficult to investigate because a behavioural response to a noxious stimulus might be a nociceptive reflex (Sherrington, 1906) that lacks an associated unpleasant experience (Crook and Walters, 2011; Elwood et al, 2009; Elwood, 2011) and because of the difficulty in gaining access to the feelings of animals (Stamp Dawkins, 2012)
The first trial of the experimental group was with the shock shelter and all 76 crabs entered on that trial, fewer crabs entered that same shelter in subsequent trials (Table 1) (χ24=16.63, P=0.002)
Pain enhances the salience of a noxious stimulus and functions to facilitate swift avoidance learning (Bateson, 1991; Elwood, 2011)
Summary
Animal pain is difficult to investigate because a behavioural response to a noxious stimulus might be a nociceptive reflex (Sherrington, 1906) that lacks an associated unpleasant experience (Crook and Walters, 2011; Elwood et al, 2009; Elwood, 2011) and because of the difficulty in gaining access to the feelings of animals (Stamp Dawkins, 2012). Pain is defined as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP, 1979); this definition is not suitable for animals because they cannot describe their experiences. Various definitions have been proposed for pain in animals such as “an aversive sensory experience caused by actual or potential injury that elicits protective and vegetative reactions, results in learned behaviour, and may modify species specific behaviour” (Zimmermann, 1986). When data for the two groups were combined more crabs moved in the same direction relative to their body in the second test than moved in the opposite direction (binomial 55 vs 28 P=0.004) but they did not go to the same type of stripe (binomial 50 vs 33 P=0.078)
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