First, responses to noxious, potentially painful events should affect neurobiology, physiology and behaviour in a different manner to innocuous stimuli and subsequent behaviour should be modified including avoidance learning and protective responses. We define two key concepts that can be used to evaluate the potential for pain in both invertebrate and vertebrate taxa. Thus clear standards to judge whether pain is likely to occur in any animal species is vital to inform whether to alleviate pain or to drive the refinement of procedures to reduce invasiveness, thereby minimizing pain. The detection and assessment of pain in animals is crucial to improving their welfare in a variety of contexts in which humans are ethically or legally bound to do so. Thus, octopuses are likely to experience the affective component of pain.
Injection site grooming occurred in all animals receiving acetic acid injections, but this was abolished by local anesthesia. In conditioned place preference assays, octopuses avoided contexts in which pain was experienced, preferred a location in which they experienced relief from pain, and showed no conditioned preference in the absence of pain. Here we show that octopuses, the most neurologically complex invertebrates, exhibit cognitive and spontaneous behaviors indicative of affective pain experience. It is generally accepted that vertebrate animals experience pain, however, there is currently inconclusive evidence that the affective component of pain occurs in any invertebrate. Because pain is aversive and its relief is innately rewarding, animals may learn to avoid a context in which pain is experienced, and prefer one where pain relief occurs. Pain is a negative affective state arising from tissue damage or inflammation.