Melatonin secretion patterns in relation to changes in light intensity and activity in Weddell seals

In many animals, seasonal events such as reproduction and changes in pelage are regulated by the alteration in length of daylight throughout the year. Weddell seals are subjected to 24 hours of sunlight during summer yet they are able to maintain appropriate timing of seasonal events throughout the year. This study examined the secretion of melatonin (the hormone which animals use to monitor the annual pattern of changes in daylight and thus regulates the physiological changes in many animals), in Weddell seals in relation to changes in light intensity and in their activity patterns. Samples of venous blood were collected from 4 adult female seals every 2 hours for 24 hours on two occasions in late November from a colony near Big Razorback Island, McMurdo Sound. A different set of 4 seals were used for each sample. Intensity of daylight was monitored on each occasion. Also a record of seals present on the surface of the ice was obtained every 2 hours over a 48 hour period. The blood samples were assayed for plasma melatonin levels.