The pineal gland signals autumn to reindeer (Rangifer tarandus tarandus) exposed to the continuous daylight of the Arctic summer

Abstract

Plasma melatonin concentrations were measured in blood samples taken hourly for 24–30 h once each month for 1 year in four male semidomesticated reindeer. The animals, which included two long-term castrates, were exposed to natural light and temperature conditions at 69°46′N throughout the study. Melatonin levels were individually variable but did not differ systematically between the castrated and the intact animals. In months characterized by a marked diel light–dark cycle, the plasma concentration of melatonin varied throughout the day; levels were high during the dark phase and low during the light phase. In summer (May, June, and July), when daylight was continuous, and in December, when there was only a short period of twilight in the middle of the day, the pooled hourly mean levels of melatonin did not change significantly across 24 h. However, though the light was continuous, the intensity varied markedly between day and night in summer and all animals showed a slight increase in the plasma concentration of melatonin around midnight in July. In addition, daily secretion of melatonin increased from an annual minimum of 66–84 pg/24 h in June and July to an annual maximum of 525 pg/24 h in August when day length began to decrease from 24 h. We suggest that exposure to continuous light throughout the polar summer reduces the sensitivity of the reindeer pineal gland to light and that, consequently, the drop in light intensity around midnight stimulates secretion of melatonin, which in turn provides photoperiodic information even during periods of continuous daylight. It appears, likewise, that the melatonin profile is enhanced in August, signalling a longer night than that actually experienced. In these two ways, reindeer may receive sufficient photoperiodic information to trigger short-day responses, such as development of the gonads, while still exposed to long days and thus compensate for the late onset of short days in autumn, which is characteristic of high latitudes.