对于北极冬天的极夜，驯鹿可能会用一种独特的方式来应对：极夜期间，它们的眼睛变得对光更加敏感。
与很多脊椎动物和大多数哺乳动物（特别是那些在夜间活动的动物）一样，驯鹿眼睛的视网膜后方有一层可以反光的含蛋白质组织。该结构被称为反光膜，使眼睛的感光神经元有第二次机会来检测微观条件下稀缺的光子。（反光膜还会产生“眼耀”，使黑暗中动物的眼睛看起来似乎在发光。）在极昼的日子里，驯鹿有着黄色的眼耀。但是在极夜期间，反光膜反射的光呈现出明显的蓝色光泽。
研究人员称，这是一种在其他哺乳动物中未注意到的季节性变化。为了研究这种不寻常的颜色变化，研究人员将一些驯鹿眼球带入实验室，并向其施加一些重量。在压力下，眼球几乎立即改变了眼耀的颜色。研究人员称，这符合在野外各种季节里发生的情况。冬天，驯鹿的瞳孔不断扩张，增加了流体的压力，而这反过来减少了反光膜中胶原纤维的间距，从而增加了眼中光的散射，将反射光改变成在黑暗中有优势的更短波长的光。这些变化使驯鹿眼睛的感光性提高了100倍到1000倍。
研究人员将报告发表在10月29日的英国《皇家学会学报B》上。他们认为，虽然这种构造降低了生物的视觉敏感度，但总体而言是一种折中方案，通过帮助驯鹿在黑暗中更好地发现捕食者， 从而促进驯鹿的生存。(生物谷Bioon.com)
生物谷推荐的英文摘要
Proceedings of the Royal Society B doi: 10.1098/rspb.2013.2451
Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer
Karl-Arne Stokkan, Lars Folkow, Juliet Dukes, Magella Neveu, Chris Hogg, Sandra Siefken, Steven C. Dakin and Glen Jeffery
Arctic reindeer experience extreme changes in environmental light from continuous summer daylight to continuous winter darkness. Here, we show that they may have a unique mechanism to cope with winter darkness by changing the wavelength reflection from their tapetum lucidum (TL). In summer, it is golden with most light reflected back directly through the retina, whereas in winter it is deep blue with less light reflected out of the eye. The blue reflection in winter is associated with significantly increased retinal sensitivity compared with summer animals. The wavelength of reflection depends on TL collagen spacing, with reduced spacing resulting in shorter wavelengths, which we confirmed in summer and winter animals. Winter animals have significantly increased intra-ocular pressure, probably produced by permanent pupil dilation blocking ocular drainage. This may explain the collagen compression. The resulting shift to a blue reflection may scatter light through photoreceptors rather than directly reflecting it, resulting in elevated retinal sensitivity via increased photon capture. This is, to our knowledge, the first description of a retinal structural adaptation to seasonal changes in environmental light. Increased sensitivity occurs at the cost of reduced acuity, but may be an important adaptation in reindeer to detect moving predators in the dark Arctic winter.
