https://phys.org/news/2018-08-mice-individuality.html

The first thing that the authors observed was that the type of behavior adopted by each individual was correlated to the activity of dopamine-producing neurons, which are especially involved in decision-making. For example, the mice that alternated the most had lower dopaminergic activity. Therefore, one can say that mice have biologically inscribed individuality.

To understand the role of mice's social environment on the development of these individualities, the researchers continued their experiments by changing the composition of the groups in Souris City. They grouped the individuals who adopted the same test strategy, those who rarely alternated on one side, and those who alternated often on the other. Surprise! After a few weeks, roles had redistributed in both groups! Some mice who had rarely varied their choice had become the more adventurous ones in their new group, and vice versa.