Baseline Testosterone Levels Peak During the Inactive Period in Male Degus (Octodon degus)

Area of Science:

• Endocrinology
• Animal Behavior
• Rodent Physiology

Background:

• Testosterone is crucial for male reproductive functions.
• Previous studies show inconsistent links between testosterone and reproductive success.
• Methodological issues, like sampling during active periods, may explain inconsistencies.

Purpose of the Study:

• To investigate diel (24-hour) variations in baseline testosterone levels in male common degus (Octodon degus).
• To test the hypothesis that testosterone levels are higher during the inactive (nighttime) period compared to the active (daytime) period.

Main Methods:

• Blood samples were collected from captive male degus every 4 hours over a 24-hour cycle.
• Testosterone levels were measured at different time points.
• Gonadotropin-releasing hormone (GnRH) challenges were administered during daytime to assess testosterone response.

Main Results:

• Significant diel variation in baseline testosterone levels was observed in male degus.
• Baseline testosterone levels were significantly higher during the inactive nighttime period than during the active daytime period.
• Nighttime baseline testosterone levels were substantially lower than post-GnRH injection levels.
• Daytime GnRH challenges did not correlate with nighttime baseline testosterone levels.

Conclusions:

• Male degus display significant diurnal rhythmicity in baseline testosterone.
• Sampling time is critical for accurate testosterone measurements in degus and potentially other species.
• Daytime GnRH challenges are not suitable for estimating nighttime baseline testosterone levels in this species.