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Long-term monitoring of circulating progesterone levels in three captive female false killer whales, Pseudorca crassidens, was conducted to characterize their reproductive events and to reveal the relationship between their estrous cycles or pregnancies and peripheral white blood cell (WBC) counts. Blood samples were collected at 2–3-day intervals or on a weekly-to-monthly basis for up to 10 years, from 2006 to 2017. In two mature females (initial body lengths of 4.22 and 4.07 m), some cyclic progesterone elevations were detected during the study period; the estimated mean (± SE) estrous cycle length was 40.5 ± 0.7 days (n=12). The seasonality of ovulation, estimated from the elevation of progesterone levels, varied among individuals or years, and ovulation did not occur every year. The third female (3.26 m) showed progesterone elevations, despite irregular cycles after sexual maturity, and became pregnant. The progesterone levels during pregnancy ranged from 7.3 to 42.2 ng/ml, and the gestation period lasted for 14 months until parturition. The mean WBC counts during estrous cycles were the lowest before the progesterone levels began to increase and then gradually increased toward the luteal phase. The WBC counts were significantly higher during pregnancy than before and were particularly high in early pregnancy. To the best of our knowledge, this is the first report of the relationship between the estrous cycle or pregnancy and WBC counts in cetaceans.

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