Effects of social isolation and the time of day on testosterone levels in plasma of C57BL/6By and BALB/cBy mice.

Adult male C57BL/6By and BALB/cBy mice were housed either in large groups (20 per cage) or individually, and levels of plasma testosterone were measured in samples taken in the morning (9 to 10 A.M.) and in the evening (9 to 10 P.M.). No significant strain differences were found in testosterone levels, but the mean testis weight was significantly higher in the BALB/cBy strain. Two-way analysis of variances of pooled plasma testosterone data showed that social isolation of males results in a significant increase in A.M. (but not P.M.) testosterone concentrations and increased testis weight in both strains. Our results suggest that differential housing of a social species can affect testicular function. Since testicular function can also be influenced by the time of day, the question is raised whether the expression of circadian variation in plasma testosterone level is dependent on population density.