Changes in plasma progesterone, estrogen, and testosterone concentrations throughout the reproductive cycle in female viviparous blue-tongued skinks, Tiliqua nigrolutea (Scincidae), in Tasmania

Changes in mean plasma concentrations of progesterone (P4), estrogen (E), and testosterone (T) in a captive population of female viviparous skinks, Tiliqua nigrolutea, were examined. Reproductively active and quiescent individuals were present in the population concurrently, allowing a comparison between these two conditions. Mean plasma progesterone concentrations were basal (1–2 ng ml⁻¹) until the start of gestation and peaked in the second trimester (12.7 ± 1.27 ng ml⁻¹), before falling significantly prior to parturition. An increase in mean plasma estrogen concentrations occurred coincident with the vitellogenic period; the mean plasma estrogen concentration peaked at 715.1 ± 106.68 pg ml⁻¹ shortly before ovulation. Mean plasma testosterone concentrations in reproductively active females peaked in the periovulatory period (6.3 ± 0.63 ng ml⁻¹) and had returned to basal concentrations (<1 ng ml⁻¹) 2 weeks later. Changes in mean plasma steroid concentrations were correlated with reproductive condition and are discussed in terms of potential in vivo steroid interactions and the multihormone control of reproduction.