Functional reduction of the southern minke whale (Balaenoptera acutorostrata) testis during the feeding season

Seasonal variation in reproductive activity in the male southern minke whales was investigated. The animals were killed and collected during the feeding season (December-March) in the Antarctic Ocean in the eastern part of Area III (35 degrees E-70 degrees E, south of 60 degrees S) and Area TV (70 degrees E-130 degrees E, south of 60 degrees S). Blood samples, testes, epididymides, and vasa deferentia were collected from 62 males, which had testes weighing over 400 g each. Changes in testicular morphology and plasma testosterone, estradiol-l-ia and LH concentrations measured by enzyme immunoassay were investigated. Reduction of testicular function associated with different capture periods was found in the summer season. From December to March, body length and body weight remained steady, but decreases in testicular weight, epididymal weight, and testicular volume were found in February. During the same period, plasma testosterone concentration also declined. A significant decrease in the number of germ cells continued during the period of feeding season. An increase in area of seminiferous tubule tended to proceed in a number of germ cells. These changes reflected the percentage of spermatozoa in the vasa deferentia; in particular, motile spermatozoa were observed in December. Based on morphological observation, spermatogenesis had also declined. These results indicate regulation of testicular function by testosterone, as in terrestrial mammals. A gradual decrease of testicular function occurred during December-January.