Abstract
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.
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.