PLASMA MELATONIN CONCENTRATION IN NEONATAL NORTHERN ELEPHANT SEALS, MIROUNGA-ANGUSTIROSTRIS

The development of pineal function in northern elephant seals was exam ined in an attempt to understand the physiological basis for previousl y observed high daytime levels of melatonin in neonatal southern eleph ant seals. Pineal glands from four northern elephant seal pups, estima ted age less than 1 week, weighed 3.0+/-0.80 g, which was significantl y less than that previously found in southern elephant seals (4.6+/-0. 35 g). Midday concentrations of plasma melatonin in pups averaged more than 3000 pmol/l in the first 5 days post-partum, but declined rapidl y to less than 400 pmol/l after day 9. Daytime melatonin levels in nor thern elephant seals tended to be lower than in southern elephant seal s, although they were very high compared with other species. A circadi an cycle of plasma melatonin concentration was observed in newborn nor thern elephant seals, with levels of 3000-5000 pmol/l during the day, rising to more than 10,000 pmol/l late in the dark phase. Soon after w eaning at 4 weeks of age, daytime and night-time levels were in the ra nge 60-100 pmol/l and 100-400 pmol/l, respectively. When approximately 10 weeks old, most samples were in the range 100-400 pmol/l with no d iscernible difference between day and night levels. The results do not support the hypothesis that the pineal gland is involved in thermogen esis in new-born southern elephant seals. Instead, the very active pin eal gland may contribute to energy conservation, by lowering body temp erature, particularly at night. As physical insulation is acquired by the deposition of blubber, the mechanism is not required and melatonin falls to adult levels.