Sex differences in daily activity have been attributed to the actions of gonadal steroid hormones. We tested this hypothesis by measuring the impact of male-typical levels of testosterone (T) on female rats’ daily activity. Analyses were conducted on pooled data from three studies in which adult female Sprague-Dawley rats were implanted with blank or T-filled capsules (and adult males were implanted with blank capsules); home cage wheel running activity was measured 23 h/day before and for 3 weeks after capsule implantation. Approximately 75–80% of running occurred during the dark phase of the light-dark cycle in all groups, throughout the study. Before capsule implantation, the total number of wheel revolutions was significantly higher in females than males at the end of the light phase and during most of the dark phase; running speed and duration of daily running were higher in females compared to males. During the 3 weeks after capsule implantation, dark phase (and end-of-light phase) wheel revolutions, and average and maximum running speeds increased in all groups; however, T exposure blunted females’ running increases during the second half of the dark phase, such that running during that period was similar to that of males by week 3. T also decreased the duration of females’ active time, primarily by shifting offset of wheel running earlier, making it comparable to that in males. T non-significantly decreased females’ running speeds. These results indicate that, within 3 weeks, T defeminizes females’ daily activity in multiple ways. Thus, in addition to producing myriad physiological changes, male-typical levels of T also alter females’ locomotor behavior.
