A changing mirror: Arctic reindeer adapt their eyes to extreme changes in environmental illumination by shifting the reflectance behind their retina

Arctic reindeer have evolved a unique mechanism for adapting to the extreme changes in environmental illumination between summer and winter by changing their tapetal reflectance. This reflectance is similar to what is commonly called 'cats eyes' that are used in road markings. It is present in a range of mammals including ungulates and carnivores. Light passing through the retina is reflected back by the tapetum, providing a second chance for photon capture. In summer the reindeer reflectance is common to that in ungulates, being gold/green, but in winter it is deep blue. This results in less light being reflected back out of the eye, rather more is scattered behind the retina. This increases further the probability of photon capture at the expense of spatial resolution. Also, the blue shift favours capture by rods, the photoreceptors used at low luminance levels. It is probable the change in reflectance is partially dependent upon the spacing of collagen fibers directly behind the retina. This seasonal change in reflectance is not found in deer at lower latitudes. As Arctic reindeer have only inhabited such high latitudes since the last ice age, about 10,000 years ago, this mechanism has evolved rapidly. The Arctic winter ground is snow covered and reflectance from it is a major component of the visual environment. With global warming in Polar Regions, the degree of snow cover is being reduced significantly, which will impact upon this animal. It is important to know what regulates this dramatic change in reindeer eyes and how such animals will cope with environmental change. We will trace the transition of the tapetal reflectance in vivo and in vitro in these animals and relate this to day length. We will ask if it is possible to induce these changes experimentally by keeping animals in winter type illumination in summer and visa versa. We will also determine whether such changes are regulated centrally or in the eye itself by patching one eye across seasons and monitoring changes in reflectance. We will also determine whether young animals born in summer need to experience an Arctic winter to induce this change or whether the shift is experience independent. We know that deer at lower latitudes do not change their tapetal reflectance seasonally. But might their be a dose dependent degree of variation at high latitudes? We will examine reindeer at progressively higher latitudes through the seasons to determine this. We will also examine other animals at such high latitudes including Arctic fox and Polar bear to reveal whether this mechanism is used widely by Arctic animals or has evolved in reindeer alone.

Arctic reindeer have evolved a unique mechanism for adapting to extreme changes in environmental illumination between summer and winter by changing their tapetal reflectance. This reflectance is similar to 'cats eyes' used in road markings. It is present in a range of mammals including ungulates and carnivores. Light passing through the retina is reflected back by the tapetum, providing a second chance for photon capture. In summer the reflectance is common to that in ungulates, gold/green, but in winter it is deep blue. This results in less light being reflected back out of the eye, rather more is scattered behind the retina, increasing further the probability of photon capture. Also, blue shifting favours capture by rods, the photoreceptors used at low luminance levels. It is probable the change in reflectance is partially dependent upon the collagen spacing behind the retina. This seasonal change is not found in deer at lower latitudes. As Arctic reindeer have only inhabited high latitudes since the last ice age, ~10,000 years ago, this mechanism has evolved rapidly. It is important to know what regulates this dramatic change in their eyes and how they cope with environmental change. We will trace transitions of the tapetal reflectance in vivo and in vitro and relate this to day length. We will ask if it is possible to induce changes experimentally by keeping animals in winter type illumination in summer and visa versa? We will also determine whether changes are regulated centrally or in the eye itself by patching one eye across seasons. We know that deer at lower latitudes do not change tapetal reflectance seasonally. But might their be a dose dependent effect at high latitudes? We will examine reindeer at progressively higher latitudes through the seasons to determine this. We will also examine other Arctic animals including Arctic fox and Polar bear to reveal whether this mechanism is used widely by them or has evolved in reindeer alone.