In northern latitudes within the Arctic Circle changes in environmental light during the year can be extreme. In Tromsø, Norway, where this research was conducted, the summer sun remains permanently above the horizon from mid-May until the end of July. In late November the sun sets below the horizon and does not reappear until late January.
Researchers from UCL and the University of Tromsø were interested in how animals that live in these extremes see in the perpetual darkness. The subject of their research, published this week, is the Arctic reindeer. The research suggests that the reindeers in Tromsø have an ingenious adaptive trait which helps them adjust their eyesight to suit the changes in light.
Reindeer, like many animals, have a layer of tissue in their eyes called the tapetum lucidum. This layer lies behind the retina, reflecting light back through it and enhancing their night vision. The tapetum lucidum is what gives animals, like cats, which see well in the dark, characteristic eye shine. Scientists have found that reindeer are able to adapt the colour of their tapetum lucidum to change the wavelengths of light it reflects back through the retina.
The scientists found that the reindeers’ tapetum lucidums tested while the sun was permanently above the horizon were golden in colour, like those of most mammals. However, tapetum lucidums tested after the sun had set for winter were deep blue. This change means the mirror-like layer reflects less light out of the eye, scattering it instead though photoreceptors at the back of the eye and increasing the reindeer’s sensitivity to the limited winter light.
The team think the colour change might occur as a result of a pressure change in the eye. In the winter the pupils of the reindeers’ eyes are dilated to let as much light in as possible. This puts the eye balls under more pressure as it prevents the normal draining of fluid. The scientists think the pressure might compress the tapetum lucidum and reduce space between collagen in the tissue which would cause shorter, bluer wavelengths of light to be reflected.
‘This is the first time a colour change of this kind has been shown in mammals,’ said lead researcher Professor Glen Jeffery of UCL. The researchers think this adaption might be advantageous for the reindeer in avoiding predators in the dark.