This article is from the WSSF 2016 AFRMA Rat & Mouse Tales news-magazine.

Did You Know?  There are 5 facial expressions to look for to see if mice are in pain—squinting eyes, nose bulge (skin and muscle contracted creating a rounded extension of skin on the bridge of the nose), cheek bulge (cheek muscle contracted and convex from normal position), ears set to the side or back, whiskers either pulled back, pointing forward, or clumped together. Other changes would be a fluffed body coat, hunched posture, abnormal walking, anorexia, aggression, hiding, etc.

You can read more in Mouse Grimace Scale (MGS), The Manual.

Study published in Nature Methods, 7, 447–449 (1 June 2010), Coding of facial expressions of pain in the laboratory mouse.