Abstract

Reproductive patterns in female dolphins are unpredictable, reportedly varying with location, climate, season, and between wild and captive animals.¹² Data from studies of reproductive physiology in bottlenose dolphins indicate this species is polyestrous and ovulates spontaneously, but also indicate different ovulatory patterns between and within individuals.¹¹ Variations occur in hormone levels, numbers of ovulations per year, and the time of year ovulations occur.

Gross anatomical and histological evaluations of bottlenose dolphin ovaries have been used to estimate individual reproductive profiles, contributing to knowledge of the species' sexual cycle^{3,6,7,9,12,13,21} and morphologic changes in the ovaries observed using ultrasonography are documented.^1,2,16 However, no correlation between hormone levels and morphologic changes in the ovaries is reported and inferences about temporal relationships between significant parameters cannot be drawn. Regular monitoring of serum hormone levels may help to indicate reproductive status or periods of estrus, but the relationship of estrus to ovulation in the dolphin is not known. Monitoring of blood hormone levels entails frequent blood sampling and assays are costly. Blood sampling is invasive and repeated skin punctures may cause phlebitis and increase the risk of infection.

Progesterone (P) has been the hormone most frequently investigated in female dolphins.^{10,11,17,18,20,27}

The initial rise and subsequent levels of progesterone can indicate ovulation and the luteal phase of the cycle, but do not provide useful information about follicular development and the exact time of ovulation. Efforts have been made to correlate total estrogen (Et) and estradiol (E2) levels and ovulation in Tursiops,^11,17,18,27 but estrogen levels have not been shown to be a useful indicator of ovulation in this species. Data available for levels of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in dolphins^19,27 are scarce and are not useful indicators of ovulation. Comprehensive data on urinary hormone levels have been reported in the killer whale^15,25 but not in Tursiops. Copulation and sexual receptivity in Tursiops are independent of hormonal or physiologic influences,^4,24 and cannot be relied upon to accurately identify estrus. It has been stated that only by exogenously inducing ovulation, or measuring urinary estrogen levels, can the time of ovulation in dolphins be accurately determined.^14,20

Diagnostic ultrasonography has a recognized role in assessing reproductive events⁵ and is now being utilized more frequently in dolphins.^{1,2,8,14,16,22,23,26} Behavioral training, ultrasonographic procedures and the sonographic appearance of the normal Tursiops uterus and ovaries are reported.^1,2 Ultrasonographic monitoring of changes in the ovary over time allows assessment of the rate of follicular growth, the time of ovulation and the duration of the corpus luteum. Comparing morphologic changes in the ovaries to changes in hormone levels would enable more accurate characterization of hormonal changes and add to knowledge of the reproductive physiology of dolphins.

The main aim of this study was to investigate the value of ultrasonography as a non-invasive method of assessing and monitoring reproductive status in a group of female Tursiops aduncus in Hong Kong, by assessing the ovaries and monitoring follicular development through the normal ovarian cycle. The specific objectives of this study were to: determine whether ultrasonography could be used to identify and monitor folliculogenesis and accurately predict ovulation; to correlate observed ovarian changes with serum hormone levels; to determine any sexual cycle or seasonal influence on reproductive status; and to use knowledge of imminent ovulation to control natural breeding and plan artificial insemination procedures.

The ovaries of 11 female Indo-Pacific bottlenose dolphins (Tursiops aduncus) were monitored for up to 12 years using ultrasonography, and appearances were correlated with serum hormone levels. A total of 137 ovarian cycles were identified and 116 were analyzed. Developing follicles were identifiable at 4 mm diameter, follicular growth was monitored and ovulation identified. The corpora lutea were also measured. Diameters of follicles at ovulation were 17-23 mm (mean 19.9 ± 1.1 mm). The corpora lutea had a mean duration of 18.6 ± 3.0 days. Mean cycle length was 30.2 ± 1.7 days and 83% of ovulations were from the left ovary. Follicular atresia, cysts and persistent corpora lutea were observed. Progesterone, E2 and LH levels varied and were not useful in assessing reproductive status, except that progesterone could indicate ovulation had taken place. Ovulations have occurred in every month of the year. Periods of ovarian activity were interspersed with periods of anestrus of 2-27 months.

Controlled natural breeding and artificial insemination using prepared fresh and frozen semen have been planned and conducted based on sonographic predictions of ovulation. Successful pregnancies have occurred from all procedures. Ultrasonography provides a reliable method of assessing reproductive status and managing breeding in live female dolphins.

Acknowledgments

This work would not have begun without the foresight of the late Dr. Derek Chow. We would also like to acknowledge Dr J. Pete Schroeder and Professor Neylan Vedros for advice and support in the early days. Many people have been a part of the team. Special thanks are due to Paul Ng, Jackson Ng, Charles Chan, Harriet Chiu, Gary Wong, Dr. Crista Rayner, Hui Suk Wai, Chan San Yuen, Dr. Stephen Leatherwood, and Dr. Natalie Rourke. The Hong Kong Polytechnic University and successive Chief Executives at Ocean Park have supported this work.

References

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