Abstract

Aspects of the general biology of many species of Delphinidae have been studied over the past century, however, despite concerns about conservation and sustainability, detailed studies of reproductive physiology have been lacking. Several species of dolphins have been maintained in captivity for a similar length of time, affording more easily accessible subjects for research, but only recently have scientists begun to investigate reproductive performance and parameters in any detail.

Now, more is known about the reproductive cycle in the female of a few species. Killer whales (Orca orcinus) appear to ovulate regularly throughout the year.^19,25 Small cetacean species, such as the dusky dolphin (Lagenorhyncus obscurus)²⁴ and the harbour porpoise, (Phocoena phocoena)^14,18 appear to have a clearly limited, seasonal pattern of reproduction. However, medium sized dolphin species, such as the bottlenose dolphin (Tursiops spp.) and the Indo-Pacific hump-backed dolphin (Sousa chinensis) do not seem to have such an easily definable pattern, with reproduction often being described as 'diffusely seasonal'. Data from both wild and captive populations show that births may occur at any time of year.^3,7,8,23,26

The maintenance of bottlenose dolphins (Tursiops truncatus and aduncus) in captivity has enabled closer study of the reproductive anatomy^{4,5,6,10,16,17} and physiology ^{12,13,20,21,22,27} of this species. The anatomy of the uterus and ovaries is reported and is similar in all cetaceans studied. Serum progesterone data were also similar and indicated an irregular pattern of ovarian cycling in Tursiops, which was difficult to predict.^20,27 Data showed that estrus varied markedly between and within individuals. Generally, P levels of < 1.0 ng/ml were determined to indicate anestrus. Elevated levels (3.0-34.0 ng/ml) indicated ovulation and the luteal phase of the cycle, but did not provide useful information about follicular development and the time of ovulation. P levels of 1-3 ng/ml were classified as indeterminate.^{11,13,20,21,22,27} Studies of serum total estrogen (Et) levels in T. truncatus showed no variation between anestrus and estrus.^{12,13,20,21,27} Yoshioka et al. (1986) reported serum estradiol (E2) levels of < 70 pg/ml during anestrus and 125-200 pg/ml during estrus. Although contributing useful information about reproductive physiology, investigations of serum steroid hormone levels did not allow direct assessment of ovarian activity in live animals.

More recently, ultrasonography has allowed observation of real time morphologic changes in the ovaries of T. aduncus and correlation of ovarian cycle phase with serum P and E2.^1,2 P levels were < 0.2-0.9 ng/ml during anestrus and < 0.2-18.5 ng/ml during estrus. E2 levels were 8.4-106.3 pg/ml during anestrus and 11.2-379.2 pg/ml during estrus. These levels are similar to those reported for T. truncatus. This was the first time real time ovarian events were correlated with serum hormone data in dolphins.

Some aspects of the reproductive biology of the Indo-Pacific humpback dolphin have been studied in the wild in South Africa^3,7,9 and Hong Kong.^7,15 However, it is difficult to collect sufficient data about reproductive physiology from wild animals and still little detailed information is known about reproduction in S. chinensis. These studies reported similar results, in that births were observed throughout the year, although more newborn calves were seen in spring/summer.^3,7 Gestation length has been estimated at about 11 months and females are believed to attain sexual maturity at 9-11 years of age.^3,7

Underwater World Singapore (UWS) opened their Dolphin Lagoon in 1999 and is one of two facilities known to maintain the Indo-Pacific humpback dolphin in sufficient numbers for breeding. The group consists of two adult males and three adult females. Underwater World Singapore is committed to controlled breeding and sustainable management of their population, as well as the collection of sound scientific data from the species.

This project to study the ovarian cycle using ultrasonography began in 2002; the aim was to assess and monitor reproductive status and patterns in the females. The specific objectives of the study were to establish the normal sonographic appearance of the ovaries in Indo-Pacific humpback dolphin, determine whether sonography could identify and monitor folliculogenesis, correlate real time ovarian changes with serum hormone levels, and determine any seasonal influence on reproductive status.

This project was aimed to collect data about the ovarian cycle of the Indo-Pacific humpback dolphin (Sousa chinensis) using sonographic examination of the ovaries and serum hormone level assessment. The ovaries of two non-pregnant females were assessed at least once weekly and one pregnant female was assessed when possible. A total of 15 normal ovarian cycles, one episode of follicular atresia and one episode of a luteinized cystic follicle were detected using ultrasonography and serum progesterone levels. Two more cycles were detected on serum progesterone levels but not on ultrasonography. The mean ovarian cycle, mean follicular phase, mean follicle diameter measured just before ovulation, mean luteal phase and average diameter of the corpus luteum were determined. Data showed an irregular pattern of ovarian cycling, varying between and within subjects. Serum progesterone and estradiol levels were measured 1-3 times per month in two females and intermittently in one other female. Recorded progesterone and estradiol levels during the follicular phase, luteal phase, and anestrus are reported. Progesterone levels during pregnancy are also included.

Acknowledgements

This project would not have been possible without the efforts of the team of marine mammal trainers and veterinary technicians at UWS, as well as the support of the managers and owners. Progesterone assays were performed by the O & G Research Laboratory, The National University of Singapore. This project is funded by the Research Grants Council of Hong Kong [Grant ref: PolyU5287/01].

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