DEVELOPMENT AND REGRESSION OF THE CORPUS-LUTEUM IN GREY SEAL (HALICHOERUS-GRYPUS) OVARIES AND ITS USE IN DETERMINING FERTILITY RATES

Adult female gray seals were sampled at the Farne Islands [England, UK] at monthly intervals, from Nov. 1980-Oct. 1981. The distribution, size and number of ovarian corpora were recorded in each case and blood samples were obtained for progesterone analysis. The concentration of plasma progesterone was .apprx. 6 ng/mL for most of gestation, including embryonic diapause; it rose to .apprx. 10 ng/mL during the final mo. Progesterone declined sharply, to < 1 ng/mL at parturition. The size of the corpus luteum was constant throughout embryonic diapause, but after implantation it grew continuously until parturition. Following parturition, it regressed rapidly at 1st, forming a corpus albicans, and then more slowly, with regression being arrested during the period of fetal growth of the following reproductive cycle. Most corpora albicantia had disappeared 1 yr after their formation. Corpora albicantia may be useful indicators of reproductive history in gray seals, providing that allowance is made for the reproductive condition of females at the time of sampling, and for several other possible errors which could arise. Retrospective calculation of reproductive statistics by > 1 yr, using corpora albicantia, is not valid for gray seals.