Mice can recognise their reflection in a mirror and detect changes in their appearance, adding to the number of known species able to form visual self-image – like humans.
In a series of experiments, researchers from the University of Texas in the US placed white and black markings on the faces of black mice and placed them in a box with a mirror.
When the ink spot was white, the mice noticed the physical change in their reflection and spent more time grooming their heads in front of the mirror, according to the study published in Neuron.
The experiment was adapted from the ‘mirror test’. The method was originally developed to examine self-recognition in chimpanzees, to see whether animals could identify and remember their own visual features in a reflection.
Visual self-image contributes to humans developing their sense of self, alongside other sensory cues like hearing, touch and smell, and cognition.
Initially the ability was identified in only a few species besides humans, including great apes, dolphins and elephants. However species-specific modifications to the mirror test have shown certain birds, fish and macaques also possess the ability.
And now mice, according to this new research.
Researchers allowed the mice 13 days to explore and become familiar with a box compartment. One half of the box contained a mirror, which was shifted to new locations each day, and half without.
Compared to a control area in the compartment with no mirror, the mice seemed to prefer the mirror zone, spending more time there.
After the familiarisation period, the experimenters anesthetised their subjects and added a white or black mark (matching their coat colour) to the head of each mouse. When the mice later returned to the box, the researchers measured the location and amount of time different mice spent grooming.
While there was no change in the amount of grooming in the control group. Head grooming behaviours in the white-marked mice markedly increased, but only when the mice were located in the mirror section.
“The increased head grooming was identified as a mark-directed behaviour because there were no changes in the duration of grooming for whiskers, body, or tail across the four test sessions,” the authors write.
In addition, the experiment also showed that mice without marks did not increase their grooming when another mouse was present with a white mark, suggesting the action is not triggered by emotional contagion.
In humans, experiments show visual self-recognition likely relies on brain activity in the prefrontal cortex, parietal cortex and medial temporal lobe, including the hippocampus regions.
In mice with white ink, brain activity increased in the prefrontal cortex and dorsal and ventral hippocampal regions with self-recognition.
The authors conclude their study, together with accumulating experimental evidence in many species, suggests “mirror-induced self-directed behaviour” most likely exists on a broad spectrum across species, rather than being an all or nothing phenomenon present in only some animals.
According to the paper, mice were deeply anaesthetised at the end of the experiment, and their brains removed.