Pineal and Thyroid Functions in Newborn Seals

Daily variations of pineal and plasma melatonin and plasma thyroid hormones were measured in harp seals ( Phoca groenlandica), grey seals ( Halichoerus grypus), and hooded seals ( Cystophora cristata), ranging in age from newborn to 14 days. In newborn harp seals the mean mass of the pineal gland was 273 mg (±45 SEM, n = 11), containing 49 ng (median) melatonin. In newborn, 4- and 10-day-old grey seals, the pineal mass was similar, weighing on average 337 mg (± 74, n = 6) and containing 90 ng melatonin. Two newborn hooded seal pups had pineals weighing 520 and 1289 mg, with 254 and 7600 ng melatonin, respectively. There were no day-night differences in the pineal contents of melatonin or in the number of pineal β-adrenergic receptors measured in newborn harp seals, and, in newborn, 4- and 10-day-old grey seals, there were no day-night or age differences in pineal melatonin content. Plasma melatonin levels were 10 times higher in newborn seals than in two 10-day-old grey seals and one 14-day-old harp seals pup. In all seal pups, the levels exhibited a 24-hr rhythmicity,with increasing night- and decreasing daytime concentrations. Plasma levels of thyroxine (T 4) and triiodothyronine (T 3) were generally higher in newborn seals than in 10- and 14-day-old seals or in adult females. There was no apparent 24-hr rhythmicity, but the thyroid hormone levels generally declined throughout each sampling sequence. High pineal and thyroid activities may play a thermoregulatory role in newborn seal, but the results do not indicate a stimulatory action of melatonin in the peripheral conversion of T 4 to T 3. It is speculated that the large and active pineal gland, particularly in newborn seals, may be related to aspects of their diving habit.