Ph.D. in Zoology 1993
Dissertation title: Photoperiod control of reproductive timing in the Pacific Harbor Seal and the California Sea Lion
xiii + 209 pages
The Pacific harbor seal (Phoca vitulina richardsi)
and the California sea lion (Zalophus californiaus) have reproductive
cycles which are characterized by seasonal breeding and delayed implantation.
Birth timing is precise, although latitudinal and regional variation may
occur. Termination of embryonic diapause in response to an environmental
stimulus has been proposed as the mechanism for precise birth timing.
This study examined the temporal patterns of birth in seals and sea lions
and tested the hypothesis that response to photoperiod sets the time of
Birth dates of pups born in captivity to identified cows were used to determine absolute and relative precision in birth timing. Individuals of both species demonstrated high precision. The presence of repeatable differences in timing between individuals suggests that birth timing has a heritable component and may evolve, thus allowing seasonal adaptation.
The effect of latitude on birth timing was evaluated using data from wild populations of harbor seals and from captive harbor seals and California sea lions. Harbor seals demonstrated three primary patterns of birth timing: a significant latitudinal trend between 30o and 50o N; late-birthing in Puget Sound; and no latitudinal effect at high latitudes. California sea lions demonstrated a significant latitudinal trend in birth timing.
Population-based photoperiod analysis was developed to assess the role of photoperiod in setting the timing of birth. Results of analyses demonstrated that response to photoperiod immediately prior to blastocyst implantation explained the high precision and latitudinal variation found in birth timing of both species. This hypothesis was tested directly by exposing captive populations of harbor seals to prolonged photoperiods between estrus and implantation. Subsequent birthing in experimental animals was significantly delayed demonstrating the causal relationship between photoperiod and implantation.
The photoperiod hypothesis implies that three distinct populations of Pacific harbor seals exist, each manifesting a specific birth timing pattern. To assess whether other differences existed among these populations, 29 metrical and 8 non-metrical cranial characters were measured in each of 154 adult skulls. Discriminant analyses demonstrated significant cranial morphometric differences between populations defined by birth timing. Hence, shifts in birth timing may allow population differentiation through seasonal reproductive isolation.