Serum melatonin, cortisol and testosterone concentrations, and rectal temperature from four captive male Indo-Pacific bottlenose dolphins (Tursiops aduncus) (estimated age: 36 to 39 y) were measured at 3-h intervals over a 24-h period on the winter solstice, spring equinox and summer solstice in order to (1) investigate diurnal variations in melatonin, cortisol and testosterone concentrations, and rectal temperatures and (2) examine the seasonal changes in parameters for discussing mechanisms of adaptation to the external environment in cetaceans. Serum melatonin, cortisol and testosterone concentrations, and rectal temperature values ranged from < 1.6 (detection limit of the assay) to 23.3 pg/mL, 0.8 to 14.0 ng/mL, 0.8 to 23.0 ng/mL, and 35.5 to 37.3º C, respectively. Melatonin measurements were not consistent with expected diurnal rhythm patterns-that is, higher values during the dark phase and lower values during the light phase. However, cortisol and testosterone concentrations and rectal temperatures showed significant diurnal rhythms with acrophases occurring at about the same time in all the seasons. The highest amplitude of testosterone diurnal rhythms, which suggests the resumption of testicular endocrine function, was at the spring equinox, and average testosterone concentrations were the highest at the summer solstice. Serum cortisol concentrations and rectal temperatures were highest at the spring equinox (lowest water temperatures of the three seasons), and there were significant negative correlations between water temperatures and cortisol concentrations and rectal temperatures. The results suggest that changes in secretion patterns and levels of hormones, as well as changes in rectal temperature, are influenced by various factors such as environmental change, seasonal breeding, and required metabolic energy in the captive dolphins.

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Heart rate monitoring in free-ranging cetaceans to understand their behavioural ecology and diving physiology is challenging. Here, we developed a simple, non-invasive method to monitor the heart rate of cetaceans in the field using an electrocardiogram-measuring device and a single suction cup equipped with an electrode. The unipolar suction cup was placed on the left lateral body surface behind the pectoral fin of Risso's dolphins ( Grampus griseus ) and a false killer whale ( Pseudorca crassidens ) in captivity; their heart rate was successfully monitored. We observed large heart rate oscillations corresponding to respiration in the motionless whales during surfacing (a false killer whale, mean 47 bpm, range 20–75 bpm; Risso's dolphins, mean ± s.d. 61 ± 15 bpm, range 28–120 bpm, n = 4 individuals), which was consistent with the sinus arrhythmia pattern (eupneic tachycardia and apneic bradycardia) observed in other cetaceans. Immediately after respiration, the heart rate rapidly increased to approximately twice that observed prior to the breath. Heart rate then gradually decreased at around 20–50 s and remained relatively constant until the next breath. Furthermore, we successfully monitored the heart rate of a free-swimming Risso's dolphin. The all-in-one suction cup device is feasible for field use without restraining animals and is helpful in further understanding the diving physiology of free-ranging cetaceans. This article is part of the theme issue ‘Measuring physiology in free-living animals (Part II)’.

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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.