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

Abstract

The steroid hormone testosterone is important for stimulating male reproductive processes including territory acquisition, mating displays, and spermatogenesis. When examining the relative effects of testosterone on reproductive processes, it is most reasonable to focus on peak baseline testosterone levels, especially for reproductive processes that may occur during specific times of day, such as mating and spermatogenesis. However, some studies have not found consistent positive relationships between circulating testosterone levels and reproductive variables. These nonsignificant relationships could be driven by methodology, as most studies in wild, free-living animals collect blood samples during an animals' active period, yet many species show peak baseline testosterone levels during their inactive period. This may be the case for the common degu (Octodon degus), as field and laboratory studies have exclusively sampled these diurnal rodents during their active period and have found little correlation between testosterone levels and reproductive success. In this study, we measured testosterone levels in captive male degus every 4 h across a 24-h cycle to test the hypothesis that male degus demonstrate diel variation in their baseline testosterone levels. We saw significant variation in male degu baseline testosterone levels over a 24-h period, and our prediction that baseline testosterone levels would be higher during nighttime (inactive period) timepoints compared to daytime (active period) timepoints was supported. However, nighttime baseline testosterone levels were still several magnitudes lower than testosterone levels after a gonadotropin-releasing hormone (GnRH) injection. While GnRH injections significantly increased circulating plasma testosterone levels during any daytime time period, we found no significant correlation between nighttime baseline testosterone levels and post-GnRH testosterone levels, which suggests GnRH-challenges during the daytime cannot be used to approximate or estimate nighttime baseline testosterone levels. These findings expand our knowledge surrounding testosterone dynamics and suggest that future studies should take into account the time of day when sampling testosterone and other hormone levels.

Fig. 1.

Experimental timeline of sampling reproductively active, singly housed, 3-year-old male degus (Octodon degus) for testosterone. Black and white bars indicate the times of day when lights were off and on, respectively. Degus were split into three groups (n = 6 males per group). Black and gray text indicate a group's first and second sampling timepoint, respectively; each group received 2 weeks of recovery in between their sampling timepoints. During lights-off sampling time points, only baseline testosterone samples were taken (one blood droplet), whereas during lights-on time points, testosterone was measured at both baseline and after injection of gonadotropin-releasing hormone (two blood droplets surrounding a syringe).

Fig. 2.

Plasma testosterone levels in male degus 0 (n = 16), 30 (n = 4), 60 (n = 4), 120 (n = 4), and 180 (n = 4) min after injection of gonadotropin-releasing hormone. Boxes represent 25th–75th percentiles, middle lines represent medians, and whiskers represent minimum and maximum values. Different letters indicate significant differences between timepoints.

Fig. 3.

Baseline plasma testosterone concentrations of male degus at six different time points (n = 6 per time point). Different letters represent significant differences between time points. Shaded and white boxes represent sampling periods during lights-off and lights-on, respectively. Boxes represent 25th–75th percentiles, middle lines represent medians, and whiskers represent minimum and maximum values.

Fig. 4.

Male degu daytime baseline testosterone levels (white boxes, n = 6 per time point) and testosterone levels 2 h after injection of gonadotropin-releasing hormone (GnRH) (stippled boxes, n = 6 per time point). *** represent significant differences (P < 0.001) between baseline and post-GnRH testosterone levels within a time point. Boxes represent 25th–75th percentiles, middle lines represent medians, and whiskers represent minimum and maximum values.

Fig. 5.

Correlations between (a) daytime baseline and daytime post-gonadotropin-releasing hormone (GnRH) testosterone levels (n = 18; r² = 0.29), (b) nighttime baseline and daytime post-GnRH testosterone levels (n = 17), and (c) daytime and nighttime baseline testosterone levels (n = 17) in individual male degus. The equation for the best-fit line in (a) is: y = 2340x–2648.