Seasonal variation in testes size and density detected in belugas (Delphinapterus leucas) using ultrasonography

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Belugas are thought to exhibit seasonal variation in testes size, but a temporal gap in postmortem sampling of wild belugas has precluded a description of the occurrence or the extent of this seasonal variation. This study aimed to utilize longitudinal monitoring of belugas in aquaria with known siring histories to assess seasonal variation in testes size, and its association with circulating testosterone concentration and testicular tissue density. Testes volume was estimated using linear measurements obtained via ultrasonography. Testicular tissue density was assessed by measuring the pixel intensity (PI) of ultrasound images of the testis. Five adult males, including 4 proven sires, were monitored for at least 1 continuous year; 2 of the males were monitored for > 2 years. A total of 154 ultrasound examinations (including 71 suitable for PI measurements) and 119 blood samples were available for analysis. Significant seasonal variation in testes volume, circulating testosterone concentration, and testicular PI were observed, with peak activity occurring between January and April. Seasonality of testicular volume was best described by a cubic function, while seasonal variations in testosterone and PI were best described by quadratic functions. Individuals differed significantly in both testes size and rate of change. On average, testes size increased by 60% from minimum to maximum values. These results are consistent with observations of reproductive seasonality both in the wild and in aquaria, and suggest a relatively low demand for sperm in this species that is consistent with their classification as induced ovulators.

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Journal of Mammalogy