Reports of the age at which male Tursiops reach sexual maturity or 'reproductive effectiveness' are largely anecdotal and the topic has not been widely investigated. Identification of the onset of sexual maturity in male dolphins is essential for controlled breeding programs so young animals may be selected to breed for wider and earlier genetic representation.
This four year study systematically monitored five male T. aduncus, four of which were captive born. Male 1 was already mature and a proven sire. Male 2 was 8½ years old and was sexually mature. Males 2-5 were 3-7½ years old and sexually immature when the study began. Sonographic examination of the testes and semen collection were performed once a week. Evaluations of individual ejaculates included motility, viability, pH, density, and volume. Ejaculates were centrifuged for examination of sediment if spermatozoa were not detected on initial wet-mount preparation. Blood samples were collected monthly for testosterone evaluation with an enzyme immunoassay kit (VIDAS bioMérieux sa, France).
Age at sexual maturity was defined by the first identification of spermatozoa in the semen. Ejaculates prior to onset of spermatogenesis were low in volume (1-3ml) and pH was >9.0, slightly higher than the pH noted in semen with spermatozoa (mean range 8.1-8.5). Onset occurred at ages 7 years and 3 months in M3, 6 years and 7 months in M4, and 6 years and 9 months in M5. Testicular size (length, volume) around the time of onset was similar in M3 (17.4cm, 172.6cm3) and M4 (17.2cm, 178.0cm3). Serum testosterone level was 1.7ng/ml in M3 and 24.1ng/ml in M4. Both testis volume and testosterone level underwent rapid increase during the 4 months preceding onset in M4. Testicular size and testosterone level at the time of onset of spermatogenesis in M5 were markedly lower; 7.5cm, 10.8cm3 and 0.2ng/ml, and did not show the same pattern of rapid increase. Ejaculate density during the 1st post-onset year reached 10.4 ± 36.2 x 106/ml in M3 and 54.6 ± 94.5 x 106/ml in M4. Spermatozoa were rare in M5's semen during the 10-month post-onset period monitored. Ejaculates of M2 and M3 reached density comparable to that of M1 (580.5 ±585.6 x106/ml) during the 3rd post-onset year. M2 sired a calf at 9 years of age through artificial insemination with cryopreserved semen collected during the 3rd post-onset year.
Marked individual differences between males were observed. Although some spermatozoa were noted in M5's semen, spermatogenesis was not fully established, as spermatozoa were frequently absent. Current findings suggest captive male T. aduncus are reproductively effective soon after onset of spermatogenesis, which occurs at the age of c.6.5-7.5 years. This is earlier than has generally been accepted for male dolphins.1,2 This data alerts curators to the possibility of earlier onset of spermatogenesis in other species and the potential to include younger males in controlled breeding plans. Although this study is the most comprehensive reported in male dolphins to date, sample size was small (N=5). Further similar study on more individuals and other species is recommended.
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 semen collection, and the Clinical Laboratory staff for the author's training in semen evaluation and other technical assistance. The completion of this Project also owes much to the continual support and commitment of the veterinarians and senior management of the Park over the years.
This project is funded by the Research Grants Council of Hong Kong (Grant ref: PolyU5287/01).
1. Cockcroft VG, GJB Ross 1990. Age, growth, and reproduction of bottlenose dolphins Tursiops truncatus from the East coast of southern Africa. Fish. Bull. 88:289-302.
2. Schroeder JP. 1990. Breeding bottlenose dolphins in captivity. In: The Bottlenose Dolphin. S. Leatherwood and R.R. Reeves,(eds). Academic Press Inc., San Diego, California. Pp.435-446.