Why Reindeer Eyes Shine Blue in the Dark Arctic Winter


Reindeer have just an amazing visual system. It’s unlike any other mammal we know about, scientists say


Similar to other animals, like cats or deer, the ungulates have light-enhancing tissue in their eyes, known as tapetum lucidum. But in reindeer, the luminescence changes colour from a golden colour in the summer to blue in the winter.

The layer of tissue [tapetum lucidum] is typically found in nocturnal animals because it increases their ability to see in dim light, but reindeer are out and about during the day.

Scientists have long puzzled over that trait, along with the reindeer’s ability to see light in the ultraviolet spectrum, The Washington Post reports.

Nathaniel Dominy, the study’s lead author and a professor of anthropology at Dartmouth College, and researchers from the University of St Andrews’ School of Psychology and Neuroscience say they have one possible explanation. They suggest that the hoofed mammals’ unique eyes, which glow a vivid blue when illuminated in colder months, may be a result of the species evolving so they can more easily find food during dark Arctic winters.

The researchers carried out their work in the Cairngorms mountains in the Scottish Highlands, home to Britain’s only reindeer herd and more than 1,500 species of lichen, including the reindeer's favourite food: a type of lichen known colloquially as “reindeer moss.”

The study is believed to be the first effort to measure the amount of light at different wavelengths that’s being reflected from the lichens that reindeer often eat, the study’s lead author said.

By studying lichen, the researchers found that the reindeer’s preferred meal, as well as several other species that the animals also enjoy feasting upon, absorb UV light. That makes it more visible to hungry reindeer scanning the snow-covered terrain for food.

The researchers note that the blue tapetum also allows up to 60 per cent of ultraviolet light to enter the eye, meaning reindeer see their surroundings in winter in shades of purple with surfaces that reflect UV giving off a glaring glow while UV-absorbing things appear dark.

Images taken with light filters adjusted to mimic reindeer sight showed that the animals probably see lichen beds as dark patches against the highly reflective snow.

They [Reindeer] can see it [lichen] from a distance and this would give them big advantages because then they don’t have to wander around the landscape looking for food, the researcher summarised. 

Source: The Washington Post