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

Abstract

Seasonal regression of testes and epididymides is described for 161 mature harbour porpoises Phocoena phocoena, L. from the Bay of Fundy and Gulf of Maine 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 end 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 spermatozoa. Ultimately, all signs of spermatogenesis were absent, but tubules retained an alternating lining of Sertoli cells and spermatogonia. Testicular and epididymal mass, testicular length and seminiferous tubular diameter 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 quadratic function. During early July when most females are ovulating, all males had active testes; variation in the degree of regression among males increased as the season progressed. This may reflect a trade-off between the costs of maintaining active testes at 4% body mass and the probability of successful fertilization. Testes are completely regressed during the winter, suggesting that few reproductive opportunities exist during this season. Unlike some other odontocete species, testicular mass of porpoises is a good indicator of breeding season.