Watanabe, S. 2023. Are mirrors aversive or rewarding for mice? Insights from the mirror preference test. Frontiers in Behavioral Neuroscience 17, 1137206.

Research investigating the effects of mirror exposure in mice found that the presence of mirrors has similar effects to the presence of cage mates. Restraint in a small holder induces hyperthermia (stress-induce hyperthermia: SIH) in mice but a restrained mouse surrounded by similarly restrained cage mates shows less SIH (Watanabe, 2015). A restrained mouse surrounded by mirrors instead of the cage mates also shows reduced SIH, suggesting that the images reflected by the mirrors are a substitute for conspecifics (Watanabe, 2016). However, there is mixed evidence on the effect of mirrors on mice. In a study on chronic mirror-image stimulation, Fuss et al. (2013) found that a mirror placed for 5 weeks in the cage of single-housed mice had no effect on anxiety and depression-like behaviors. Nevertheless, the presence of the mirror increased exploratory behavior, enhancing both rearing in the novel cage exploration test and head-dipping in the hole-board test. Conversely, pharmacological studies have used mirrors to induce anxiety in mice. This suggests that mirrors have contrasting effects on mice. In the present article, we will examine mirror-based rodent behavioral tests and compare their individual characteristics to understand the effect of mirrors on mice. Moreover, we will describe under which conditions mirrors could be used as rewards. Indeed, mirror-based behavioral tests would be particularly useful for behavioral neuroscience research. Since the mirror reward does not require previous starvation or water deprivation, it would be an animal-friendly alternative to classical appetitively motivated learning tests employing food or water as reward. Refinement of current behavioral tests is important both to maximize animal welfare and to reduce stress-associated variability, hence improving reproducibility of scientific results.