Seasonal changes in the testis of the North Pacific common minke whale

Seasonal changes in the testis of the common minke whale in the North Pacific from April to October were analyzed. Testis weight was analyzed in 268 individuals, 70 of which (10 per month) underwent histological examination. An increase in testis weight was observed in only individuals with body length greater than 7 m and only after July. Expansion of seminiferous tubule diameter was detected after a reduction in diameter observed from May to June. Spermatocytes, which are developed germ cells, were most decreased in July and increased thereafter. These results suggest that the spermatogenetic activity of the common minke whale has seasonal changes, namely, that it reduces from May to June and is activated in August in preparation for the next breeding season.