The effects of melatonin implants on Weddell seals pelage (Leptonychotes weddelli)

In many animals, seasonal events such as reproduction and changes in pelage are regulated by the alteration in length of daylight throughout the year. These changes in lighting become manifested as changes in circulating levels of melatonin (a hormone), and the physiological responses of such animals are due to changes in melatonin levels. Weddell seals experience 24 hours of sunlight during the summer in Antarctica and are adapted to a polar environment. The role of melatonin in the Weddell seal was investigated to determine if Weddell seals are actually sensitive to melatonin, the hormone which other animals use to monitor the annual pattern of changes in daylight. 34 lactating female seals were selected from a single pupping colony near Big Razorback Island, McMurdo Sound in November. Half of the seals received two implants containing melatonin, injected subcutaneously. The other half were controls. Samples of venous blood and skin, plus vaginal smears, were obtained from all seals immediately prior to implantation and once again (31 seals only) between 11 and 15 days later.