Abstract:
Birth and pupping season records of Pacific harbor seals
(Phoca vitulina richardsi) were used in an analysis of
photoperiod effect on the timing of birth in this species. Dates
and latitudes of births were converted into photoperiods using a
photoperiod equation. Sequential dates from parturition were
converted into photoperiods and plotted as a function of
latitude. The resulting functions of latitude vs. photoperiod
were regressed to determine the point during the reproductive
cycle at which latitude did not have an effect on photoperiod.
The Pacific harbor seal can be divided into two geographic
subgroups by their respective pupping seasons. Seals inhabiting
the Puget Sound / Strait of Georgia (PS/SG) System give birth to
pups an average of 88 days (P < 0.01) later than do seals of the
North American west coast. The dine in the pupping season of
coastal harbor seals can be defined by a 13.8 h/day photoperiod
occurring at 68 days post partum. PS/SG harbor seals experience
a significantly different photoperiod (P <0.001) of 10.8 h/day
at 68 days post partum. Progesterone and testosterone levels
support an hypothesis that these two populations may be
reproductively isolated, as coastal male seals show low
testosterone levels during the breeding season for PS/Sc seals.
A proposed photoresponse at 68 days post partum occurs at the
midpoint between estrus and implantation. This period is
characterized by secondary follicular activity in the ipsilateral
ovary and other changes occurring in the uterus and corpus
luteum. A response to a specific photoperiod terminating the
follicular activity and leading to a specific implantation date
is suggested.
