Facial expressions of mice in aggressive and fearful contexts.

Some animals display a variety of context dependent facial expressions. Previous studies have shown that rodents display a facial grimace while in pain. To determine if the facial expressions of mice extend beyond pain, facial expressions were analyzed in the presence of non-social, social and predator stimuli. In a vibrissae contact test, the whiskers of mice were stroked by the bristles of a brush. In a social proximity test, two mice were placed together in a small chamber where contact was virtually unavoidable. In a resident-intruder test of aggression, an unknown mouse was placed into the homecage of another mouse. In a cat odor exposure test and in a live rat exposure test, mice were presented with the respective stimuli. Results from this study indicated that mice showed two patterns of expression, either a full display of changes in the measured facial components, characterized by tightened eyes, flattened ears, nose swells and cheek swells; or a more limited display of these facial changes. The full display of changes occurred in the vibrissae contact test, the social proximity test, and in resident mice in the resident-intruder test. The more limited display of facial changes occurred in the cat odor exposure test, the rat exposure test and in intruder mice in the resident-intruder test. The differential display of facial changes across conditions indicated that mice showed tightened eyes and flattened ears in situations that provided the immediate potential for contact, suggesting that such changes are involved in protection of sensitive and/or vulnerable body parts. Furthermore, the display of facial expressions by mice indicates that these expressions are widely distributed across evolution.