Plasma melatonin in the Adelie penguin ( Pygoscelis adeliae ) under continuous daylight in Antarctica

Cockrem JF, Plasma melatonin in the Adelie penguin (Pygoscelis adeliae) under continuous daylight in Antarctica. J. Pineal Res. 199 1 10:2‐8. Abstract: Circadian rhythms of melatonin secretion in birds are influenced by daylength and light intensity. Daily patterns of melatonin secretion were examined in Adelie penguins ( Pygoscelis adeliae ) under natural continuous daylight at Cape Bird, Antarctica (77$S). Although daylight is continuous during the Antarctic summer there was a marked daily cycle of light intensity. However, there was no relationship between mean plasma melatonin levels and time of day in groups of 2–10 penguins sampled at 2–3 h intervals in November, December, or January. Mean melatonin levels over 24 h in groups of birds from which single samples were collected, or in groups of birds sampled repeatedly through cannulae, were low (12.4 ± 1.2 pg/ml‐28.8 ± 4.4 pg/ml for 4 sampling periods; n = 22‐163). Levels in individual birds were, however, quite variable and ranged from 5.0–68.1 pg/ml. Some birds had periods of increased melatonin levels that tended to occur during the time of day when light intensity was least. One bird had a clear low amplitude melatonin rhythm with a peak during the time of least light intensity. These results, the first for any bird under a natural photoperiod, indicate that melatonin secretion is inhibited by natural continuous daylight, but that it is not abolished.