Researchers have recently discovered that the eye color of Arctic reindeer changes between the seasons; in turn, this enhances their vision in response to the ever-shifting light intensities throughout the Arctic year. To achieve maximal sight during winter, reindeer eyes transition from gold to blue, adapting to the radical changes of light intensity within their environment.
The research was conducted by a team of scientists working from the University College London (UCL) and the University of Tromsø Norway, with the findings published in the Proceedings of the Royal Society B: Biological Sciences journal. funding was granted by the Biotechnology and Biological Sciences Research Council (BBSRC).
The Tapetum Lucidum
The teams collaborative efforts identified a rather unusual change in a tissue layer of reindeer eyes, called the tapetum lucidum (TL), which means “bright tapestry” in Latin. The TL lies directly behind the retina and aids in reflecting light back towards a special type of neuron that is densely packed within the retina, called a photoreceptor. The TL provides
the light-detecting cells of the retina a second opportunity to intercept stray photons of light, which it had missed first time round.
These photoreceptors contain proteins that “catch” photons of light, the results of which are interpreted within the animal’s brain, depending upon the electrical signals fired along the optic nerve.
Ultimately, the TL provides an animal with superior night vision capabilities, boosting the amount of light that is available to photoreceptors. This has a perceptible affect on the eyes of animals that possess tapetum lucidum; when light shines into the eyes of animals that have this specialized tissue layer, their pupils appear to glow ominously.
We’ve all seen this phenomenon, called eyeshine, in a variety of different creatures – the color of which depends upon the animal being looked at, since the minerals present within the TL crystals can differ slightly. For example, eyeshine is typically green in cats and dogs, but red in rodents and birds.
Arctic Reindeer and the Color-Changing TL
Before beginning their studies, the team was curious as to how the reindeer were able to cope with three months of intense summer sunlight versus, at the opposite end of the spectrum, three months of perpetual winter gloom.
In fact, Professor Glen Jeffrey, a neuroscientist from the University College London, and lead researcher on the project, didn’t believe the study was worth conducting, initially. Jeffrey considered the idea “dumb,” suspecting the eyes of the Arctic reindeer conferred little advantage, and all the adaptive changes to light were part of some neurological trick.
Some 12 years ago, a Norwegian contingent sent over reindeer specimens, which had been obtained by local Sami herders. The Sami people’s homeland is spread out across Finland, Norway, Russia and Sweden, and their livelihood is dependent upon these Arctic reindeer. Jeffrey was shocked to find, after splitting into groups the eyes of reindeer that were killed during the summer from the eyes of reindeer killed during the winter, there was an obvious difference in color.
Stoking Jeffrey’s curiosity, he then embarked upon a series of studies, along with Karl-Arne Stokkan of the University of Tromsø, which sought to explain this aberration. This spurred a great number of quests to the Arctic.
The researchers found, by changing the color of this TL layer, Arctic reindeer were able to reflect different wavelengths of light. During the summer period, the TL layer is a gold color; this is a similar color to that of many other animals, and reflects a large proportion of the light back out of the eye.
Meanwhile, during the murky winter months, the TL layer transitions to a brilliant blue color, thereby reflecting less light out of the eye. Instead, this “winter TL layer” scatters more light through the photoreceptors within the retina, and essentially serves to increase the sensitivity of the retina, in response to the restricted winter light.
This miraculous discovery was the first time that any research group has shown the TL layer of a mammal to alter color in such a coordinated manner. The team theorize that this color change will provide Arctic reindeer a significant advantage during winter, improving their ability to detect and evade predation. Furthermore, these changes also enable the reindeer to forage for food across the darkened terrain of the cold, winter months.
Jeffrey briefly explained the advantage to the dynamically changing TL layer:
“By changing the colour of the TL in the eye reindeer have flexibility to cope better with the extreme differences between light levels in their habitat between seasons.”
Jeffrey then goes on to explain that this property provides an advantage that improves their odds of survival when pitted against predators, such as wolves.
The findings substantiate a study the group had previously performed, which investigated the ability of these majestic creatures to detect ultraviolet light. Arctic light is abundant in UV rays; again, the group concluded that the reindeer use the UV light to detect potential predators. Tying the two studies together, the researchers conjecture that the blue color of the TL layer could favor improved UV sensitivity.
How did this Adaptation in the Tapetum Lucidum Arise?
In explaining how the TL adapts to these different conditions, the researchers sought to understand the changes that take place within the eye during the winter months.
In conditions of low light intensity, the irises contract to allow more light into the eyes, by dilating the pupils. During the winter season, the Arctic reindeers’ pupils are persistently dilated. This blocks some of the small blood vessels that naturally drains fluid from the eyes, causing them to swell.
This swelling then applies pressure to the tapetum lucidum, squeezing fluid out from the ordered collagen fibers, contained within. The collagen fibers then become more densely packed, allowing the tapetum to reflect blue wavelengths of light, instead of yellow wavelengths.
Meanwhile, speaking to the National Geographic, Dan-Eric Nilsson, a vision specialist based at the Lund University, disagrees with this explanation. Nilsson suggests that the reindeer eyes have light-sensitive pigments in the retina that could be responsible for this adaptive change.
We now understand that reindeer eyes shift to a brilliant blue color, during the winter months. However, it seems further evidence will be required to definitively link the afore-mentioned color change with the creatures’ visual capabilities.
By James Fenner