Assessment of Testicular Growth and Sexual Maturity in the Indo-Pacific Bottlenose Dolphin (Tursiops aduncus)
Despite many reports of captive bottlenose dolphin births, there is still a lack of information about the reproductive physiology of the bottlenose dolphin, particularly the male dolphin. Ultrasonography of the testes is a technique that yields important information on male reproduction.2 Studies that investigate basic reproductive patterns, influence of season, and collection and evaluation of gametes, will yield information that can be applied to the management of captive dolphins. This includes controlled breeding to maintain self-sustaining groups that are genetically fit. Assisted reproductive techniques (ART), such as artificial insemination (AI), may be used to enhance controlled breeding. Again, understanding of reproductive physiology, specific to the species of interest, is required if AI is to be successful.3
This study aims to gather specific information about the reproductive development of male Tursiops aduncus. Data are collected from five animals of different reproductive status, four of which are captive born and so of known-age. The size (length and volume) of the testes are measured weekly using B-mode ultrasonography.1 Immediately following ultrasonography, semen samples are collected. Both procedures are carried out under trained voluntary behaviours. Ejaculates are assessed for basic semen quality parameters, including density, motility, viability, pH and volume, within an hour of collection. Blood samples are taken on a monthly basis, on the same day as ultrasound examination and semen collection, for serum testosterone level evaluation.
Preliminary results (October 2002 to November 2003), show that the testes size of a 19-year old adult male (MO) remained constant (right testis: length 20.16 ±0.91 cm, volume 493.49 ± 57.79 cm3; left testis: length 19.85 ± 0.85 cm, volume 498.41 ±75.77 cm3). In contrast, testes size in a sexually mature 9-year old male (PO) and a maturing 7-year-old (TO), fluctuated markedly. The testes of two immature males have increased in size in proportion to their increasing body lengths.
Spermatozoa were first found in the semen of the maturing male (TO) at the age of 7 years and 3 months. The semen quality of this male is poorer than that of the mature males. The semen quality of the 9-year old male (PO) is good, and comparable to that of the 19-year old male. A lack of spermatozoa in the semen occurred only once in MO and PO, but more frequently in TO. Such lack in spermatozoa does not appear to follow a pattern and any possible seasonal changes in density remain to be investigated. Spermatozoa have not been found in the ejaculates of the younger males, now 4 and 5 years of age.
The difference in testosterone levels between the three sexually mature males is approximately 10-fold; MO has exhibited the highest level (70.24 ng/ml); the highest level recorded in PO is 7.89 ng/ml and in TO is 6.27 ng/ml. Similar and possible seasonal patterns were found in these dolphins, with peak levels in April / May and July / August and lowest levels in December, however, further data are required to confirm this.
Continuation of the study will provide further information that will help in the understanding of the reproductive physiology during sexual maturation in dolphins. Additional data will establish a more reliable age, or age range, at which spermatogenesis occur in Tursiops aduncus and will further document testosterone changes and the rapid growth of the testes prior to the onset of spermatogenesis previously reported.1 The study will provide, for the first time, information on the quality of semen immediately following the onset of spermatogenesis, until such time the animal is deemed 'reproductively effective'. Longitudinal data will also provide further evidence about the male dolphin reproductive cycle and any effects of seasonality, particularly changes in testis size and spermatogenesis, where a lack of consensus between investigators remain.
The authors are grateful to Gary Wong, Harriet Chiu and the trainers of Ocean Park's Marine Mammal Department for their invaluable contributions in dolphin training, husbandry and handling, and the Clinical Laboratory staff for their support and assistance in semen evaluation.
This project is funded by the Research Grants Council of Hong Kong (Grant ref: PolyU5287/01).
1. Brook FM. 1997. The use of diagnostic ultrasound in assessment of the reproductive status of the bottlenose dolphin, Tursiops aduncas, in captivity & applications in management of a controlled breeding programme. Ph.D. dissertation. The Hong Kong Polytechnic University, Hong Kong.
2. Robeck TR, BE Curry, JF McBain, DC Kraemer. 1994. Reproductive biology of the bottlenose dolphin (Tursiops truncatus) and the potential application of advanced reproductive technologies. J. Zoo & Wildlife Medicine. 25. Pp. 321-336.
3. Wildt DE. 1989. Reproductive research in conservation biology priorities and avenues for support. J. Zoo & Wildlife Medicine. 20. Pp. 391-395.