Department of Arctic and Marine Biology, University of Tromsø, , Tromsø, Norway, Institute of Ophthalmology, University College London, , 11-43 Bath Street, London EC1V 9EL, UK, Moorfields Eye Hospital, , London, UK.
Proc Biol Sci. 2013 Oct 30;280(1773):20132451. doi: 10.1098/rspb.2013.2451. Print 2013 Dec 22.
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.
北极驯鹿经历了环境光照从连续夏季日光到连续冬季黑暗的极端变化。在这里，我们表明它们可能有一种独特的机制来应对冬季黑暗，即通过改变它们的脉络膜（TL）的反射波长。在夏季，它是金黄色的，大部分光线直接通过视网膜反射回来，而在冬季，它是深蓝色的，反射出的光线较少。与夏季动物相比，冬季的蓝色反射与视网膜敏感性的显著提高有关。反射的波长取决于 TL 胶原的间隔，间隔减小导致较短的波长，我们在夏季和冬季动物中得到了证实。冬季动物的眼内压显著升高，可能是由于瞳孔永久性扩张阻塞了眼内排水造成的。这可能解释了胶原的压缩。由此产生的向蓝色反射的转变可能会使光线散射通过光感受器，而不是直接反射，从而通过增加光子捕获来提高视网膜敏感性。据我们所知，这是对季节性环境光照变化的视网膜结构适应性的首次描述。敏感性的提高是以降低分辨率为代价的，但这可能是驯鹿在黑暗的北极冬季探测移动捕食者的重要适应。
