Secondary Corpora Lutea Enable Early Conception in the Breeding Season in Sika Deer (Cervus nippon)

In sika deer Cervus nippon, the corpus luteum (CL), which is formed after ovulation, secretes progesterone, which is essential for establishing and maintaining pregnancy. Typically, one CL develops per pregnancy; however, some cervids exhibit multiple CLs even when carrying a singleton. In sika deer, secondary CLs are observed in approximately 80% of pregnant females; however, their absence in some females suggests that they may not be essential for maintaining pregnancy. In this study, we investigated whether secondary CLs support progesterone secretion and facilitate earlier conception. This study was conducted in Nishiokoppe-mura, Hokkaido, Japan. Uteri and ovaries were collected from 1620 wild sika deer (C. n. yezoensis) (> 1 year old), legally harvested through hunting and population control operations, and the number and size of CLs were measured. Fetal size was measured to estimate conception dates. The pregnancy rate was 94.1%, and the twinning rate was 0.93%. Females carrying a single fetus had 1-5 CLs, whereas those carrying twins had 2-4 CLs. The mean number of CLs in females carrying one fetus was 1.86, with secondary CLs occurring in 83.1% of all cases. Notably, the estimated median conception date was significantly earlier in females with secondary CLs than in those with only one CL, with a difference of approximately 20 days. These results suggest that when luteal function is inadequate during the early breeding season, sika deer develop secondary CLs to support progesterone secretion and allow for early conception. This adaptation may result in optimal birth timing and serve to enhance reproductive success.