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12 13 14 15 2The detection and assessment of pain in animals is crucial to improving their welfare in a 16 variety of contexts where humans are ethically or legally bound to do so. Thus clear standards 17 to judge whether pain is likely to occur in any animal species is vital to inform whether to 18 alleviate pain or to drive the refinement of procedures to reduce invasiveness thereby 19 minimising pain. We define two key concepts that can be used to evaluate the potential for 20 pain in both invertebrate and vertebrate taxa. Firstly, responses to noxious, potentially painful 21 events should affect neurobiology, physiology and behaviour in a different manner to 22 innocuous stimuli and subsequent behaviour should be modified including avoidance learning 23 and protective responses. Secondly, animals should show a change in motivational state after 24 experiencing a painful event such that future behavioural decision making is altered and can 25 be measured as a change in conditioned place preference, self-administration of analgesia, 26 paying a cost to accessing analgesia or avoidance of painful stimuli and reduced performance 27 in concurrent events. The extent to which vertebrate and selected invertebrate groups fulfil 28 these criteria is discussed in light of the empirical evidence and where there are gaps in our 29 knowledge we propose future studies are vital to improve our assessment of pain. This review 30 highlights arguments regarding animal pain and defines criteria that demonstrate, beyond a 31 reasonable doubt, whether animals of a given species experience pain. 32 33 34 35 36 3Bateson's (1991) seminal review on the assessment of pain has been influential in 37 inspiring numerous researchers investigating pain in animals. Bateson set out a clear 38 framework upon which hypothesis driven research questions could be derived regarding the 39 capacity for pain in any species. Indeed the criteria suggested have been applied to numerous 40 species particularly non-mammalian vertebrates (e.g. fish, Sneddon, 2011) and more recently 41 to invertebrates (e.g. crustaceans, Barr et al., 2008). Well-defined criteria were proposed and 42 it was suggested that animals that fulfilled all criteria should be considered capable of pain. 43These criteria were possession of nociceptors, receptors that detect damaging stimuli on or in 44 the body; pathways from nociceptors to the brain; brain structures analogous to the human 45 cerebral cortex that process pain; opioid receptors and endogenous opioid substances in 46 nociceptive neural system; a reduction in adverse behavioural and physiological effects after 47 administration of analgesics or painkillers; learning to avoid potentially painful stimuli and 48 that this learning is rapid and inelastic; Sneddon (2004) added that normal behaviour should 49 be suspended for a prolonged period rather than a reflex response with adverse changes in 50 behaviour reflective of signs of "discomfort" as shown by long-term motivational change. 51These robust scientific appro...

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