In this paper we describe the existence and consequences of subspecific and individual variation in the genetic make-up of house mice. The purpose is to illustrate forms of variation that are often neglected in discussions about animal care and experimental design. Towards this end,
different inbred mouse strains as well as genetically selected mouse lines are compared in relation to their ecological origin. Firstly, the behaviour of BALB/c, C57BL/6J and CBA mice is described in relation to different habitats. Furthermore, their aggression is compared, as measured by
two paradigms. It appears that some inbred lines (eg BALB/c and C57BL/6J) clearly show behaviour that reflects the functional adaptation to the natural habitats in which their ancestors lived. Other strains (eg CBA) show a lack of such behavioural adaptation and their phenotypes appear to
be very unstable over time. Secondly, two fundamentally different characters, both present in populations of wild house mice and under genetic control, are described: on the one hand, active copers are characterized by aggressive behaviour; on the other hand, passive copers are reluctant to
attack. The active, aggressive animals (manipulators) are well adapted to an invariant environment like their own territory, whereas the passive, non-aggressive copers (adjustors) are well adapted to a changing environment, eg when roaming. We discuss to what extent these coping styles are
present in laboratory strains of mice. The major conclusion with regard to both phenomena is that individual and subspecific variation may have significant implications for experimental design and the welfare of the experimental animals.