Let’s explore why grimace scales were developed for laboratory animals. Effective pain alleviation in lab animals relies on recognizing and assessing pain severity. Traditional methods based on behavior and clinical signs (e.g., weight loss) are time-consuming and can have limitations, as these signs may not be specific to pain.

Research by Dr. Jeffrey Mogil and colleagues at McGill University has shown that changes in facial expression provide a reliable and rapid way to assess pain in mice and rats. They developed “grimace scales” for these species, focusing on changes in several “facial action units” like narrowing of the eyes (orbital tightening) or changes in the whiskers’ position and shape.  

When using grimace scales to assess pain in real-time at the cage or pen side, each animal should be observed for a short period to avoid scoring brief changes in facial expression that are unrelated to the animal’s welfare. These scales should only be used with awake animals.