What key ability do fancy mice retain from their wild ancestors that requires them to have vertical space in their housing?

They are excellent climbers and possess the agility to jump distances equivalent to many times their body length.

Despite being domesticated, fancy mice retain several athletic traits inherited from their wild relatives, which were necessary for survival, particularly evading predators. One prominent retained capability is their agility, specifically noted by their ability to jump distances spanning many times their own body length. This athletic prowess translates directly into housing requirements; they are identified as excellent climbers. Consequently, appropriate housing must account for this, necessitating the provision of vertical space within their enclosures to allow them to express these natural, high-reaching movements.