Reindeer eyes undergo a striking seasonal change that helps them cope with the unusual light of the Arctic winter. During summer, the reflective layer behind the retina, known as the tapetum lucidum, has a gold-turquoise colour. In winter, it becomes deep blue, matching the extremely blue twilight that lasts for many hours at high latitudes. According to research published in Proceedings of the Royal Society B: Biological Sciences titled ‘Reindeer eyes seasonally adapt to ozone-blue Arctic twilight by tuning a photonic tapetum lucidum’, found, this change is linked to the photonic nanostructure of the tapetum, which is made from parallel collagen fibrils.
How does Arctic winter change the colour of reindeer eyes
Researchers found that the winter tapetum of reindeer changes its reflection from the gold-turquoise colour seen in summer to a deep blue. The winter reflection is well matched to the spectral irradiance of Arctic twilight, with a peak near 450 nanometres. Reindeer living around latitude 70° north experience 8-11 hours of very blue twilight each day from early September through April.In snow-covered landscapes, reflected light also retains a strong blue and ultraviolet component because snow has high albedo at these wavelengths. The researchers indicate that increased visual sensitivity at the blue end of the visible and near-ultraviolet spectrum can help reindeer forage and detect predators when winter twilight provides very little light, while also improving contrast against snow overall.
PC: Wikipedia
How does ozone absorption change the colour of Arctic twilight
The unusual colour of Arctic twilight is caused mainly by ozone absorption in the atmosphere. When the Sun is close to or below the horizon, sunlight travels through a much longer atmospheric path. This increases absorption in the Chappuis band of ozone, removing much of the yellow and orange light.Rayleigh scattering in the upper atmosphere further shifts the remaining light towards shorter wavelengths. The effect becomes strongest during mid-twilight, when the Sun is about 7 degrees below the horizon. Researchers measured a twilight spectrum with a peak spectral irradiance near 450 nanometres.
The collagen fibres that give the reindeer eye its seasonal colour
The tapetum lucidum is a reflective layer behind the retina that can increase the fraction of incoming photons detected by the eye. In reindeer, it contains collagen fibrils arranged in an approximately hexagonal close-packed structure. The researchers describe this as a two-dimensional photonic crystal that can reflect visible light efficiently through Bragg diffraction. Their model proposes that the spacing and order of the fibrils change when the volume of fluid between them changes.The experiments tested this idea by allowing fluid to evaporate from summer and winter tapetum surfaces while measuring changes in their reflectance spectra. The results indicate that changes in fibril spacing and packing can transform the reflector from an approximately achromatic summer mirror to a more strongly blue-reflecting winter structure.
Why the reindeer eye changes its reflector every season
The researchers note that the seasonal change may be connected to changes in intra-ocular pressure, although they did not test the biochemical pathway that triggers the transformation. Winter reindeer have significantly increased intra-ocular pressure, probably because prolonged pupil dilation in low-light conditions can block ocular drainage.The authors propose that this pressure may alter fluid balance within the eye and drain the tapetum, changing collagen fibril spacing. The seasonal transformation must also be reversible. The study notes that reindeer are the only mammal known to undergo this seasonal change in the tapetum.
