Seasonal changes in circulating gonadal steroid levels and physiological evidence for the presence of intrinsic circannual reproductive cycles in captive finless porpoises <i>Neophocaena asiaeorientalis</i> from the western Inland Sea, Japan

We monitored annual fluctuations of gonadal steroid levels in three sexually mature captive finless porpoises (Neophocaena asiaeorientalis; two males and one female) from two different facilities over 56-91 months. Two animals (one male and one female) were held in an indoor tank with a sunroof (facility A) and the other male was held in an indoor tank without a sunroof (facility B). Water temperatures in both facilities reflected seasonal changes during the study period with a minor difference in the fluctuation pattern. Testosterone levels of the male in facility A were higher from spring to summer every year and exhibited a 12-month cycle. The female showed estrus cycles in 1-month intervals from summer to winter, excluding 2 anestrus years. In contrast, the period of higher testosterone levels of the male in facility B gradually initiated earlier over the years under a constant photoperiod (11.5L:12.5D) and exhibited a 9-month cycle during the first 52 months. After changing the light conditions to a natural photoperiod, its testosterone levels were high from early spring to summer for 3 consecutive years and exhibited a 12-month cycle. Our results showed that under a constant artificial photoperiod, the male in facility B failed to recognize the seasonal changes of a natural external environment, resulting in a 9-month, free-running hormone cycle.