Reindeer eyes seasonally adapt to ozone-blue Arctic twilight by tuning a photonic tapetum lucidum.

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.

Color-coordinate system from a 13th-century account of rainbows.

We present a new analysis of Robert Grosseteste's account of color in his treatise De iride (On the Rainbow), dating from the early 13th century. The work explores color within the 3D framework set out in Grosseteste's De colore [see J. Opt. Soc. Am. A29, A346 (2012)], but now links the axes of variation to observable properties of rainbows. We combine a modern understanding of the physics of rainbows and of human color perception to resolve the linguistic ambiguities of the medieval text and to interpret Grosseteste's key terms.

Why aye-ayes see blue.

The capacity for cone-mediated color vision varies among nocturnal primates. Some species are colorblind, having lost the functionality of their short-wavelength-sensitive-1 (SWS1) opsin pigment gene. In other species, such as the aye-aye (Daubentonia madagascariensis), the SWS1 gene remains intact. Recent studies focused on aye-ayes indicate that this gene has been maintained by natural selection and that the pigment has a peak sensitivity (lambda(max)) of 406 nm, which is -20 nm closer to the ultraviolet region of the spectrum than in most primates. The functional significance behind the retention and unusual lambda(max) of this opsin pigment is unknown, and it is perplexing given that all mammals are presumed to be colorblind in the dark. Here we comment on this puzzle and discuss recent findings on the color vision intensity thresholds of terrestrial vertebrates with comparable optics to aye-ayes. We draw attention to the twilight activities of aye-ayes and report that twilight is enriched in short-wavelength (bluish) light. We also show that the intensity of twilight and full moonlight is probably sufficient to support cone-mediated color vision. We speculate that the intact SWS1 opsin pigment gene of aye-ayes is a crepuscular adaptation and we report on the blueness of potential visual targets, such as scent marks and the brilliant blue arils of Ravenala madagascariensis.