Age-associated changes in plasma testosterone levels in male mice and their relation to social dominance or subordinance

Plasma testosterone (T) levels were determined in male mice of the CD-1 (ICR) strain from 30 to 680 days of age. The mean plasma T was 5.2 ± 1.0 ng/ml for adult male mice (70–400 days old, 102 mice) and 1.8 ± 0.9 ng/ml for old males (450–680 days old, 11 mice). Through 140 determinations, the highest T value obtained was 52.3 ng/ml in an 80-day-old individual while the lowest was 0.08 ng/ml in a 680-day-old animal. It was found that plasma T was significantly decreased in the old age, although individual variation in T levels was considerable. On the other hand, social dominance or subordinance was examined among male mice in each cage and its relation to plasma T was investigated. Dominance or subordinance was verified by frequency of chases or attacks delivered or received, or number of fights won or lost. It was observed that dominant-subordinate relationships were present in 6 out of 10 cages and that the dominant individual had a significantly higher T level than the subordinate male; the mean T level was 10.5 ± 2.5 ng/ml for the dominant individuals vs 2.2 ± 0.8 ng/ml for the subordinate ones. Marked individual variation in plasma T titers in male mice seems to be related to the dominance/subordinance rank within a group.