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Reproduction in the female fur seal Callorhinus ursinus (Linn.) Craig, Allison Maud

Abstract

Gross and histological analyses of reproductive tracts from multiparous, nulliparous, and non-pregnant females of Pribilof Islands (eastern Pacific) origin have been used to determine the histology and physiology of the estrus cycle, the age of sexual maturity, and certain causes of prenatal mortality. The ovaries alternate in function, one ovary ovulating in one breeding season, the opposite ovary ovulating in the next. An undetermined factor suppresses follicular development in the ovary containing a corpus luteum of pregnancy; suppression maintains for 6 months after parturition. Graafian follicular development is greatest in July, with an average 15 follicles in the ovary destined to ovulate. About 4 of these will enlarge abruptly prior to ovulation; one will reach ovulation size (10 mm or greater in diameter) and rupture; the rest will become atretic. Mating occurs 3-5 days after parturition in mid-July. If fertilization is accomplished, the resulting blastocyst remains free in the uterus until early or mid-November, when it implants in the mucosa. The newly formed corpus luteum is actively secretory for a month after ovulation. During this time, follicular development is suppressed in both1 ovaries, and the uterine mucosa is progesteronic. From the end of August, the corpus luteum is regressive; the luteal cells are vacuolated, and secretion is minimal. This is reflected in a recrudescence of follicular activity in the ovary containing the corpus luteum, and regression of the associated uterine mucosa. Immediately prior to implantation the corpus luteum resumes secretion, and the mucosa is prepared for implantation. Placentation is established during December. Luteal degeneration begins in January and is progressive until parturition, when the corpus luteum appears to be non-secretory. Luteal degeneration has no affect on the pregnant uterine horn; it is assumed that the placenta secretes sufficient hormone to replace the luteal hormones. Prenatal mortalities take 3 forms: "missed” pregnancies, abortions, and resorptions. On the basis of histological examinations, "missed" pregnancies are ultimately due to failure of implantation. Three conditions are responsible: failure of fertilization, failure of implantation, or malfunction of the reproductive tract which precludes pregnancy. Abortion results in the expulsion of a conceptus from the uterus before term; resorption is a gradual process of dissolution of the conceptus within the uterus. On the basis of histological evidence, three causative agents of abortion and resorption are evident: degeneration of the corpus luteum after implantation, malfunction of the uterine mucosa, and intra-uterine mortality of the conceptus. Annovulatory cycles, dominated by Follicle Stimulating Hormone, occur among females 1 to 3 years of age. A cycle increasingly dominated by Luteinizing Hormone and culminating in ovulation occurs among the majority of 4-year-old females. The follicular cycle resulting in the first ovulation is later than subsequent cycles; the peak of first ovulations occurs around August 25th. On the basis of gross examinations, 60% of females ovulate for the first time at 4 years of age. Since the pregnancy rates of 5-year-old females are consistently about 50%, probably a number of 4-year-olds do not mate, either because ovulation occurs after the breeding season, or because breeding males are not available. Gross and histological analyses of reproductive tracts from nulliparous females of Robben and Commander Islands (western Pacific) origin have been used to compare the reproductive potential of two breeding populations of fur seals,; Pregnancy rates among females 4 years of age of Robben and Commander Islands origin are approximately 50%; those of 4-year old-females of Pribilof Islands origin are approximately 5%. The disparity in the age of reproductive maturity between females of the two populations is based on a physiological delay of one year in the maturation of the endocrine system controlling reproduction among females of Pribilof Islands origin.

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