Summary. An equation for determination of the photoperiod at any given latitude for any given date is presented and used in an analysis of reproductive timing in the northern fur seal in which there is an obligatory delay of implantation. Fur seals breeding on San Miguel Island, California (33°N) displayed a mean date of parturition that was 14 days earlier (P < 0·001) than that of the parent stock on the Pribilof Islands, Alaska (57°N). Previous studies have shown that changes occur in the corpus luteum, in follicles in the ovary containing the corpus luteum, in concentrations of plasma progesterone and oestradiol-17β, and in the uterine lining when there is a mean photoperiod of 12·5 h/day. This photoperiod occurs at both locations at 62 days after the mean dates of parturition, and may act as a cue for the initiation of implantation in these seals.
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