Shock Avoidance By Discrimination Learning In The Shore Crab (Carcinus Maenas) Is Consistent With A Key Criterion For Pain
It is a strange paradox of some types of animal testing: in the search to “discover” whether certain animals feel pain, scientists subject them to painful, damaging experiments and then observe the results. In this study, scientists took shore crabs from the wild and subjected them to a series of painful scenarios involving electric shocks, in order to measure their “nociception” (their ability to learn lessons from painful experiences.) More than teaching us something about shore crabs – the study finds that crabs do indeed react to pain stimuli beyond just reflex, and demonstrate learning – the study speaks to an entrenched disconnect in scientific inquiry into the non-human animal world.
Though many advocates take sentience in animals as a given, the subject of whether or not non-human animals feel pain is still an issue of a great deal of scientific inquiry. It is widely accepted that mammals and other vertebrates feel pain, but the status of invertebrates is still questioned by some scientists. In this study, researchers wanted to determine whether or not shore crabs feel pain beyond just a stimulus / response dynamic, in a way that allows them to learn to change their behavior. The authors say that “pain is a powerful motivating factor and it enables the organism to learn to avoid the situation that led to tissue damage,” but also they note that “assessing pain in animals is difficult because of their lack of verbal communication.” To complete their study, they captured nearly 90 European shore crabs using baited pots, and placed them in a pair of artificial shelters to be tested on. Using copper wire, the researchers shocked crabs randomly, noting which shelter they were in at the time, and whether or not the crabs changed their behavior later to avoid their “shock shelter”.
Of particular concern for advocates is the fact that these tests which deliberately cause pain to animals are allowed to be conducted at all; it speaks to the lack of protection that many laboratory animals face, especially invertebrates. Buried towards the end of the paper, the authors note that “no licence was required for this work as crustaceans are not included in UK scientific legislation. We used electric shock as the stimulus because this has been shown to be aversive in a wide variety of taxa and it can be delivered to freely moving well-protected animals such as the shore crab in a reliable manner.” In what they might consider as an ethical concession, the authors also note that “crabs could terminate the shock by exiting the shock shelter and could avoid shock either by not entering either shelter or by selecting the non-shock shelter.” The only positive is that the researchers decided to spare these animals (that were captured from the wild) their deaths: “all crabs were returned to a suitable shore near to the collecting site after the experiments were conducted.”
At the end of their paper, the scientists state that their findings indicate that crabs do have “swift avoidance learning, which is a key criterion/expectation for pain experience, but this alone does not prove that crabs can experience pain.” In their view, crabs’ behavior goes beyond a simple reflex action, and shows “long-term motivational change that enables discrimination learning has been demonstrated. Perhaps such motivational changes and learning can arise without any an unpleasant experience, although that is doubted by [others].” Of course, it begs the question that, if crabs have the ability to feel pain, doesn’t it mean that this type of testing in and of itself is unethical?
Nociception allows for immediate reflex withdrawal whereas pain allows for longer-term protection via rapid learning. We examine here whether shore crabs placed within a brightly lit chamber learn to avoid one of two dark shelters when that shelter consistently results in shock. Crabs were randomly selected to receive shock or not prior to making their first choice and were tested again over 10 trials. Those that received shock in trial 2, irrespective of shock in trial 1, were more likely to switch shelter choice in the next trial and thus showed rapid discrimination. During trial 1, many crabs emerged from the shock shelter and an increasing proportion emerged in later trials, thus avoiding shock by entering a normally avoided light area. In a final test we switched distinctive visual stimuli positioned above each shelter and/or changed the orientation of the crab when placed in the chamber for the test. The visual stimuli had no effect on choice, but crabs with altered orientation now selected the shock shelter, indicating that they had discriminated between the two shelters on the basis of movement direction. These data, and those of other recent experiments, are consistent with key criteria for pain experience and are broadly similar to those from vertebrate studies.