The individuality paradigm: Automated longitudinal activity tracking of large cohorts of genetically identical mice in an enriched environment.

Personalized medicine intensifies interest in experimental paradigms that delineate sources of phenotypic variation. The paradigm of environmental enrichment allows for comparisons among differently housed laboratory rodents to unravel environmental effects on brain plasticity and related phenotypes. We have developed a new longitudinal variant of this paradigm, which allows to investigate the emergence of individuality, the divergence of individual behavioral trajectories under a constant genetic background and in a shared environment. We here describe this novel method, the "Individuality Paradigm," which allows to investigate mechanisms that drive individuality. Various aspects of individual activity are tracked over time to identify the contribution of the non-shared environment, that is the extent to which the experience of an environment differs between individual members of a population. We describe the design of this paradigm in detail, lay out its scientific potential beyond the published studies and discuss how it differs from other approaches to study individuality. The custom-built cage system, commercially marketed as "ColonyRack", allows mice to roam freely between 70 cages through connector tubes equipped with ring antennas that detect each animal's ID from an RFID transponder implanted in the animal's neck. The system has a total floor area of 2.74 m and its spatial resolution corresponds to the size of the individual cages. Spatiotemporally resolved antenna contacts yield longitudinal measures of individual behavior, including the powerful measure of roaming entropy (RE). The Individuality Paradigm provides a rodent model of the making of individuality and the impact of the 'non-shared' environment on life-course development.