Seasonal regression in testicular size and histology in harbour porpoises (Phocoena phocoena, L.) from the Bay of Fundy and Gulf of Maine

Seasonal regression of testes and epididymides is described for 161 mature harbour porpoises Phocoena phocoena, L. from the Bay of Fundy and Gulf of M aine from June to December 1984-1995. Based on histological appearance and size of gonads, testes are fully active from late June until at least the e nd of July, spanning the estimated period of conceptions. During testicular regression, spermatocytes and round spermatids disappeared first from the lumina of seminiferous tubules, followed by the gradual disappearance of sp ermatozoa. Ultimately, all signs of spermatogenesis were absent, but tubule s retained an alternating lining of Sertoli cells and spermatogonia. Testic ular and epididymal mass, testicular length and seminiferous tubular diamet er decreased approximately 3.5, 1.5 and 1.5 times, respectively, from peak production to full regression and this decrease was best described by a qua dratic function. During early July when most females are ovulating, all mal es had active testes; variation in the degree of regression among males inc reased as the season progressed. This may reflect a trade-off between the c osts of maintaining active testes at 4% body mass and the probability of su ccessful fertilization. Testes are completely regressed during the winter, suggesting that few reproductive opportunities exist during this season. Un like some other odontocete species, testicular mass of porpoises is a good indicator of breeding season.