An assessment of reproductive development of the male Indo-Pacific bottlenose dolphin, Tursiops aduncus, in captivity.

摘要

The reproductive parameters, testis size, serum testosterone levels and ejaculate traits, of five male Tursiops aduncus (MI--M5) were monitored for periods of between three and five years, to investigate reproductive development and determine at what age the onset of sexual maturity occurs. The testosterone level of a sixth male was also investigated. Of the five main study subjects, four males (M2--M5), were born in captivity and therefore of known age. Weekly ultrasonographic appearance and measurements of the testes were compared to monthly serum testosterone levels to evaluate the reproductive development and status of each individual. This study defined the onset of sexual maturity as the first release of sperm in the semen, which signifies the onset and establishment of spermatogenesis. Weekly ejaculate traits, in conjunction with ultrasonographic and testosterone data, were used to monitor the reproductive development of the recently matured individuals and investigate their reproductive potential, or 'effectiveness'. Sperm from recently matured males was cryopreserved, thawed and examined for any difference in freezability compared to those of the older mature males.;The age of onset of spermatogenesis ranged from 6y 7m to 7y 4m. The onset of sexual maturity in two males occurred before the age of 7yr, which is younger than previously reported. The changes in ultrasonographic appearance of the testis during sexual maturation were consistent with one previous report in that the testicular echopattem became more echogenic throughout the organ. The testes also changed shape from cylindrical to being 'cigar-shaped', or expanded at the caudal end. The cranial and caudal aspects of the epididymis became more readily visualised 4--5 months prior to onset of spermatogenesis. Testis size increased rapidly during a 4-month period preceding onset. In M3 and M4, testis size at onset was 17--18cm in length and 170--190cm 3 in volume. In M2--M4, serum testosterone level at onset varied widely, from 1.7--24ng/ml, therefore, assignment of a particular level to mark sexual maturity was not possible. Endocrine profiles proximal to onset suggest a threshold level of 3ng/ml may be useful to identify recently matured males. Overall sperm density from one collection session during the first year after onset ranged from 0--713 x 106/ml and the highest density recorded in a single ejaculate was 1,475 x 10 6/ml. After onset, spermatogenesis continued to increase in efficiency, and azoospermia or extremely low sperm density became progressively rare. Data of the reproductive parameters of M5 was not consistent with the other subjects, although the onset of sexual maturity occurred within the same age range. Further monitoring is being conducted on this individual.;Ejaculates of recently matured males (1st post-onset year) were of good quality and generally comparable in sperm density (> 200--500 x 106/m1), motility (> 80%) and viability (> 90%) to those of the oldest male (M1, 19+yr). Ejaculates collected as early as one month after onset tolerated cryopreservation well. Post-thaw results of these ejaculates were comparable to ejaculates collected from M1, a fully mature, proven sire. Semen taken from M2 during the 3rd post-onset year, at the age of 9y 7m, was cryopreserved, used for AI in one female and produced a healthy calf.;Testis size and testosterone levels showed seasonal changes, but sperm density did not. Testis size and testosterone levels tended to be higher between March and August, (spring and summer in Hong Kong) and lower between October and February (autumn and winter). Although, overall density was lower in January and February, ejaculates of high density (> 500--600 x 10 6/ml) were often found.;Data strongly suggest testicular activities were affected by illness. Testis size and testosterone levels both decreased during episodes of illness. There was also evidence of decline in semen parameters, sperm density motility and viability during illness.;Results indicate that social structure also may impact reproductive development and status, however further investigation of this was beyond the scope of the present study.;This study has demonstrated the combined use of ultrasonography of the testes, serum testosterone and ejaculate traits to accurately evaluate reproductive development and identify sexual maturation in the bottlenose dolphin, Tursiops aduncus. Results have also contributed towards improving Ocean Park's controlled breeding programme. Although the sample size of this study was small, it is the largest and longest term study of its type reported to date. It is, however, important to note that marked individual differences were found and further questions about reproductive development, particularly as affected by social structure and illness, were raised. Continued monitoring of this group and further research on more individuals are required in order to better understand the complex reproductive physiology of male dolphins.