Little is known about reproductive physiology in the two-toed sloth (Choloepus didactylus). Therefore, the aim of this study was to obtain detailed information about the ovarian cycle. Measurements of reliable gonadal steroids in the feces of this species were undertaken. For this purpose, fecal samples were collected one to three times per week from nonpregnant captive females (n = 2) over a 16-month period. Before assay analysis, the fecal samples were extracted with methanol. Radioimmunoassays and enzyme immunoassays for fecal progesterone, estradiol-17β, pregnanediol-glucuronide (PdG), and estrone sulfate were tested for their ability to detect the ovarian activity. Using the lowest and highest progesterone values, the ovarian cycle length was comparatively analyzed. The ovarian cycle (n = 26) averaged between 31.4 ± 9.1 days (lowest progesterone) and 32.5 ± 7.5 days (highest progesterone) throughout the whole year. The length of the follicular phase, as indicated by low progesterone levels, was 18.1 ± 4.4 days (range 12-25 days), and the length of the luteal phase, as characterized by elevated progesterone levels, was 13.2 ± 1.8 days (range 11-16 days). In contrast, estradiol-17β and estrone sulfate were not suitable to detect the cycle due to irregular collection intervals. Fecal progesterone and PdG, as well as estradiol-17β and estrone sulfate, significantly correlated (r = 0.621, P < 0.01 and r = 0.606, P < 0.01). PdG concentrations (dilution factor (DF) 1:40) were considerably higher than progesterone concentrations (DF 1:10), PdG amounted in the range of 1326.7 ± 320.2 ng/g wet feces (animal S1) and 1373.8 ± 468.3 ng/g wet feces (animal S2) compared with progesterone concentrations in the range of 98.0 ± 17.0 ng/g (S1) and 105.9 ± 30.0 ng/g (S2). The estrone sulfate levels (DF 1:2) were similar to estradiol-17β (DF 1:22). The mean fecal estradiol-17β concentrations were 6.7 ± 0.9 ng/g for animal S1 and 7.5 ± 1.6 ng/g for animal S2. In conclusion, the ovarian activity of the two-toed sloth was studied using the noninvasive method by means of the fecal steroid monitoring. Progesterone was the most reliable fecal steroid hormone to determine the duration of the ovarian cycle independent of the weekly defecation rate. The course of progesterone concentrations resulted in a cycle length of 4-5 weeks length in the two-toed sloth. Thus, the reproductive activity of the two-toed sloth does not show any seasonality like the three-toed sloth.
Keywords: Estrogen; Fecal hormone concentrations; Ovarian cycle; Progesterone; Two-toed sloth.
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