Melatonin and seasonal cycles in muskoxen
The objectives of this study were to characterize the annual reproductive cycle and melatonin rhythm in muskoxen at 52°N and to investigate the role of photoperiod in timing seasonality. During the first part of the study, changes in reproductive function and circulating melatonin patterns were studied in 15 tame muskoxen. Radioimmunoassays were used to measure serum concentrations of gonadotrophins, gonadal steroids, prolactin and melatonin. Informal observations of dominance and breeding behaviour of males were recorded, semen was collected and assessed for gross motility, sperm concentration and abnormalities, and dimensions of male accessory genital glands were measured in vivo using transrectal ultrasonography. Seasonal changes in reproductive function were evident in all animals. In intact males, mean luteinizing hormone (LH) concentrations rose from March to May, and declined by September; pulses of LH became smaller by September, near the breeding season. Testosterone concentrations peaked soon after the summer solstice, followed by enlargement of accessory genital glands and improved seminal quality, culminating in peak breeding condition between late August and early October. In all females, mean LH and follicle-stimulating hormone (FSH) concentrations peaked in September, during the breeding season, when LH pulses were smallest and most frequent. The daily light cycle influenced patterns of LH and prolactin release, as well as melatonin. In intact males, a disproportionately large number of LH pulses occurred during civil twilight before sunrise ($\chi\sp2:$ P = 0.009). Prolactin concentrations, measured in samples collected over 24 hours in both May and September from one female rose soon after dawn and decreased before sunset. Serum melatonin rhythms were coupled to the natural light:dark cycle throughout the year. The amplitude, duration and phase of the nightly rise changed with daylength. In the second part of the study, circadian and circannual rhythms were examined by manipulating the ambient photoperiod and by administering exogenous melatonin. Finally, the influence of photoperiodic cues in spring on the timing of subsequent seasonal cycles was examined. (Abstract shortened by UMI.)
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