Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, UK.
European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany.
Proc Biol Sci. 2022 Jun 29;289(1977):20221002. doi: 10.1098/rspb.2022.1002.
Reindeer are the only mammal known to seasonally adapt their eyes to the extremely blue colour of the extended twilight that occupies a large part of the winter 24 h cycle in their Arctic habitat. We describe the atmospheric phenomenon that results in this extreme spectral change in light environment. Reflectance spectroscopy is used to characterize the photonic nanostructure that generates the reflective region of the tapetum lucidum behind the retina. A model is proposed to explain the reversible reformatting of the reflector by seasonal changes in the volume of interstitial fluid within the two-dimensional photonic crystal of parallel collagen fibrils. This model is tested by allowing slow evaporation of the fluid from both summer and winter tapetum surfaces while monitoring changes in the reflectance spectrum. Coupled variations in the spacing and the degree of order of the fibril packing can transform the typical gold-turquoise colour of such a tapetal reflector to a deep blue that matches the peak spectral irradiance of twilight. The mechanism we describe might be applied by other animals with similar tapeta that experience prolonged changes in light environment.
驯鹿是已知的唯一一种会季节性地调整眼睛，以适应其北极栖息地 24 小时周期中大部分冬季延长的黄昏极蓝颜色的哺乳动物。我们描述了导致这种极端光谱变化的大气现象。反射光谱用于描述在视网膜后面的明毯反射区域产生的光子纳米结构。提出了一个模型来解释通过二维光子晶体中平行胶原纤维的间质液体积的季节性变化来实现反射器的可逆格式化。通过允许夏季和冬季明毯表面的液体缓慢蒸发，同时监测反射率光谱的变化，来测试该模型。纤维包装的间距和有序程度的耦合变化可以将这种明毯反射器的典型金绿颜色转变为与黄昏峰值光谱辐照度匹配的深蓝色。我们描述的机制可能适用于其他经历长时间光照环境变化的具有类似明毯的动物。
