Abstract

Belugas have a seasonal reproductive pattern in the wild as well as in zoological facilities, with conceptions generally occurring in the spring and births in the summer of the following year.^1,2 Some species of seasonally breeding odontocetes manifest substantial seasonal variation in testes size.³ Male belugas have seasonal changes in testosterone production, but there is not adequate data to fully characterize any potential seasonality in testes size in this species.⁴ Based on postmortem studies, some authors have suggested that beluga testes may be larger during the breeding season.^4,5 This study aimed to determine if testis size varies seasonally in belugas. The testicular volumes (measured via ultrasonography) of 3 male belugas (2 proven sires) were monitored longitudinally for one calendar year, and a fourth male (also a proven sire) was monitored for two consecutive years. Blood testosterone concentration was also monitored longitudinally in the same individuals over the same period. A total of 99 testicular volume measurements and 97 testosterone concentration measurements were included in the analysis. Total testicular volume (sum of the volumes of the right and left testes) varied seasonally, with a peak in March (1279.8 ± 96.9 cm³, mean ± SE) and a nadir in August (988.2 ± 64.3 cm³). Total testicular volume was significantly higher during the months of December through May (1215.1 ± 36.4 cm³) than during the months of June through November (1056.3 ± 34.3 cm³) (p < 0.01). The right and left testes underwent the same size changes at the same time, but the right testis was significantly larger than the left (p < 0.001). On average, an individual's total testicular volume increased by a factor of 1.5, although the degree of increase ranged from 1.2 to 2.1 in the individuals studied. Seasonality of blood testosterone concentration agreed with previously published results, with a peak in February (5.22 ± 92 ng/ml) that was approximately 8 times the lowest concentration in June (0.65 ± 0.15 ng/ml). On average, testosterone concentration was below 1.0 ng/ml from June through October. Both mean testicular volume (p < 0.01) and mean blood testosterone concentration (p < 0.05) differed significantly among individuals. Additionally, mean testicular volume varied significantly from one year to the next in the same individual (p < 0.01). The findings of this study agree with the previously established seasonal pattern of reproduction in this species. Although the degree of seasonality in testes size is not as dramatic as in some other odontocetes, these results have implications for identifying potential breeding males, facilitating voluntary semen collection training for use in artificial insemination, and managing beluga reproduction in zoos and aquaria.

Acknowledgements

This project would not have been possible without the contributions of the beluga trainers at Mystic Aquarium (especially Kristine Magao and Kelly O'Neil), Brian Sheehan and the beluga husbandry staff at Vancouver Aquarium, and the beluga training staff at SeaWorld San Diego. We would also like to acknowledge the help of Dr. Scott Jacques of the Texas A&M Veterinary Diagnostic Laboratory in validating an assay for beluga testosterone. We thank the Research and Veterinary Services team at Mystic Aquarium, veterinary technician Jen Rego at SeaWorld San Diego, and veterinary technicians Chelsea DeColle and Gwyneth Nordstrom at Vancouver Aquarium for their support of the project. Drs. Cara Field and Jennifer Meegan gave their expert advice in developing the methodology for the ultrasonography portion of the research. We are especially grateful to Lynn Marcoux for her help in initiating the research, and for the support of Dr. Tracy Romano throughout the project.

References

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