Control of annual reproduction in the female harbor seal, Phoca vitulina
Creators: name: Bigg, Michael Andrew
Publisher: name: University of British Columbia
Year: 2011

description: It was noted recently that the annual timing of the birth season varies regionally in the harbor seal Phoca vitulina. In any one area births occur once a year during a 1 - 2 1/2 month period but between areas vary by up to 5 or 6 months with clinal variations over much of the species distribution. To explain these variations, this study describes the reproductive cycle of female harbor seals and investigates the factors which control reproductive timing. The study is divided into one field project and two laboratory experiments. The field project decribes the annual pregnancy and nonpregnancy cycles and discusses the role of negative steroid feedbacks from the corpus luteum and/or placenta in annual reproductive timing. It is based on a macroscopic and microscopic examination of 135 adult genital organs collected in southeastern Vancouver Island. The first laboratory experiment describes the estrous cycle of 15 captive adults based on vaginal smears and vaginal and uterine biopsies. These females were representatives of three regions, southeastern.and northeastern Vancouver Island and northern California, and were kept under the same outdoor conditions for up to 4 years. From this the roles of nutrition, temperature and genetic variation in reproductive timing are discussed. In the second experiment, the effect of photoperiod on the timing of estrus is tested and the existence of an endogenous cycle is discussed for females from southeastern Vancouver Island and northern California. The results show that females in all populations have a monestrous cycle. In southeastern Vancouver Island the pregnancy cycle consists of 2 - 2 1/2 months delay of blastocyst implantation, 8 - 8 1/2 months placentation and 6 weeks of lactation. The follicular phase extends from shortly after parturition to the end of lactation and the luteal phase from conception to term. A nonpregnancy cycle occurs when: 1) no ovulation takes place, which results in 10 months of anestrus followed by 3-4 weeks of proestrus; 2) ovulation but no fertilization occurs, which results in 2-3 months pseudopregnancy and 7 months of anestrus; and 3) ovulation and fertilization are followed by a prematurely terminated pregnancy, which results in anestrus. The main reproductive difference between populations is that the whole cycle is shifted either earlier or later and is in phase with a different time of the year. Within each population, photoperiod and an endogenous cycle are the main proximate controls for the annual timing of estrus. Photoperiod probably sets the annual timing of the endogenous cycle so that estrus occurs at a particular time of the year. Between populations, genetic variation maintains differences in the timing of estrus. Differences in timing may be maintained by population-specific responses to the annual light cycle. In this way each population would set the timing of the endogenous cycle differently so that estrus occurs at a different time of the year. The adaptive value of a population-specific response to photoperiod could be to permit each population to set the timing of the entire cycle so that estrus, conception, implantation, birth and weaning occur at the optimum times for survival. The ultimate factor in determining the timing of the whole cycle may be food (shrimp, Crangon) for the newly weaned pups.