Seasonal changes in the Tapetum lucidum as an adaptation to winter darkness in reindeer

Abstract

Reindeer (Rangifer tarandus tarandus) live at high latitudes and are therefore exposed to extreme changes in environmental illumination. In winter they spend months in relative darkness, in summer they are exposed to months of continuous brightness. Previous investigations have revealed a shift in the wavelength of the light being reflected by the tapetum lucidum (TL) between winter and summer. Since no seasonal changes in the TL have ever been documented in any species before, this finding raised several questions. In the present study the following issues have been dealt with: Is this chromatic shift triggered by changing light conditions between summer and winter or regulated by an endogenous clock? Are there other parts of the visual system that undergo seasonal changes? What are the physical mechanisms behind this shift? To find out what is regulating the change in reflected light eyes from animals killed in summer and from animals killed in winter were compared to each other and to eyes from animals kept in altered light conditions.