While grimace scale based tools are much simpler to learn and use than animalFACS-based methods, they remain subject to bias due to the use of human raters. They are species-specific and have been developed for several species, such as mice 1, rats 2, cows 21, lambs 22 and cats 23. However, major limitations of FACS are that it requires extensive human training for reliable application, requires dynamic change in the face, for which it can be difficult to clearly identify an apex, is time consuming and in some cases can be costly to undertake and may be prone to human error or bias 20.ДЄ simplified method, applied mainly in the context of pain, are grimace scales. FACS has recently been adapted for several non-human species, including several non-human primates 16, dogs 17 and cats 18, aiming also at facilitating cross-species comparison 19. The gold standard of objectively assessing facial expressions in human emotion research is the Facial Action Coding System 15 (FACS), which measures the individual movements or вЂAction UnitsвЂ™ (AUs) among the muscles of the face, assigning codes to activity of individual muscles or groups of muscles. Such factors may also contribute to cats tending to be prescribed less analgesic drugs by veterinarians compared with dogs 13, 14. Optimal pain management in cats presents various challenges due to a lack of well-established management strategies for certain painful conditions 9, reservations about the adverse effects of analgesics 10, uncertainty around the reliability and specificity of certain behavioural indicators of pain 11 and a limited ability of humans to accurately interpret facial expressions associated with negative affect in this species 12.

Such tools have been created and validated for various species, including rats 2, rabbits 3, horses 4, pigs 5, sheep 6, ferrets 7 and cats 8. Since Langford et al 1 first reported on facial expressions associated with pain in mice, there has been growing interest in tools for assessment of facial expressions linked to pain in a range of non-human animal species.

Facial expressions are identified as one of the most common and specific indicators of pain in (human and non-human) animals. Accurate recognition and assessment of pain in animals is crucial for pain management and welfare assessment.

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