Facial expression analysis

The facial expression of animals provides valuable information on their affective state. Pain faces can be analyzed by means of the so-called grimace scales, which consist of different facial action units (e.g. orbital tightening). The intensity of each facial action unit is scored and then all scores are added or averaged. After Dr Jeffrey Mogil and colleagues from the McGill University in Canada had developed a grimace scale to be used to detect acute pain in mice, grimace scales were developed for several other species:

• Mouse Grimace Scale (Langford et al. 2010)
• Rat Grimace Scale (Sotocina et al. 2011)
• Rabbit Grimace Scale (Keating et al. 2012)
• Horse Grimace Scale (Costa et al. 2014, Gleerup et al. 2015)
• Cat Grimace Scale (Holden et al. 2014, Evangelista et al. 2019)
• Cow Grimace Scale (Gleerup et al. 2015)
• Sheep Grimace Scale (McLennan et al. 2016)
• Piglet Grimace Scale (Di Giminiani et al. 2016)
• Lamb Grimace Scale (Guesgen et al. 2016)

Besides pain, positive affective states can also change the facial expression as demonstrated in rats (Finlayson et al. 2016).

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