The phase of the estrous cycle modifies the endocrine, metabolic and behavior rhythms in ewes

The aim of this study was to compare circadian rhythms of serum cortisol and insulin concentrations, glycemia, glycemia:insulin ratio (GLY:INS), rectal, vaginal and surface (ST) temperatures as well behavioral patterns during the follicular (FOL) and luteal (LUT) phases in ewes. All the variables were measured during both phases of the estrous cycle in 8 West African ewes during a 24 h period. Cortisol concentrations were greater during FOL than LUT (30.9 ± 2.1 ng mL^-1 vs 24.1 ± 2.1 ng mL^-1; P < 0.01); cosinor analysis revealed also differences in the circadian pattern: while the acrophase of the cortisol secretion during the FOL occurred at 10:00 ± 0:49 h, coinciding with higher environmental temperatures during the LUT was observed at 05:07 ± 0:49 h (P < 0.05). There were no differences in insulin concentrations according to the phase of the estrous cycle (FOL: 43.3 ± 6.6 μg mL^-1 vs LUT: 44.9 ± 6.6 μg mL^-1); however, insulin secretion varied according to the time of the day (P < 0.01) and showed a strong robustness in both phases of the estrous cycle (>40%), with acrophase occurring at sunset. Glycemia was greater during the FOL than the LUT (69.9 ± 1.1 mg dL^-1 vs 63.9 ± 1.1 mg dL^-1, respectively; P < 0.01), and varied according to the time of the day (P < 0.05), without interaction between both effects. However, the strength of rhythmicity was <40% during both phases. GLY:INS ratio was greater in FOL than LUT (2.2 ± 0.2 vs 1.8 ± 0.2, respectively; P = 0.04), indicating that the greater glycemia observed during FOL did not trigger the same secretion of INS than during LUT. Rectal and vaginal temperatures, as well as nasal vein, front and neck ST were greater during FOL than during LUT (P < 0.05). There was no circadian rhythm for vaginal temperature but, both rectal temperature and the mean surface temperature showed a strong robustness (>65%). The mesor of the rectal temperature was greater during the FOL than during the LUT (39.2 ± 0.1 °C vs 38.8 ± 0.1 °C, respectively; P < 0.05). Ewes were observed urinating more times during the FOL than during the LUT (7.3 ± 1.5% vs 1.7 ± 1.5%, respectively, P < 0.05). Ewes were observed standing, drinking water and in bipedal position more times during FOL than during the LUT (P = 0.07, P = 0.08 and P = 0.07, respectively). In conclusion, the phase of the estrous cycle affected the secretion and the circadian rhythm of cortisol and in consequence the metabolic activity, thermoregulatory response and behavior pattern in ewes.