Environmental and physiological factors controlling the reproductive cycles of pinnipeds
The reproductive pattern of pinnipeds is characterised by embryonic diapause, annual reproduction, and synchronous breeding cycles. Active gestation lasts about 8 months in most species, but the duration of embryonic diapause varies indirectly with the length of the postpartum oestrous cycle. Data on reproductive endocrinology are limited to a few species, owing to difficulties involved in obtaining serial samples from individuals. The postpartum oestrus is marked by elevated oestradiol concentrations followed by a rise in progesterone levels after ovulation. An infertile cycle may lead to pseudo-pregnancy, although on rare occasions there may be a further period of oestrus. Progesterone concentrations remain elevated for the duration of pregnancy and chorionic gonadotrophin concentration in the placenta increases early in postimplantation. Sex steroids do not appear to be directly involved in blastocyst reactivation. The environmental factors influencing implantation appear to be the most significant proximate factors in timing pinniped reproductive cycles. In most species, implantation occurs when day length is declining, and a photoperiod of about 12 h may provide the signal for implantation in some species. Other proximate factors may include sea temperature, at least for grey seals. Pinnipeds have reproductive cycles with an active and an inactive phase. The inactive phase may be equivalent to the period of embryonic diapause. Transition from the inactive to the active phase and reactivation of embryonic growth are probably controlled by the same environmental cues.