Reading Mice’s Emotional States

In any given population, whether human or nonhuman, individuals responses to stress can vary greatly; everybody has different coping mechanisms for stressful situations. Still, researchers have done their best to find consistencies and commonalities, and often describe them in terms of two kinds of individuals: proactive (bolder and aggressive and with a higher activation of the “sympathetic adrenomedullary system”), and more reactive individuals (passive and with a higher activity in the “hypothalamic–pituitary–adrenocortical axis”).

Recently, researchers have been looking at the possibility that facial expressions may be able to tell us something about animal stress, with studies already published relating to the emotional or stress states of dogs, horses, and others. This study, which used wild mice in a laboratory setting, set out to determine if facial expressions could show a continuum of emotional states. Using a “novel odor” test (with a “predator odor”) in combination with a standard “elevated plus maze” setting, the researchers wanted to see if they could differentiate individuals according to coping styles. Wild mice were used because the researchers said they “have been shown to express more inter-individual differences and comparatively stronger emotional reactivity compared to laboratory animals.”

The study found that in general, the “ear forward” posture was unambiguous and could be easily rated by “naïve observers.” Ears forward, in this case, is a sign of extra attention, as can be seen in other species such as dogs, horses, and sheep, “when particular attention is required.” The researchers conclude that the ears forward posture is a “vigilance posture” shared by many animals in stressful situations. Likewise, they note that differences in ear posture may be related to “inter-individual differences in emotional reactivity.”

The results of this study, though specifically observed in wild mice in a lab setting, could have broader implications. It implies that even untrained observers such as lab technicians could be able to approximately gauge the stress levels of mice in a lab, and, in an ideal world, be able to make adjustments to their environments to alleviate that stress.