Testicular morphology was compared between sexually immature, pubertal and mature male North Eastern Atlantic short-beaked common dolphins (Delphinus delphis) and over the months. Reproduction requires a high metabolism level and high quality habitats. A short mating season of only a few months saves animals energy. The seminiferous tubule diameter, combined testes weights, interstitial tissue amount, tubule shape and the proportion of tubules with and the amount of spermatocytes, spermatids and spermatozoa differed over the months. The highest reproductive activity period for the species was from June until September. May and October were transition months and November until March was a period of low reproductive activity. Sperm was present the entire year and seasonal testicular regression in Winter varied between individuals. In Winter, the large majority had small testes, with small angular tubules and large amount of interstitial tissue, and decrease in proportion of tubules and amount of each spermatogenesis stage. However, approximately 20% remained more active, thus did not have a decrease as much in testes weights, tubule diameter, amount and proportion of specific spermatogenesis stages, and their tubules remained round in Winter. Three causes are proposed why some remained more active: food availability, testosterone concentration threshold and reproductive behaviour.
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