Facial expressions of mice in aggressive and fearful contexts

Abstract

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.

Introduction

In The expression of the emotions in man and animals, Darwin emphasized the functional value of facial expressions and asserted that these expressions and their functions may be conserved across evolution [1]. Since then, others have continued to characterize the facial expressions of humans [2], nonhuman primates [3], canids [4] and rodents [5], [6].

Facial expressions are capable of conveying emotions [7] and/or intentions [8], providing useful information during social interactions. Many types of information are available in social contexts including tactile, acoustic, olfactory and visual signals. In social interactions of mice, visual signals are generally considered secondary to olfactory information [9]. For example, male mice will attack intact males, but will not attack females or castrated males that enter their territory. However, if urine from an intact male is placed on a female or a castrated male, these animals will be attacked by the male territory owner [10]. Similarly, female urine placed on a male will decrease attack by the male territory owner and can also induce sexual behavior [10], [11].

While olfaction has been shown to be critical in social interactions of mice [9], [12], [13], the role of visual signals, such as facial expressions, remains unclear. Langford et al. recently (2010) described a coding system, the Mouse Grimace Scale, for measuring facial changes in response to pain [5]. They showed that mice displayed a ‘pain face’ when exposed to various noxious stimuli. In addition, they suggested that the facial changes induced by pain mediate the preference shown by females to be near familiar mice in pain [14], implying a communicative function for mouse facial expressions [5].

The current study investigated the facial expressions of mice in non-social, social and predator contexts. For each experimental context, facial expressions of mice were assessed in a pretest trial and were then immediately assessed in a test trial, where experimental stimuli were introduced. Facial expressions scored from the pretest and test trials were compared. To determine if facial changes occur in response to stimulation of the mystacial vibrissae, mice were tested in a vibrissae contact test [15]. To determine if facial changes occur in social contexts, mice were tested in a social proximity test [16] and a resident–intruder test [17], [18]. To determine if facial changes occur in response to predator stimuli, mice were tested in a cat odor exposure test [19], [20], [21] and a rat exposure test [22].