Melatonin levels from blood samples collected from adelie penguins (Pygoscelis adeliae) held under different light conditions to investigate the circadian physiology of adelie penguins

All animals have a biological clock that controls the timing of their daily activities. This internal clock is reset each day to a 24-hour period by the regular cycle of day and night. Animals in Antarctica are exposed to continuous daylight for four months each summer. How they maintain a sense of time the absence of a regular light/dark cycle was investigated. The pattern of plasma melatonin under natural daylight and experimental light conditions was measured in Adelie penguins to determine the physiological basis of the daily circadian rhythm of animals that live at high latitudes in the absence of regular light/dark cycles. Similar experiments were conducted over two seasons.

In the first season blood samples were collected from five experimental conditions:
a) mean melatonin levels over 24 hours with samples for at least five birds every 3 hours,
b) Midday vs Midnight - five blood samples from different birds at midday and five at midnight over 3 days,
c) Natural Light - Repeated blood samples collected from 15 birds kept in captivity under natural light and,
d) Light/Dark cycle - Three birds were held under an LD12:12 light/dark cycle and repeated blood samples were collected.

In the second season blood samples were collected from penguins in natural continuous daylight and four experimental light conditions including:
a)12 hours of light and 12 hours of dark,
b)12 hours of light and 12 hours of dark with a 30 minute light pulse midway through their third night
c)12 hours of dark and 12 hours of light with a 30 minute light pulse midway through their sixth night and
d)constant darkness.

Blood samples were collected every 2 hours from each bird. Plasma levels of the hormone melatonin was measured in the laboratory to reveal the response of pineal gland rhythms to these unusual conditions.