Plasma progesterone levels and corpus luteum morphology in the female prairie dog (Cynomys ludovicianus).

Plasma progesterone levels in female prairie dogs were determined by a radioimmunoassay specific for progesterone. Plasma progesterone levels were determined in samples taken before estrus, at estrus, during the luteal phase, and during anestrus from females maintained all year in the laboratory. Progesterone levels were also determined in plasma samples taken in the laboratory from two pregnant and three postparturient females captured in the field. Progesterone levels were low before estrus and continued low during estrus. They rose on the first week after estrus to 0.8 ng/ml or above and continued at or above this level for 9-14 weeks following estrus. Gestation in prairie dogs is 35 days in this species. Progesterone levels of three postparturient females were above 1.0 ng/ml for 7 weeks after their arrival in the laboratory. These females all had uterine scars showing that they had delivered their litters before they were captured. Two females were determined to be pregnant at the time of their capture. These females later reabsorbed their fetuses (determined by laparotomy). Progesterone values of samples from these females were all above 1.0 ng/ml except for one low value in one female which occurred 3 weeks after her capture and after reabsorption of her fetuses was in progress. The cells of the corpus lutea (CL) of nonpregnant, pregnant, and postparturient females had well-developed rings of cytoplasmic basophilia but as the CL regressed this pattern became disorganized and disappeared. The function of this basophilia is not known. The long luteal phase found in female prairie dogs is compared to those found in other species of mammals. This is the first annually breeding rodent reported to have a longer luteal phase that the period of gestation.