Abstract

The California sea lion (Zalophus californianus) has a synchronized, annual, reproductive cycle. Females give birth in May and June and mate in July. After fertilization, there is a period of delayed implantation until October when the blastocyst implants and active gestation begins. To determine whether progesterone could be used to predict pregnancy, we characterized the annual progesterone profile of the California sea lion.

At The Marine Mammal Center (TMMC) in Sausalito, CA, sick and injured sea lions are brought in for treatment and returned to the wild. From 1998 through February 2001, serum was banked from 209 adult female sea lions. Multiple samples were obtained from some animals resulting in a total of 367 serum samples. Animals were sampled throughout the year, but most samples were clustered around 2 outbreaks of domoic acid toxicity when greater numbers of adult females were admitted to the center. These outbreaks occurred in May 1998 around the time of parturition and July 2000 during estrus and mating.

Progesterone concentrations were measured by radioimmunoassay at the Alaska SeaLife Center in Seward, AK. Concentrations ranged from 0.03 to 30.34 ng/ml. Mean concentrations for all females were elevated in October at the time of implantation and depressed in May toward the end of the gestation period (Fig. 1). This is consistent with histological results from fur seals (Craig 1964) and California sea lions (Buckles 2001) which found increased luteal activity at the time of implantation and regression of the corpus luteum in February.

In February, elevated progesterone concentrations measured in pregnant sea lions compared with non-pregnant sea lions might indicate a fetal or placental source of progesterone rather than a luteal source. The two pregnant animals sampled in February indicate a potential for pregnancy prediction during the last half of gestation, but more samples are needed from pregnant animals in January through April.

Figure 1. Mean progesterone concentration, averaged by month, for 209 serum samples from 209 adult female California sea lions. Error bars represent standard error. Sample size is equal to one for March and April.

References

1.  Craig AM. (1964) Histology of reproduction and the estrus cycle in the female fur seal, Callorhinus ursinus. J. Fish Res. Bd. Canada, 21(4): 773-811

2.  Buckles E, Greig D, Gulland F, Lowenstine L. (2001) Histologic morphology of the california sea lion (Zalophus californianus) reproductive track throughout the estrous cycle. Poster abstract from the 14th Biennial conference on the Biology of Marine Mammals, November 28-December 3, Vancouver, Canada.