Varying Sensitivity of C57BL/Crg1 Mice to Grouping

Mice responded differentially to increases in population size, depending on their prior experience. Compared to animals grouped prior to an increase in population size, animals housed as isolates responded to grouping with more fighting, a greater diminuation of testis weight, and a higher level of locomotor activity. They also had lower testis weight and higher locomotor activity than individually housed, nongrouped controls.