Study on the estrous cycle in the Mongolian gerbil (Meriones unguiculatus)

The paucity of information and the unagreed consensus about the estrous cycle of the Mongolian gerbil (Meriones unguiculatus) have rendered such studies rather difficult. The estrous cycle of the Mongolian gerbils in this report were divided into 5 stages: Stage I, proestrus; Stage II, estrus (scattering); Stage III, estrus (gathering); Stage IV, metestrus and Stage V, diestrus. The normal estrous cycle in the Mongolian gerbils was 4-6 days long. In our experimental conditions, 67.9% of virgin females had a 4-6 day cycle, whereas 26.4% had an unsettled cycle and 5.7% assumed pseudopregnancy. The changing pattern of the estrous cycle and copulatory behavior of the Mongolian gerbils after hormone (PMSG: pregnant mare serum gonadotropin, 10 IU; hCG: human chorionic gonadotropin, 10 IU) injection were examined. The change in the estrous stage after hormone injection could be roughly classified into two types. The vaginal smear of virgin females injected with PMSG at Stage I, II or III changed to Stage V the next day and to Stage I or Stage II after 48 hr, but in the case of Stage IV or V, the smear changed to Stage I after 48 hr. The females injected with PMSG at Stage I, II or III copulated at from 13:00 to 23:00, whereas others injected at Stage IV or V scarcely copulated at all between 13:00 and 23:00. Mating of these females with the male midnight was not observed.