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Seasonal Difference of Diurnal Variations in Serum Melatonin, Cortisol, Testosterone, and Rectal Temperature in Indo-Pacific Bottlenose Dolphins (Tursiops aduncus)
Abstract: 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.
Key Words: Indo-Pacific bottlenose dolphin, Tursiops aduncus, melatonin, cortisol, seasonal changes, diurnal rhythms, testosterone, cetacean, rectal temperature, environment
Document Type: Research article
Page Numbers: 433-442